HK1156015A - Fly ash based lightweight cementitious composition with high compressive strength and fast set - Google Patents

Description

Fly ash based lightweight cement compositions having high compressive strength and fast setting

Cross Reference to Related Applications

This application claims priority to U.S. patent application No. 12/237,634, filed on 25/9/2008, which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates generally to fast setting cementitious compositions that can be used in a variety of applications where rapid hardening and early strength is desirable. In particular, the present invention relates to cementitious compositions having excellent moisture resistance that can be used to make concrete products for use in wet and dry locations in buildings. Precast concrete products such as cement slabs are manufactured under the following conditions: these conditions provide for a rapid setting of the cement mixture so that the panels can be immediately processed after the cement mixture is poured into a stationary or moving form or a continuously moving belt. Ideally, setting of such a cement mixture can be achieved immediately after mixing the cement mixture with a suitable amount of water for about 20 minutes, preferably 10 to 13 minutes, more preferably 4 to 6 minutes.

Background

U.S. patent 6,869,474 to Perez-Pena et al, which is incorporated herein by reference, discusses the extremely rapid setting of cement compositions used to produce cement-based products (e.g., cement boards) by adding an alkanolamine to hydraulic cement, such as portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90 ° F (32 ℃). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. Extremely rapid curing allows rapid production of cementitious products. It has been found that the added triethanolamine is a very powerful accelerator that can produce formulations with relatively short final set times, with increased levels of fly ash and gypsum, and without the need for calcium aluminate cement. However, the formulation with triethanolamine also had a relatively low early compressive strength compared to the cement board formulation containing calcium aluminate cement.

U.S. patent application No. 11/758,947 filed 6.6.2007 by Perez-Pena et al, which is incorporated herein by reference, discusses the extremely rapid curing of cementitious compositions having early compressive strength for the production of cement-based products (e.g., cement boards) by adding an alkanolamine and a phosphate to hydraulic cement, such as portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90 ° F (32 ℃). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. Again, all compositions contain a significant amount of hydraulic cement as well as gypsum.

U.S. patent 4,488,909 to Galer et al, which is incorporated herein by reference, discusses cementitious compositions that are capable of rapid setting. Such compositions allow for high speed production of carbon dioxide resistant cement boards by forming substantially all of the potential glauberite within about 20 minutes after the composition is mixed with water. The basic components of such a cement composition are portland cement, high alumina cement, calcium sulfate, and lime. Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth, and pumice can be added in amounts up to about 25%. The cement composition comprises about 14 to 21% by weight of high alumina cement in combination with other components which enable the early formation of calcium aluminous and other calcium aluminate hydrates responsible for the early setting of the cement mixture. Galer et al in their invention provide for the use of aluminates of High Alumina Cement (HAC)) and the use of sulfate ions of gypsum to form the aluminosilicates and achieve rapid setting of their cement mixtures.

Aluminate is a calcium aluminum sulfate compound with the chemical formula of Ca₆Al₂(SO₄)₃·32H₂O or alternatively 3 CaO. Al₂O₃·3CaSO₄·32 H₂And O. The glauberite forms as long needle crystals and provides the cement board with a rapid early strength so that it can be handled rapidly after pouring them into a mould or onto a continuously cast and shaped strip.

In general, the rapidly curing formulations of Galer et al suffer from several limitations. As highlighted below, these limitations constitute a more serious concern for the production of cementitious products (e.g., cement boards).

U.S. patent No. 5,536,310 to Brook et al discloses a cement composition comprising 10-30 parts by weight (pbw) of a hydraulic cement such as portland cement, 50-80pbw of fly ash, and 0.5-8.0pbw (expressed as free acid) of a carboxylic acid (e.g., citric acid) or their alkali metal salts such as tri-potassium citrate or tri-sodium citrate, along with other conventional additives including retarder additives such as boric acid or borax, which are used to accelerate the reaction and cure time of the composition to overcome the disclosed disadvantage of using a high fly ash content in cement compositions.

U.S. patent No. 5,536,458 to Brook et al discloses a cement composition comprising a hydraulic cement such as portland cement, 70-80 parts by weight of fly ash, and 0.5-8.0pbw of a free carboxylic acid such as citric acid or an alkali metal salt thereof, e.g., potassium or sodium citrate, with other conventional additives including retarder additives such as boric acid or borax, which are used to accelerate the reaction and setting time of the composition to overcome the known disadvantages of using a high fly ash content in cement compositions.

U.S. patent No. 4,494,990 to Harris et al discloses a cement mixture of portland cement (e.g., 25-60pbw), fly ash (e.g., 3-50pbw), and less than 1pbw sodium citrate.

U.S. patent No. 6,827,776 to Boggs et al discloses a hydraulic cement composition containing portland cement, fly ash, having a set time controlled by the pH of an activator slurry of an acid (preferably citric acid) and a base (which may be a strong oxide of an alkali or alkaline earth metal or a salt of the acid component).

U.S. 5,490,889 to Kirkpatrick et al discloses a blended hydraulic cement consisting of: water, fly ash (50.33-83.63pbw), portland cement, ground silica, boric acid, borax, citric acid (0.04-2.85pbw), and an alkali metal activator such as lithium hydroxide (LiOH) or potassium hydroxide.

U.S. patent No. 5,997,632 to Styron discloses a hydraulic cement composition containing 88-98 wt.% fly ash, 1-10 wt.% portland cement, and from about 0.1-4.0 wt.% citric acid. The minimum lime content to achieve the desired 21% is provided by the sub-fume fly ash or the sub-fume fly ash and a beneficiation reagent. In addition to citric acid, Styron uses a source of base, such as potassium hydroxide or sodium hydroxide.

The final set time of prior art cement mixes is typically greater than 9 minutes and can be extended to 2-3 hours for standard concrete products. The final set time is generally defined as the time in which the cement mixture sets to the point where the concrete product made therefrom can be handled and stacked, although the chemical reaction can last for an extended period of time.

The content of high alumina cement (also called calcium aluminate cement) in such reactive powder blends is also very high in prior art concrete products. Typically, the aluminous cement is greater than 14 wt% of the reactive powder blend.

Summary of The Invention

It is an object of the present invention to provide a method for producing a fast setting cementitious slurry.

It is another object of the present invention to provide a lightweight cementitious composition having improved early and final compressive strength. These cement compositions comprise potassium citrate, sodium citrate, or mixtures thereof.

The present invention includes a method of providing a lightweight cementitious mixture having rapid setting, improved compressive strength, and water resistance, comprising: mixing water, reactive powder, a set-promoting amount of alkali metal citrate, and a lightweight aggregate at or above ambient temperature, wherein the ratio of water to reactive powder solids is about 0.17 to 0.35: 1.0 and more preferably about 0.20 to 0.23: 1.0, the reactive powder mixture comprising 75 to 100 wt.% fly ash and 0 to 25 wt.% hydraulic cement and gypsum.

Preferably, the reactive powder is free of hydraulic cement and free of gypsum (hydrated calcium sulfate).

The cementitious active powder includes at least fly ash and may also include hydraulic cement, such as portland cement or Calcium Aluminate Cement (CAC) (also commonly referred to as aluminous or aluminous cement), calcium sulfate, and a non-fly ash mineral additive.

Up to 25% by weight of the cement reactive powder blend of the cement composition may be a non-fly ash mineral additive with substantially little or no cementitious properties.

Such cement reactive powders generally contain about 10 to 40 wt.% lime and more typically 20 to 30 wt.% lime. However, if the composition of the reactive powder already contains sufficient lime, the addition of lime is not required to obtain rapid setting. For example, class C fly ash generally includes lime. Thus, the reactive powder blend of such cementitious compositions is typically free of externally added lime.

