GB2168695A

GB2168695A - Settable compositions

Info

Publication number: GB2168695A
Application number: GB08526752A
Authority: GB
Inventors: Peter Shelley Mills
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1984-10-30
Filing date: 1985-10-30
Publication date: 1986-06-25
Also published as: GB2168695B; GB8526752D0

Abstract

A settable composition is formed by a mixture of a first slurry of a solid component comprising at least 60% calcium monoaluminate cement and water, and a second slurry comprising a source of calcium sulphate, calcium oxide and/or calcium hydroxide, and water. The mixture setting to form a hardened mass containing ettringite, especially useful in mining. A single pack composition is also disclosed.

Description

SPECIFICATION Settable compositions This invention relates to settable compositions components of which when mixed with water form pumpable slurries and which slurries when mixed together set into hardened masses containing ettringite.

A procedure known as "pump packing" has become widely adopted in British coal mines.

A cementitious material is prepared and pumped into a cavity, conveniently defined by a large bag, but possibly defined by shutterings of different forms, where it is allowed to set to form a supportive and/or sealing mass.

In particular, although not exclusively, the present invention relates to settable compositions for stowing cavities in underground mines, the compositions being formed by mixing a first component and water to form a first slurry, mixing together beta-anhydrite, calcium oxide and/or calcium hydroxide and water to form a second slurry, pumping the two slurries to the cavity site, mixing the two slurries and allowing the mixture to set in the cavity to form a hardened mass.

It is known to form a cementitious first slurry using a high alumina cement as defined by British Standard 915, Part 2. One requirement of such a high alumina cement is that the setting time of the cement when tested by methods described in the British Standard shall be as follows: initial setting time not less than two hours and no more than six hours with a final setting time not more than two hours after the initial set.

Unfortunately, this maximum initial setting time requirement tends to cause problems when filling cavities in underground mines where rigorous control and supervision of the mixing and pumping procedures are difficult.

The relatively short defined initial setting time requires that the cement slurry mixing and pumping system has to be flushed clean after every cavity filling operation. Otherwise, the cement slurry system tends to become blocked with set material. Such an occurrence may require parts of or a complete mixing and pumping system to be renewed or mechanically cleaned out. As will be appreciated, this can be a time-consuming and expensive task, particularly if production of the mine district is halted. Despite this, high alumina cement has advantages in the underground mine environment compared with the cheaper Ordinary Portland Cement because it is much less aggressive to the skin, and therefore handling is facilitated.

An object of the present invention is to provide a settable composition having the advantages associated with high alumina cement but tending to overcome the above-mentioned problems associated with relatively short initial setting times.

Accordingly, the present invention provides a settable composition formed by a mixture of a first slurry of a solid component comprising at least 60% calcium monoaluminate and water, and a second slurry comprising betaanhydrite as a source of calcium sulphate, calcium oxide and/or calcium hydroxide, and water, the mixture setting to form a hardened mass containing ettringite.

The solid component comprising at least 60% calcium monoaluminate is not commercially available in quantity, unlike high alumina cements, but may be manufactured by the application of known criteria and the use of suitable raw materials by a competent cement manufacturer, under conditions that at least 60% calcium monoaluminate is formed.

Advantageously, the solid component does not contain the phases 12 CaO. 7Al2Os or 2Ca0 . SiO2.

Advantageously, the solid component does not cotain a ferrite phase.

Conveniently, the second slurry contains bentonite.

Calcium sulphate may also be present in the form of gypsum dihydrate, hemihydrate or other forms of anhydrite.

The second slurry may comprise one or more accelerators; this may be conveniently an alkali metal salt, e.g., sodium carbonate and/or potassium carbonate and lithium carbonate is especially preferred.

The settable compositions of the invention are capable of providing good performance with high water contents; these may be conveniently from 100 to 500% by weight of the solids content.

By way of example, the present invention will be described with reference to the accompanying flow circuit diagram illustrating the use of cement compositions according to the present invention, stowing cavities in underground mines.