Typically, the slurry has an initial temperature of from room temperature to about 100 ° F to 115 ° F (24 ℃ to about 38 ° -46 ℃).

The final set time of the cement composition (i.e. the time after which the cement board can be treated) as measured according to Gilmore needle (Gilmore needle) should be at most 20 minutes, preferably 10 to 13 minutes or less, more preferably about 4 to 6 minutes after mixing it with an appropriate amount of water. The shorter set time and higher early compressive strength help increase production yield and reduce manufacturing costs of the product.

The very fast setting cement compositions of the present invention can be used in a variety of applications where rapid hardening and early strength is desired. The use of alkali metal citrates, such as potassium citrate and/or sodium citrate, to promote setting of the cement composition makes it possible to increase the production rate of cement products, such as cement boards, when the slurry is formed at elevated temperatures.

The amount of alkali metal citrate in the slurry is preferably in the range of about 1.5 wt.% to 6 wt.%, preferably about 1.5 wt.% to 4.0 wt.%, more preferably about 2 wt.% to 3.5 wt.% and most preferably about 3.5 wt.%, based on the cement reactive components of the present invention. Potassium citrate or sodium citrate is preferred. As mentioned above, these weight percentages are based on 100 parts by weight of this reactive component (cement reactive powder). Thus, for example, for 100 pounds of cement reactive powder, there is about 1.5 to 4.0 total pounds of potassium and/or sodium citrate.

A typical cement reactive powder of the invention comprises from 75 to 100 wt% fly ash and from 0 to 25 wt% hydraulic cement, such as portland cement or gypsum. Typically, at least half of the fly ash is class C fly ash.

Another typical cement reactive powder comprises, based on the weight of the reactive powder, from 75 to 100 wt% fly ash, from zero to 20 wt% calcium aluminate cement, from zero to 7 wt% calcium sulfate, no gypsum and no hydraulic cement other than calcium aluminate cement.

There is a synergy between the alkali metal citrate and fly ash. For compositions containing high amounts of fly ash, the addition of alkali metal salts has the following benefits compared to comparable compositions using promoters like calcium aluminate cement, triethanolamine or corrosive alkali metal hydroxides: increased early and long term compressive strength is achieved.

In addition, the addition of alkali metal citrate improves the mix fluidity compared to other accelerators (e.g., aluminum sulfate) which may cause premature setting of the concrete mix.

Other additives, such as inert aggregates, which are not considered to be cement reactive powders, may also be present as part of the overall cement composition. Such other additives include one or more of sand, aggregate, lightweight filler, water reducing agents, such as superplasticizers, set accelerators, set retarders, air entraining agents, foaming agents, shrinkage control agents, slurry viscosity modifiers (thickeners), colorants, and internal curing agents, which may be included as desired depending on the processing capabilities and application of the cementitious compositions of the invention.

Such lightweight cement compositions of the present invention can be used to manufacture prefabricated concrete building products, such as cement boards with excellent moisture resistance for use in wet and dry locations of a building. Such prefabricated concrete products, such as cement slabs, are manufactured under the following conditions: these conditions provide for rapid curing of the cement mixture so that the panels can be rapidly handled after the cement mixture is poured into a stationary or moving form or onto a continuously moving belt.

The lightweight cement composition can be used in any concrete product application, including concrete panels, floors, overlays, facings, pile caps, as well as repair mixtures for concrete roads. Concrete products made using the lightweight compositions of the invention are of particular advantage for applications requiring water resistance compared to gypsum-containing compositions and for applications requiring higher compressive strength than cement-containing compositions having a higher carbon footprint.

All percentages, ratios, and proportions herein are by weight unless otherwise specified.

Brief description of the drawings

Figure 1 is a graph of the results of example 1 showing the effect of increasing sodium citrate on the rate of temperature rise for a mixture with borax, boric acid and citric acid.

Figure 2 is a graph of the results of example 1 showing the effect of increasing sodium citrate on temperature rise for a mixture with boric acid and citric acid.

Figure 3 is a graph of the results of example 2 showing the effect of increasing potassium hydroxide on temperature rise for a mixture with citric acid and sodium citrate.

Figure 4 is a graph of the results of example 4 showing the temperature rise for the mixture with potassium citrate and no potassium hydroxide.

Figure 5 is a graph of the results of example 5 showing the temperature rise for a mixture comprising potassium citrate or sodium citrate mixed with water at room temperature.

FIG. 6 is a graph of the results of example 8 showing the temperature rise using a water to cement weight ratio of 0.30: 1 for mixtures containing different proportions of fly ash and Portland type III cement.

FIG. 7 is a graph of the results of example 9, showing the effect on the temperature rise of mixtures 1-4 in this example with different ratios of water to fly ash without the use of Portland cement.

FIG. 8 is a graph of the results of example 9 showing the temperature rise for mixtures 3,5, 6 and 7 having different ratios of fly ash and Portland cement type III with citrate where the weight ratio of water to the combined weight of fly ash and Portland cement is 0.20: 1.

Figure 9 is a graph of the results for the mixtures of example 10 having different amounts of potassium citrate (only fly ash without portland cement) and showing that the addition of potassium citrate significantly increases the temperature ramp rate of the fly ash based mixture.

Detailed description of the invention

The present invention includes a method of providing a lightweight cementitious mixture having improved compressive strength and water resistance, comprising: water, active powder, an alkali metal citrate, and lightweight aggregate are mixed, wherein the ratio of water to reactive powder solids is about 0.17 to 0.35: 1.0, typically about 0.17 to 0.30: 1.0, and more preferably about 0.2 to 0.23: 1.0. The reactive powder comprises 75 to 100 wt.% fly ash and 0 to 25 wt.% hydraulic cement and/or gypsum. Typically, the present invention mixes the cement reactive powder comprising fly ash with potassium citrate and/or sodium citrate and water at an initial slurry temperature of at least room temperature to 115 ° F (24 ℃ to 41 ℃) to produce a rapid set of preferably less than 10 to 13 minutes, more preferably about 4 to 6 minutes or less.

The present invention also provides a cement composition having enhanced rapid final set properties and enhanced early compressive strength.

Typical ingredients are listed in table a below.

Generally, the weight ratio of water to cement reactive powder is about 0.15-0.3: 1.0. The inert lightweight aggregate is not part of the cement reactive powder.

While not wishing to be bound to a particular theory, it is theorized that increased early and compressive strength is achieved while rapidly setting by providing a high fly ash mineral content of 75 to 100 wt% to the cement reactive powder and preferably without portland or calcium aluminate cement or gypsum and mixing the cement reactive powder, alkali metal citrate and water to form a slurry at elevated temperatures above 20 ℃, such that alkali metal aluminosilicate hydrates and/or calcium aluminosilicate compounds present in the fly ash are formed due to hydration of the reactive powder blend with the alkali metal citrate.

Thus, a suitable amount of water is provided to hydrate the cement reactive powder and rapidly form alkali aluminosilicate hydrates as well as other hydrates present in the fly ash. In general, the amount of water added will be greater than theoretically required for hydration of the cement reactive powder. This increased water content contributes to the workability of the cementitious slurry. Typically, the weight ratio of water to reactive powder blend in the slurry is about 0.20 to 0.35: 1, more typically about 0.20 to 0.30: 1, preferably about 0.20 to 0.23: 1. The amount of water depends on the desired value of the individual materials present in the cement composition.

These alkali metal aluminosilicate hydrates and/or hydrates of other aluminosilicate and/or calcium aluminosilicate compounds form very rapidly during hydration, thus imparting rapid setting and stiffness to the mixture made with the cement reactive powder blend of the cement composition of the invention. In the manufacture of cement-based, for example cement boards, mainly alkali aluminosilicate hydrates and/or other hydrates of aluminosilicates and/or calcium aluminosilicate compounds are formed, which makes it possible to treat the cement boards within a few minutes after mixing the cement composition of the invention with a suitable amount of water.

The setting of the composition is characterized by initial and final set times as measured using a Gilmo needle as specified in ASTM C266 test method. The final set time also corresponds to the time a concrete product (e.g. a concrete panel) has hardened sufficiently that it (in the case of a concrete floor or road) can be handled or transported. A relatively higher early (3 to 5 hours) compressive strength may be an advantage for concrete materials because it can withstand higher stresses without deforming. One of ordinary skill in the art will appreciate that the curing reaction continues for an extended period of time after the final cure time has been reached.

The early strength of the composition is characterized by measuring the compressive strength after 3 to 5 hours of cure as specified in ASTM C109. Achieving high early strength allows for ease of handling of these stacked panels.

Reactive powder for cement

Such cement reactive powders include fly ash and optionally non-fly ash mineral additives, hydraulic cement and optionally gypsum. Such cement reactive powders typically comprise from 75% to 100% fly ash and from 0 to 25 wt.% of a member selected from the group consisting of: hydraulic cement, gypsum and non-fly ash mineral additives. Such cement reactive powder preferably comprises 88.5 wt% to 100 wt% fly ash. More preferably, the cement reactive powder comprises 88.5 wt% to 100 wt% fly ash and is free of hydraulic cement and free of gypsum.

Preferably, the cement reactive powder comprises 10 to 40 wt.% lime. However, this lime is not added lime as a whole. But it is contained in another component of the cement reactive powder, for example fly ash.

The main component of the cement reactive powder of the cement composition of the invention is a fly ash mineral additive, preferably class C fly ash. Fly ash is described in the following section entitled fly ash and non-fly ash mineral additives.

In addition to fly ash, this cement reactive powder comprises 0 to 25 wt.% of an optional cement additive, such as portland cement, calcium aluminate cement, calcium sulfate or gypsum (landplaster). However, the lower water content cement compositions of the invention (i.e., cement compositions having a water to reactive powder weight ratio of about 0.17 to 0.35: 1.0) with these optional cement additives have significantly reduced compressive strength compared to the same lower water content compositions of the invention without additional cement additives.

For example, in some cement reactive powder blends, when compressive strength is not required or when higher water to reactive powder ratios are to be used (e.g., at a ratio of about 0.35: 1.0), about 0 to 25 wt% portland cement and 75 to 100 wt% fly ash can be used.

Fly ash and non-fly ash mineral additives

The hydraulic cement of the conventional reactive powder composition is essentially replaced by fly ash having pozzolanic properties (especially class C fly ash) along with other optional non-fly ash mineral additives having substantially little or no cementitious properties. Mineral additives other than fly ash having pozzolanic properties are particularly preferred in the cement reactive powder of the invention.

ASTM C618-97 defines pozzolanic materials as "siliceous or siliceous and aluminum materials that have little or no cementitious value by themselves, but will chemically react with calcium hydroxide at ordinary temperatures in a finely dispersed form and in the presence of moisture to form compounds with cementitious properties". Different natural and man-made materials are known as pozzolanic materials with pozzolanic properties. Some examples of pozzolanic materials include pumice, perlite, diatomaceous earth, silica fume, tuff, volcanic earth, rice hulls, metakaolin, ground granulated blast furnace slag, and fly ash.

All of these pozzolanic materials may be used alone or in combination as part of the cement reactive powder of the present invention.

Fly ash is a preferred pozzolan in the cement reactive powder blend of the present invention. Fly ash containing high calcium oxide and calcium aluminate content (e.g., class C fly ash of ASTM C618 standard) is preferred, as explained below. Other mineral additives such as calcium carbonate, vermiculite, clay, and ground mica may also be included as mineral additives.

Fly ash is a by-product of fine powders formed from the combustion of coal. Pulverized coal fired power plant utility boilers produce most commercially available fly ash. These fly ashes are mainly composed of vitreous spherical particles together with residual hematite and magnetite, char and some crystalline phases formed during cooling. The structure, composition and characteristics of the fly ash particles depend on the structure and composition of the coal and the combustion process in which the fly ash is formed. The ASTM C618 standard identifies two major grades of fly ash (grade C and grade F) for concrete. These two grades of fly ash are generally derived from different kinds of coal due to the different processes of coal formation that occur during the course of the geological time period. Class F fly ash is typically produced from the combustion of anthracite or bituminous coal, while class C fly ash is typically produced from lignite or bituminous coal.

The ASTM C618 standard differentiates primarily into class F and class C fly ashes based on their pozzolanic properties. Thus, in the ASTM C618 standard, the primary specification difference between class F fly ash and class C fly ash is SiO in the composition₂+Al₂O₃+Fe₂O₃To the minimum of (c). For class F fly ash, SiO₂+Al₂O₃+Fe₂O₃The minimum of (A) is 70% and for class C fly ash is 50%. Thus, class F fly ash is more pozzolanic than class C fly ash. Although not specifically identified in the ASTM C618 standard, class C fly ash typically has a high calcium oxide (lime) content.

Class C fly ash generally has cementitious properties in addition to pozzolanic properties due to free lime (calcium oxide), while class F has little cementitious properties when mixed with water alone. The presence of high calcium oxide content gives class C fly ash cementitious properties leading to the formation of calcium silicate and calcium aluminate hydrates when mixed with water. As seen in the examples below, class C fly ash has been found to provide excellent results, particularly in preferred formulations that do not use calcium aluminate cement and gypsum.

Typically, at least 50 wt.% of the fly ash in the cement reactive powder is class C fly ash. More typically, at least 75 wt.% of the cement reactive powder is class C fly ash. Still more preferably, at least 88.5 wt.% of the cement reactive powder is class C fly ash.

A typical mineral found in fly ash is (among others) quartz (SiO₂) Mullite (Al)₂Si₂O₁₃) Calcium-aluminium-yellow feldspar (Ca)₂Al₂SiO₇) Hematite (Fe)₂O₃) Magnetite (Fe)₃O₄). In addition, aluminum silicate polymorphous minerals (e.g., sillimanite, kyanite, and andalusite, all of which are commonly found in rock, pass throughMolecular formula Al₂SiO₅Representation) are also found in fly ash.

A typical suitable class C fly ash produced from bituminous coal has the composition listed below in table B.

The fine precision of such Fly Ash is typically such that less than about 34% is retained in a 325 mesh screen (U.S. sieve) when tested in ASTM test method C-311 ("sampling Testing Procedures for Fly as Mineral addition for Portland Cement"). This fly ash is preferably recovered and used at the time of drying due to its self-setting nature.

Hydraulic cement

Fly ash constitutes essentially all of the cementitious material of the reactive powder of the invention. In some cases, such reactive powders may also include optional cement additives, for example, hard cement or gypsum may be added. However, these optional cement additives are not preferred because they reduce the ultimate compressive strength of the lightweight aggregate composition of the present invention.

Hydraulic cement is a material that sets and hardens due to a chemical reaction with mixed water after being combined with water, and maintains strength and stability (even under water) after hardening. Portland cement is a typical hydraulic cement. It should be understood that as used herein, "hydraulic cement" does not include gypsum, which does not gain strength under water, although some gypsum is typically included in portland cement. The ASTM C150 standard specification for portland cement defines portland cement as a hydraulic cement produced by grinding clinker, which consists primarily of hydraulic calcium silicate, usually with one or more forms of calcium sulfate as an interground addition.