In the cavity stowing procedure calcium monoaluminate cement and water are mixed into a first slurry in mixer/pump means 1 and pumped along a first pipe line 2 towards a discharge outlet 3 adjacent to a cavity site 4 in an underground mine. Other materials including calcium sulphate in the form of betaanydrite, calcium oxide and/or calcium hydroxide, bentonite and water are mixed into a second slurry in further mixer/pump means 5 and pumped along a secone pipe line 6 which is connected to the first pipe line 2 by a 'Y' adaptor 7 located in the vicinity of the cavity site. In use, when the two mixer/pump means are operating simultaneously, the two slurries are splash-mixed adjacent to the discharge outlet 3 upon entering the cavity. The resulting mixture hydrates and sets to a hardened mass filling or partly filling the cavity, the hardened mass containing ettringite.

The cement is at least 60% CaO.Al2O3. This constituent has the advantage over high alumina cements as defined by British Standard 915 Part 2 in that its initial setting time is relatively long. Thus, in operation when stowing cavities in underground mines it tends not be necessary to flush clear the cement slurry mixing and pumping system after each stowing operation. The first slurry can be left in the system for over a working shift without fear of blockage due to setting, and setting times may be up to a week.

With this advantage, the solid component should not contain significant amounts of the phases 12CaO.7Al2O3 or 2CaO.SiQ2 or of a ferrite phase as their presence would tend to reduce the setting time. The first slurry may contain a retarder to increase the pumping life of the slurry, suitable retarders are well known in the art and citric acid is especially convenient.

The second slurry may contain accelerators for the cement. The bentonite, if required, is added to the second slurry to maintain the constituents of the slurry and the settable composition in suspension. The second slurry may contain alkali sulphates to promote the formation of ettringite.

The second slurry may comprise 2.5% potassium sulphate, 0.3% lithium carbonate, 10% bentonite, 15% hydrated lime and 72.2% of finely ground beta-anhydrite. The mixture is formed into a slurry with 2.5 times its weight of water.

The invention provides settable compositions whose properties may be tailored to particular applications in supporting and/or sealing in mining or may be adapted for uses in building, civil engineering, etc. The settable compositions may be foamed according to our published UK Patent Application, No.

2,139,517A.

Claims

1. A settable composition formed by a mixture of a first slurry of a solid component comprising at least 60% calcium monoaluminate and water, and a second slurry comprising beta anhydrite as a source of calcium sulphate, calcium oxide and/or calcium hydroxide, and water, the mixture setting to form a hardened mass containing ettringite.

2. A composition as claimed in claim 1, in which the solid component does not contain the phases 1 2CaO. 7Al2O3 or 2CaO. SiO2.

3. A composition as claimed in claim 1 or 2, in which the solid component does not contain a ferrite phase.

4. A composition as claimed in any one of the preceding claims, in which the second slurry contains bentonite.

5. A composition as claimed in any one of the preceding claims, in which calcium sulphate is also present in the second slurry in the form of gypsum.

6. A composition as claimed in any one of the preceding claims, in which the calcium sulphate is also present in the second slurry in the form of hemihydrate gypsum.

7. A composition as claimed in any one of the preceding claims, in which the second slurry contains an alkali metal sulphate.

8. A composition as claimed in any one of the preceding claims, in which the second slurry contains an accelerating agent for the settable composition.

9. A composition as claimed in any one of the preceding claims, in which the accelerating agent is a salt of lithium.

10. A composition as claimed in any one of the preceding claims, in which the first slurry contains a retarder to increase the pumping life of the slurry.

11. A composition according to claim 1, substantially as described herein.

12. A two component dry mixture for forming a settable composition according to any one of the preceding claims upon mixing with water.

13. A single component dry mixture comprising a solid component comprising at least 60% calcium monoaluminate, beta-anhydrite and calcium oxide and/or hydroxide, for forming a settable composition on admixture with water.

14. A method of forming a hardened cementitious mass comprising admixing a first slurry of a solid component comprising at least 60% calcium monoaluminate and water, and a second slurry comprising beta-anhydrite as a source of calcium sulphate, calcium oxide and/or calcium hydroxide and water, and allowing the mixture to form a mass containing ettringite.