To manufacture portland cement, an intimate mixture of limestone and clay is ignited in a kiln to form portland cement clinker. In this clinker there are the following four main phases of portland cement-tricalcium silicate (3CaO · SiO)₂Also known as C₃S), dicalcium silicate (2 CaO. SiO)₂Is called C₂S), tricalcium aluminate (3 CaO. Al)₂O₃Or C₃A) And tetracalcium aluminoferrite (4 CaO. Al)₂O₃·Fe₂O₃Or C₄AF). The resulting clinker containing the above compounds is interground with calcium sulfate to the desired fineness to produce portland cement.

Other compounds present in small amounts in portland cement include double salts of alkali metal sulfates, calcium oxide, and magnesium oxide. When cement panels are made using portland cement, such portland cement will typically be in the form of very fine particles, such that the surface area of the particles is greater than 4,000cm²Per gram and typically between 5,000 and 6,000cm²Between/gram as measured by the Blaine surface area method (ASTMC 204). Among the different classes of recognized portland cements, the astm type iii portland cement is most preferred among the cement reactive powders of the cement compositions of the invention. This is due to its relatively faster reactivity and high early strength development.

In the present invention, the need for the use of hydraulic cement (e.g. type III portland cement) is avoided and a relatively faster development of early strength can be obtained by using only fly ash instead of the mixture containing type III portland cement. Other recognized cements that are not required in the compositions of the present invention include class I portland cements or other hydraulic cements, including class II portland cements, white cements, slag cements such as blast furnace slag cement and pozzolan blended cements, expansive cements, calcium sulfoaluminate cements, and oil well cements.

Calcium aluminate cement

Calcium Aluminate Cement (CAC) is another type of hydraulic cement that may form a component of the reactive powder blend of certain embodiments of the present invention when higher compressive strength is not required for low water content slurries containing substantial amounts of fly ash.

Calcium Aluminate Cement (CAC) is also commonly referred to as aluminous or aluminous cement. Calcium aluminate cements have a high alumina content, typically about 36 wt% to 42 wt%. Higher purity calcium aluminate cements, in which the alumina content ranges up to 80 wt.%, are also commercially available. These higher purity calcium aluminate cements tend to be very expensive relative to other cements. The calcium aluminate cement used in the compositions of certain embodiments of the present invention is finely ground to help drive the aluminate into the aqueous phase so that rapid formation of calcium aluminosilicates and other calcium aluminate hydrates can occur. The surface area of the calcium aluminate cement that may be used in certain embodiments of the compositions of the present invention will be greater than 3,000cm²Per gram, and typically about 4,000 to 6,000cm²Per gram as measured by Blaine surface area method (ASTM C204).

Several manufacturing processes have also emerged to produce calcium aluminate cements globally. Typically, the main raw materials used in the manufacture of calcium aluminate cement are bauxite and limestone. One manufacturing process used in the united states for the production of calcium aluminate cement is as described below. Bauxite ore is first crushed and dried, then ground with limestone. The dry powder comprising bauxite and limestone is then fed into a rotary kiln. A pulverized low ash coal is used as a fuel for the drying oven. The reaction between bauxite and limestone takes place in the kiln and the molten product collects at the lower end of the kiln and is poured into a chute located at the bottom. This molten clinker is quenched with water to form clinker pellets, which are then transported to a stockpile. This particulate is then ground to the desired fineness to produce the final cement.

Several calcium aluminate compounds are formed during the manufacturing process of the calcium aluminate cement. Form aThe main compound of (A) is monocalcium aluminate (CaO. Al)₂O₃Also known as CA). Other calcium aluminate and calcium silicate compounds formed include 12CaO 7Al₂O₃Also known as C₁₂A₇；CaO·2Al₂O₃Also known as CA₂(ii) a Dicalcium silicate (2 CaO. SiO)₂Is called C₂S); dicalcium aluminosilicate (2 CaO. Al)₂O₃·SiO₂Referred to as C₂AS). Several other compounds containing relatively high proportions of iron oxides are also formed. These include calcium ferrites, e.g. CaO. Fe₂O₃Or CF and 2 CaO. Fe₂O₃Or C₂F and calcium aluminoferrite, e.g. tetracalcium aluminoferrite (4 CaO. Al)₂O₃·Fe₂O₃Or C₄AF)，6CaO·Al₂O₃·2Fe₂O₃Or C₆AF₂) And 6CaO 2Al₂O₃·Fe₂O₃Or C₆A₂F) In that respect Other minor components present in calcium aluminate cements include magnesium oxide (MgO), titanium dioxide (TiO)₂) Sulfate, and base.

Calcium sulfate

Different forms of calcium sulfate, as shown below, may be used in the present invention to provide sulfate ions for the formation of glauberite and other calcium sulfoaluminate hydrate compounds:

dihydrate-CaSO₄·2H₂O (also commonly known as gypsum or landplaster)

hemihydrate-CaSO₄·1/2 H₂O (also commonly referred to as stucco or plaster of paris or simply plaster)

anhydrite-CaSO₄(also known as anhydrous calcium sulfate)

Landplaster is a relatively low purity gypsum and is preferred for economic reasons, although higher purity grades of gypsum may be used. The gypsum powder is made from excavated gypsum andis ground into relatively small particles so that the specific surface area is more than 2,000cm²Per gram and typically about 4,000 to 6,000cm²Per gram as measured by Blaine surface area method (ASTM C204). Such fine particles are readily soluble and provide the gypsum required to form the glauberite. Synthetic gypsum obtained as a by-product from various manufacturing industries may also be used as a preferred calcium sulfate for the present invention. Two other forms of calcium sulfate, hemihydrate and anhydrite, may also be used in the present invention in place of gypsum, the hydrate form of calcium dithioate.

Alkali metal salts of citric acid

In the present invention, the use of an alkali metal salt of citric acid, such as sodium citrate or potassium citrate, produces a mixture that has relatively good flow properties and does not become too fast (i.e., does not become stiff faster than 5-10 minutes after mixing above room temperature) while achieving good early compressive strength.

The amount of alkali metal citrate (e.g., potassium citrate or sodium citrate) used is about 1.5 wt.% to 6.0 wt.%, preferably about 1.5 wt.% to 4.0 wt.%, more preferably about 2.0 wt.% to 3.5 wt.% and most preferably about 3.5 wt.%, based on 100 parts of the cement reactive component of the present invention. Thus, for example, for 100 pounds of cement reactive powder, about 1.5 to 4.0 total pounds of potassium and/or sodium citrate may be present. Preferred alkali metal citrates are potassium citrate and sodium citrate and in particular tripotassium citrate monohydrate and trisodium citrate monohydrate.

Coagulation retarder

The use of a set retarder as a component of the composition of the present invention is particularly useful where the temperature of the initial slurry used to form the cement-based product is particularly high, typically greater than 100 ° F (38 ℃). At such relatively high initial slurry temperatures, the retarder promotes physical and chemical reactions between the different reactive components in the composition, resulting in a favorable slurry temperature rise response and rapid setting behavior. Without the addition of a retarder, the stiffening of the reactive powder blend of the invention can occur very rapidly (immediately after the addition of water to the mixture). Rapid hardening of the mixture, also known as "false setting", is undesirable because it interferes with the proper and complete formation of the calcium aluminosilicates, prevents the normal formation of calcium silicate hydrates at a later stage, and results in the development of an extremely poor and weak microstructure of the hardened cement mortar.

The primary function of the retarder in the composition is to keep the slurry mixture from rigidifying too rapidly to promote synergistic physical interactions and chemical reactions between the different reactive components. Other secondary benefits obtained by adding a retarder to the composition include a reduction in the amount of superplasticizer and/or water required to achieve a slurry mixture of workable consistency. All the above advantages are achieved due to the suppression of false coagulation. Examples of set retarders include boric acid, borax, citric acid, potassium tartrate, sodium tartrate, and the like.

Furthermore, since set retarders prevent the slurry mixture from rigidifying too rapidly, their addition plays an important role and helps to form good edges during the cement board manufacturing process. The weight ratio of set retarder to cement reactive powder blend is generally less than 1.0 wt%, preferably about 0.04 wt% to 0.3 wt%.

In the present invention, it has been found that the use of conventional retarders such as citric acid, tartaric acid, malic acid, acetic acid, boric acid, and the like can be avoided by using only alkali metal citrates such as sodium or potassium citrates, and that the use of these alkali metal citrates in the absence of these conventional set retarders provides good flow properties and prevents the cement slurry from rigidifying too rapidly.

Secondary inorganic setting accelerator

As discussed above, such alkali metal citrate salts are primarily responsible for imparting extremely rapid setting characteristics along with compressive strength to these cement mixtures. However, in combination with the alkali metal citrate, other inorganic set accelerators may be added as secondary inorganic set accelerators in the cement composition of the invention.

It is expected that the addition of these secondary inorganic set accelerators gives only a small reduction in set time compared to the reduction achieved due to the addition of alkali metal citrate. Examples of such secondary inorganic solidification promoters include sodium carbonate, potassium carbonate, calcium nitrate, calcium nitrite, calcium formate, calcium acetate, calcium chloride, lithium carbonate, lithium nitrate, lithium nitrite, aluminum sulfate, alkanolamine, polyphosphate, sodium hydroxide, potassium hydroxide, and the like. When corrosion of cement panel fasteners is a concern, potassium hydroxide, sodium hydroxide, and calcium chloride should be avoided. Secondary inorganic set accelerators are generally not required. The use of a secondary set accelerator is not required and is not part of the preferred compositions of the present invention. If used, the weight ratio of secondary inorganic set accelerator to 100 parts by weight of cement reactive powder blend will typically be less than about 1.0 wt.%, preferably less than about 0.25 wt.%. These secondary inorganic setting accelerators may be used alone or as a combination.

Preferably, lithium carbonate and potassium carbonate are not used.

Other chemical additives and ingredients

Chemical additives, such as water reducing agents (superplasticizers), may be included in the compositions of the present invention. They can be added in dry form or in the form of a solution. The superplasticizer helps to reduce the water requirement of the mixture. Examples of superplasticizers include polynaphthalene sulfonates, polyacrylates, polycarbonates, lignosulfonates, melamine sulfonates, and the like. Depending on the type of superplasticizer used, the weight ratio of the superplasticizer (based on dry powder weight) to the reactive powder blend is typically about 2 wt.% or less, preferably about 0.1 wt.% to 1.0 wt.%.

When it is desired to produce a lightweight product, such as lightweight cement board, air entraining agents (or foaming agents) can be added to lighten the product.

Air entraining agents are added to the cement slurry to form air bubbles (foam) in situ. Air-entraining agents are typically surfactants used to purposefully trap microscopic air bubbles in the concrete. Alternatively, an air entraining agent is used to produce the foam externally, which is introduced into the composition mixture of the present invention during the mixing operation to reduce the density of the product. To produce the foam externally it is typical to mix air-entraining agent (also known as a liquid foaming agent), air and water to form the foam in a suitable foam-producing device and then add the foam to the cement slurry.

Examples of air entraining/foaming agents include, among others, alkyl sulfonates, alkyl benzene sulfonates, and alkyl ether sulfate oligomers. Details on the general formula of these blowing agents can be found in U.S. Pat. No. 5,643,510, which is incorporated herein by reference.

Use may be made of (blowing agents) such as those which comply with the standards as set forth in ASTM C260 "Standard Specification for air-advancing additives for Concrete" (8/1/2006). Such Air-entraining agents are well known to those of ordinary skill in the art and are described in "Design and Control of concentrate Mixtures," fourth Edition, Portland center Association, specific Chapter 8 entry, "Air EntrainedConcrete," Kosmatka et al (referenced in U.S. patent application publication No. 2007/0079733A 1). Commercially available air-entraining materials include: rosin wood resins, sulfonated hydrocarbons, fatty and resinous acids, aliphatic substituted aryl sulfonates, such as sulfonated lignins and various other interface-active materials (which are typically in the form of anionic or nonionic surfactants), sodium abietate, saturated or unsaturated fatty acids and their salts, surfactants, alkyl-aryl-sulfonates, phenol ethoxylates, lignosulfonates, resin soaps, sodium hydroxystearate, lauryl sulfate, ABS (alkylbenzene sulfonate), LAS (linear alkylbenzene sulfonate), alkanesulfonates, polyoxyethylene alkyl (phenyl) ethers, esters of polyoxyethylene alkyl (phenyl) ether sulfates or their salts, esters of polyoxyethylene alkyl (phenyl) ether phosphates or their salts, proteinaceous materials, alkenyl sulfosuccinates, esters of aliphatic substituted aryl sulfonates, esters of aliphatic and unsaturated fatty acids and their salts, alkyl-aryl-sulfonates, esters of phenol ethoxylates, esters of lignin sulfonates, resin soaps, sodium hydroxystearate, lauryl sulfate, ABS (alkylbenzene sulfonate), LAS (linear alkylbenzene sulfonate), alkanesulfonates, polyoxyethylene alkyl (phenyl, Alpha olefin sulfonate, a sodium salt of an alpha olefin sulfonate, or sodium lauryl sulfate, and mixtures thereof.

Typically, the air entraining (foaming) agent is about 0.01 wt.% to 1 wt.% by weight of the total cement composition.

Other chemical mixtures, such as shrinkage control agents, colorants, viscosity modifiers (thickeners), and internal curing agents, may also be added to the compositions of the present invention, if desired.

Coarse cloth

Different types of discrete reinforcing fibers may also be included in the cementitious compositions of the present invention. Depending on the function and application of the product, scrim made of a variety of materials such as polymer coated fiberglass, and polymeric materials such as polypropylene, and nylon may be used to reinforce the cement-based product. The cementitious panels produced according to the present invention are typically reinforced with a scrim of polymer-coated glass fibers.

Aggregate and filler

Although the cement reactive powder blends of the present disclosure define the rapidly setting component of the cement composition of the present invention, it will be understood by those of ordinary skill in the art that other materials may be included in the composition depending on the intended use and application of the composition.

For example, for cementitious board applications, it is desirable to produce a lightweight board without unduly compromising the desired mechanical properties of the product. This object is achieved by the addition of lightweight aggregates and fillers. Examples of useful lightweight aggregates and fillers include blast furnace slag, volcanic tuff, pumice, expanded forms of clay, shale and perlite, hollow ceramic spheres, hollow plastic spheres, expanded plastic beads, and the like. Expanded clay and shale aggregates are particularly useful for producing cement boards. When used in this composition, the expanded beads as well as the hollow plastic spheres require very small amounts due to their very low bulk density.

The weight ratio of the lightweight aggregate or filler to the reactive powder blend may be about 1/100 to 200/100, preferably about 2/100 to 125/100, depending on the lightweight aggregate or filler selected. For example, to make lightweight cement board, the weight ratio of the lightweight aggregate or filler to the reactive powder blend is preferably from about 2/100 to 125/100. In applications where lightweight product characteristics are not a critical indicator, river sand and coarse aggregate commonly used in concrete buildings can be utilized as part of the composition of the present invention.

Initial slurry temperature

In the present invention, it has been found that forming the slurry under conditions that provide an initially high slurry temperature is important to achieve rapid setting and hardening of the cement formulation. The initial slurry temperature should be at least about room temperature to about 35 ℃. Slurry temperatures in the range of 38 ℃ to 41 ℃ give short setting times. The initial slurry temperature is preferably about 38 ℃ to 41 ℃.

Generally, increasing the initial temperature of the slurry within this range increases the rate of temperature rise and reduces the set time as the reaction proceeds. Thus, an initial slurry temperature of 95 ° F (35 ℃) is better than an initial slurry temperature of 90 ° F (32.2 ℃), a temperature of 100 ° F (37.7 ℃) is better than 95 ° F (35 ℃), a temperature of 110 ° F (40.6 ℃) is better than 105 ° F (41.1 ℃), and so on. It is believed that the benefit of increasing the initial slurry temperature decreases when the upper end of the wide temperature range is reached.

As understood by those of ordinary skill in the art, achieving an initial slurry temperature may be accomplished by more than one method. Perhaps the most convenient method is to heat one or more components of the slurry. In these examples, the invention provides water heated to a temperature such that the resulting slurry is at the desired temperature when the dry reactive powder and non-reactive solids are added. Alternatively, the solid may be provided at a temperature above ambient if desired. Another possible method can be used when steam is used to provide heat to the slurry.

Although perhaps slower, a slurry can be prepared at ambient temperature and heated rapidly (e.g., within about 10, 5, 2, or 1 minute) to raise the temperature to about 90 ° F or higher (or any other of the ranges listed above) and still achieve the benefits of the invention.

Making prefabricated concrete products, e.g. cement slabs

Precast concrete products such as cement slabs are most effectively manufactured in a continuous process in which the reactive powder blend is mixed with aggregate, fillers and other necessary ingredients, followed by the addition of water and other chemical additives just prior to placing the mixture in a mold or on a continuous casting and forming belt.

Due to the rapid setting characteristics of the cement mixture, it is understood that mixing of the dry components of the cement blend with water is typically performed just prior to the casting operation. As a result of the formation of alkali aluminosilicate hydrates and/or aluminosilicate and/or calcium aluminosilicate compounds other hydrates, the concrete product hardens, is easily cut, handled and stacked for further curing.

Examples of the invention

The following example illustrates the effect of the addition of potassium citrate and sodium citrate on the temperature rise behavior, setting characteristics and Cubic Compressive Strength (CCS) of a slurry of the cementitious composition of the invention comprising as components of the reactive powder a mixture of portland cement, class C fly ash and calcium sulfate dihydrate (lime fertilizer).

The mixture used to activate the fly ash, e.g., potassium citrate, sodium citrate, and optional additives (e.g., citric acid, borax, boric acid), is added to the mix water prior to mixing with the fly ash, cement, and optional lightweight aggregate.

The compositions described herein use expanded clay aggregate in a 0.56: 1.0 weight ratio in combination with cement (reactive powder).

The temperature of the liquid is adjusted to obtain a specific mixing temperature before mixing with the cement. After mixing in a Hobart mixer, approximately 280 grams of the mixture was placed in a 6 ounce styrofoam cup and in an insulated styrofoam box. The temperature response WAs measured continuously as part of HYDRA SERIES portable data collection products using a computerized data collection application provided by fluke corporation, Everett, WA 98203.

The final set time was determined using a gilmor needle according to the procedure set forth in ASTM C266. The cubes were kept in a sealed plastic bag containing a wet towel at a temperature of 68 ℃ until the 3 hour test was performed, and the cubes used for the 14 day test were cured at 68 ℃ for 24 hours and then removed from an incubator and further cured at room temperature. In some cases, the example mixtures were cast using water at room temperature and the cubes were kept at room temperature until the test time. The maximum load required to crush the cubes was measured using a SATEC UTC 120HVL compression machine programmed to meet the load rate specified by the program in ASTM C109.

The pH of some of these mixtures was measured after the compressive strength test measurements described above by grinding the samples into a powder using a FRITSCH powdering machine. Only the interior of the crushed cube sample was used. The pH of the pulverized material was measured by preparing a 1: 1 ratio of dry powder sample to water and testing at room temperature using a Fisher Scientific ACCUMET BASIC AB-15pH meter while stirring the solution at a speed such that mixing occurs relative to the consistency of the solution. When the pH changed by no more than 0.02pH over 1 minute, the pH was recorded (approximately 5 minutes).

The compositions included in examples 1 to 5 were combined using a weight ratio of water to reactive powder of 0.56/1 and a weight ratio of expanded clay aggregate to fly ash, cement and gypsum (reactive powder) of about 0.56/1.

The temperature of the liquid is adjusted to obtain a specific mixing temperature before mixing with the cement. After mixing in a Hobart mixer, the mixture (about 280 grams) was placed in a 6 ounce styrofoam cup and in an insulated styrofoam box. A computerized data acquisition program is used to measure the temperature response. The maximum temperature rise rate, along with the maximum temperature and time to maximum temperature, is used as an indication of the reactivity of the experimental mixture.

Initial and final set times were determined using a Gilmo needle according to ASTM C266. The aim is to achieve final setting in less than 10 minutes, preferably 5 to 7 minutes after mixing. For compressive strength, test cubes (2 inches by 2 inches) (5.1cm by 5.1cm) were held at a temperature of 68 ℃ (154 ° F) within one sealed plastic bag containing the wet towels until the test time. The compressive strength of 3 cubes from each mixture was determined 5 hours after the addition of the mixture liquid. The maximum load required for the mill cubes was measured using a SATEC UTC 120HVL compression machine programmed to meet the load rates specified by the ASTM C109 program.

The raw materials and ingredients used in these examples are as follows:

class III Portland cement

Gypsum (e.g. gesso)

Class C fly ash

Expanded clay aggregate

Boric acid

Borax

Citric acid

Sodium citrate (trisodium citrate monohydrate)

Potassium citrate (tripotassium citrate monohydrate)

Potassium hydroxide

In the examples below, the dry reactive powder ingredients, as well as any aggregate used, are mixed with water under conditions that provide an initial slurry temperature above ambient. Typically, the hot water used has a temperature that produces a slurry with an initial temperature in the range of 90 ° F to 115 ° F (32 ℃ to 41 ℃).

When the reactive powder is substantially 100% by weight fly ash and portland cement and gypsum are minimized according to the preferred practice of the invention, the weight ratio of water to reactive powder is typically in the range of 0.2 to 0.30: 1.0, with lower weight ratios of 0.2 to 0.23: 1 being preferred.

These examples report the setting of the compositions characterized by initial and final set times as measured using the aforementioned gilmor pin as specified in the ASTM C266 test procedure along with high initial compressive strength according to ASTM C109.

Example 1 (mixtures 1 to 8)

Table 1 shows compositions containing class III portland cement and class C fly ash in a weight ratio of 20/100, and varying amounts of sodium citrate in combination with boric acid, borax, or citric acid. In these compositions, the level of potassium hydroxide remained unchanged at 1.8% by weight of fly ash and portland cement. From table 1, these data show that increasing sodium citrate shortens the final set time and improves the early compressive strength. Comparison of mixtures 1, 3 and 4 with sodium citrate dosages of 5.4, 10.8 and 16.2 grams, respectively, showed that the final set time was reduced to 11, 8.1 and 5.5 minutes, respectively. In the compression strength comparison after 3 hours (early compression strength) and 14 days, mixtures 2, 5 and 7, which contain the same amount of boric acid but in which the level of sodium citrate is 10.8, 16.2 and 21.8 grams, respectively, show an increase in compression strength after 3 hours and 14 days with increasing sodium citrate.

The data in table 1 also show that the effect of sodium citrate is reduced in the presence of borax compared to the effect of the mixture containing boric acid. In a comparison of mixtures 6 and 7 containing the same level (21.8g) of sodium citrate but using (7.2g) citric acid in the case of mixture 6 and boric acid (7.2g) in the case of mixture 7, the mixture containing citric acid had a slightly better 3 hour compressive strength but a similar 14 day compressive strength.

The effect of increasing the content of sodium citrate on the temperature rise of the mixture with borax, boric acid and citric acid is shown in the graphs plotted in fig. 1 and 2. As can be seen from fig. 1, the mixture with the higher amount of sodium citrate has a sharper temperature rise during the first 5-10 minutes. In fig. 2, it is noted that the mixture containing citric acid achieved a significantly higher temperature rise (about 230F-230F) during the first 45 to 90 minutes after mixing. It is known in the art that the rate of temperature increase is related to the reaction rate of the mixture as well as the setting time. The results of observing mixtures 6 and 8 in figures 1 and 2 containing 16.2 and 21.6 grams of sodium citrate and 7.2 grams of citric acid, which mixtures had two different inflection points at about 2-3 minutes in figure 1 and at about 15 to 30 minutes in figure 2.

In the case of mixtures 5 and 7 containing the same amounts of sodium citrate and boric acid instead of citric acid, the second inflection point in fig. 2 is not as well defined as in mixtures 6 and 8. The first peak of the reaction is understood in the art to relate to the final compressive strength of the mixture, while the second peak is known to relate to the early compressive strength of the mixture. This comparison shows that the presence of citric acid contributes to a second reaction related to the relatively higher early compressive strength measured for the citric acid containing mixture compared to the boric acid containing mixture.

Example 2

Another set of mixtures labeled 1-5 was prepared. Table 2 shows these compositions containing: 900 grams of class III portland cement, 180 grams of class C fly ash, 250 grams of water, and 608 grams of expanded clay lightweight aggregate.

Table 2 shows a composition containing 20/100 weight ratio of class III portland cement and class C fly ash, varying levels of potassium hydroxide, and a constant amount of sodium citrate (16.2g) and citric acid (7.2g) held constant at 0.67 wt.% and 1.5% (by weight of fly ash and portland cement reactive powder).

The results from table 2 show that as the content of potassium hydroxide increases, the set time decreases and the early strength increases together with the compressive strength measured after 14 days. Mixture 5 with 19.7g (1.8 wt.%) potassium hydroxide had a compressive strength of 8604psi after 14 days and the set time dropped to 4.0 minutes. The 3 hour compressive strength 5072psi for blend 3 with 1% potassium hydroxide is about twice the 2482psi compressive strength for blend 1 with 0.32 wt.% potassium hydroxide.

The effect of increasing potassium hydroxide content on the increase in mixture temperature for these mixtures in table 2 is plotted in the graphs in figures 3 and 4. As shown in fig. 3, the rate of temperature rise for mixtures 1 and 2 containing 3.5g (0.32%) and 5.6g (0.52%) of potassium hydroxide, respectively, was more gradual than the relatively sharper rate of temperature rise during the first 5 minutes for mixtures 3, 4 and 5 containing 11.2g (1.0%), 15.5g (1.4%) and 19.7g (1.8%) of potassium hydroxide. The rate of temperature increase is related to the reaction rate and the set time.

The graph in fig. 4 shows that increasing the potassium hydroxide significantly increases the temperature rise of about 205 ° F to 210 ° F within 1 hour after mixing.

Example 3 (mixtures 1 to 9)

Table 3 shows detailed compositions of portland cement type III and class C fly ash with different weight ratios along with different ratios of water to reactive solids. The weight of potassium citrate, sodium citrate and citric acid were kept constant at 1.8%, 1.5% and 0.67% by weight of fly ash and portland cement, respectively. 600 grams of expanded clay lightweight aggregate was added to each mixture. As shown in table 3, increasing the fly ash content and decreasing the water content reduced the set time to about 6 minutes and increased the 3 hour compressive strength to almost 6000 psi. It was also observed that the effect of reducing the water to cement ratio had a significant effect on the compressive strength of the mixture containing fly ash and without portland cement.

Example 4 (mixtures 1 to 5)

Another set of lightweight aggregate cement composition mixes was made, labeled mixes 1-5. The compositions shown in table 4 comprise different amounts of potassium or sodium citrate for mixtures containing different fly ash and portland cement weight ratios.

As shown in table 4, the mixture containing only potassium citrate and no potassium hydroxide or citric acid (e.g., 4 and 5) achieved a final set time within about 5 minutes and had a 3 hour compressive strength from 6000 to 7800psi that was over 60% of the strength over 10,000psi achieved after 14 days. Comparing mixes 4 and 3, it is noted that mix 4, which has 100 wt.% fly ash and no portland cement, has a higher compressive strength 7823psi than 5987psi for mix 3, which contains 86.4% fly ash and 11.6% portland cement. Both mixtures 3 and 4 had a potassium citrate content of 4.0 wt% by weight of the total fly ash and portland cement reactive powder.

In the case of mixtures 3 and 5, the temperature of the mixing water was reduced to 35 ℃ compared to 75 ℃ to prevent rapid solidification. Cubes tested after 14 days were kept at 65 ℃ for a period of 24 hours and then kept at room temperature until the time of the test. The weight ratio of water to reactive powder was kept at 0.2/1.0 for all mixtures.

The use of portland cement under these test conditions produced mortars with lower compressive strengths with increasing amounts of potassium citrate. For example, blend 3 with 4.0 wt.% potassium citrate has a compressive strength of 5987psi compared to 6927psi measured for blend 5 containing only 2.5 wt.% potassium citrate. After the 3 hour strength and 14 day strength increase to over 10,000psi, there was an additional increase in compressive strength.

The data in table 4 show that final set times of 4.8 to 5.1 minutes can be achieved without the use of potassium hydroxide according to the present invention wherein compressive strengths in the range of above 5900 to above 7800psi can be obtained.

The graph in fig. 4 shows that the mixture with potassium citrate or sodium citrate achieves a relatively high temperature during the first few minutes, similar to the mixture comprising potassium hydroxide and citric acid in the previous example.

Example 5 (mixtures 1 to 7)

Another set of mixtures 1-7 of lightweight cement compositions was made. The mixture in this example contains sodium or potassium citrate without potassium hydroxide. The water used in the mixture was 216g of water at 24 ℃ at room temperature, as compared to 75 ℃ used in most of the above examples. The results shown in table 5 indicate that the mixture can achieve relatively high compressive strength without the need for hot water. Mixtures 1-5 contained fly ash and portland cement in a weight ratio of 88.4: 11.6, while mixtures 6 and 7 had fly ash and portland cement weight ratios of 63.4: 36.6 and 75.6: 24.1, respectively.

As shown in table 5, blend 1-2 with potassium citrate or blend 3-5 with sodium citrate achieved a final set time in the range of 5 to 8 minutes and a 3 hour compressive strength in the range of 5268 to above 5757 psi. Note that for mixes 3-5 containing 11.6 wt% portland cement, no benefit was obtained by increasing the potassium citrate content above 2.4 wt.%. The weight ratio of water to total reactive powder was 0.2/1.0.

For mixtures 6 and 7 containing fly ash and gypsum, the final set time increased to 16 to 20 minutes and the 3 hour compressive strength dropped significantly to 3352psi and 4271psi, respectively, with increasing amounts of gypsum. This indicates the worst degree of interaction between gypsum, fly ash and potassium citrate. To a lesser extent, the 14-day compressive strength also decreased with increasing amounts of gypsum.

The graph in fig. 5 shows the temperature rise of a mixture containing no potassium hydroxide and using water at room temperature, the mixture with potassium citrate and sodium citrate still achieving a relatively high temperature during the first few minutes.

Example 6

This example summarizes the effect of adding portland cement and/or silica fume on the compressive strength of fly ash/potassium citrate based compositions. The total weight ratio of water to total reactive powder was maintained at 0.23/1.0. Table 6 shows the final set time, density, compressive strength for these mixtures. Table 6 shows that the density of these mixtures ranged between 112 and 117 pcf. The data contained in table 6 shows that mixture 4, which contains 100% fly ash and zero percent portland cement or silica fume, has a 3 hour compressive strength that is more than 20% higher than mixtures 1-3, which contain about 83% fly ash and about 17% blend of portland cement and silica fume. The 14 day compressive strength data shows about 30% to 40% higher compressive strength for mixture 4 with 100% fly ash.

Example 7

Five mixtures shown in table 7 were prepared for pH testing. The mixtures 1-3 do not include silica fume or gypsum and have a higher compressive strength of 3 hours and 14 days than the mixture comprising portland cement and gypsum along with the mixture 5 comprising silica fume. The pH of the mixtures 1-3 is about 12.7 to 12.8. Mixture 4, which contains fly ash and gypsum in a weight ratio of 63.4 to 36.6, has a pH of about 11 and mixture 5, which has a fly ash to silica ash weight ratio of 94.4 to 5.6, has a relatively low pH of 11.5. The weight ratio of water to total reactive powder was maintained at 0.20/1.0.

Thus, in compositions where pH is more of a concern than compressive strength (e.g., glass fiber reinforced concrete), a mixture of fly ash with gypsum or silica fume can be used to provide a lower pH product.

Example 8

Details of the formulations used in this example are included in table 8. For these mixtures, the fly ash to portland cement ratios were different, the potassium citrate was used at 3.5% (by weight of fly ash plus portland cement) and the water to cement material ratio (water: fly ash + portland cement) was 0.26 for mixtures 1-4 and 0.30 for mixtures 5-8. The results of compressive strength clearly show that higher amounts of fly ash increased the compressive strength for 3 hours.

Furthermore, the temperature rise curves measured for mixtures 4-7 are shown in fig. 6. Figure 6 shows that the temperature reached during the first 15 minutes is higher with increasing fly ash content and the amount of portland cement decreases at the same water to reactive powder ratio. To clearly show the data points, the data were measured continuously and plotted at 1 minute intervals.

Example 9

The details of the formulation used in this example are contained in table 9. Two sets of results are included herein. For the first four mixtures, only fly ash was added without any portland cement and the water to fly ash ratio varied from 0.26 to 0.17, with the amount of potassium citrate kept constant at 4% (by weight of fly ash). The compressive strength results show that reducing water content significantly increases the 3 hour compressive strength.

The second set of results included mixtures 5-7 containing a blend of fly ash and portland cement. For mixtures 5 to 7, the compressive strength decreases with decreasing amount of fly ash and with increasing amount of portland cement. Furthermore, the setting time of the mixture with portland cement falls below 5 minutes, indicating a fast setting.

Fig. 7 shows the temperature increase of the mixtures 1 to 4 in this example. Figure 7 shows that reducing water content increases the maximum temperature for a mixture containing fly ash without portland cement.

Fig. 8 shows the temperature increase for mixtures 3,5, 6 and 7. Figure 8 shows that increasing portland cement increases the second inflection point of the temperature response, which further increases the rate of temperature rise at about 30 minutes after the reaction begins.

An increase in temperature with mixtures having lower water content correlates with higher compressive strength. By contrast, the temperature increase obtained with increased portland cement does not translate into increased compressive strength. Thus, a different mechanism is responsible for the development of strength in mixtures with a blend of fly ash and portland cement than mixtures containing only fly ash.

Example 10

The details of the formulation used in this example are contained in table 10. For these mixtures, only fly ash was added without any portland cement. The amount of potassium citrate used varies between 2% and 6% (by weight of the fly ash) and the water to fly ash ratio remains constant at 0.20. The results in table 10 show that overall, the compressive strength of the fly ash mixture increases with increasing potassium citrate usage. The increase in 3 hour strength appears to level off at 5 wt.%, with the mixture with 5 wt.% potassium achieving a 3 hour strength comparable to the mixture with 6 wt.% potassium citrate. The 14-day compressive strength appears to peak at about 3.0 wt.% to 4.0 wt.%.

Figure 9 shows the temperature rise for mixtures with different potassium citrate usage using only fly ash without portland cement. These data show that the addition of potassium citrate significantly increases the temperature rise of the fly ash based mixture. However, the maximum temperature achieved is relatively lower than the value of the portland cement-containing mixture discussed in the example above.

While we have described the preferred embodiments for carrying our invention, it will be understood by those skilled in the art that the present disclosure is directed to changes and additions which may be made to the invention without departing from the spirit and scope thereof.

Claims (10)

1. A method of providing a lightweight cementitious mixture having improved compressive strength and water resistance, comprising:

mixing the following substances:

the amount of water is controlled by the amount of water,

a reactive powder, which is a mixture of a reactive powder,

an alkali metal salt of citric acid, and

lightweight aggregate

Wherein the weight ratio of water to reactive powder is about 0.17 to 0.35: 1.0, the reactive powder comprising 75 to 100 wt.% fly ash and 0 to 25 wt.% hydraulic cement and gypsum.

2. The method of claim 1, wherein the reactive powder comprises 88.5% to 100% fly ash, no hydraulic cement and no gypsum based on the weight of the reactive powder.

3. The method of claim 1, wherein the initial temperature of the mixture is about 24 ℃ to 41 ℃.

4. The method of claim 1, wherein the reactive powder comprises 10 wt.% to 40 wt.% lime.

5. The method of claim 1, wherein the amount of the alkali metal citrate is about 1.5 wt.% to 6 wt.% based on the weight of the cement reactive powder.

6. The method of claim 1, wherein the cement reactive powder further comprises silica fume.

7. The method of claim 1, wherein the cement reactive powder and water are present in a weight ratio of about 0.20-0.23: 1 parts by weight of water to reactive powder.

8. A composition for making lightweight cementitious panels comprising a mixture of:

cement active powder;

an alkali metal salt of citric acid as an accelerator for the reactive powder;

a lightweight aggregate, and

the amount of water is controlled by the amount of water,

wherein the ratio of water to cement reactive powder solids in the mixture is about 0.17 to 0.35: 1,

the reactive powder comprises 75 to 100 wt.% fly ash and 0 to 25 wt.% hydraulic cement and gypsum.

9. The composition of claim 8, wherein the mixture comprises about 1.5 wt.% to 6.0 wt.%, based on the weight of the cement powder, of at least one alkali metal salt of citric acid selected from the group consisting of: sodium citrate, potassium citrate, and mixtures thereof.

10. The composition of claim 8, wherein the mixture comprises about 1.5 wt.% to 4.0 wt.%, based on the weight of the cement powder, of at least one alkali metal salt of citric acid selected from the group consisting of: sodium citrate, potassium citrate, and mixtures thereof.