HK1145021A - Cement composition for self-levelling concrete and self-levelling concrete containing such composition - Google Patents

Description

Cement composition for self-leveling concrete and self-leveling concrete containing the same

Technical Field

The present invention relates to a cement composition for self-levelling concrete, which contains a mixture of rheology modifying adjuvants, and to self-levelling concrete containing such a composition.

Background

Self-leveling concrete is very high, uniform and stable in fluidity, does not need vibration when used, and can be densified only under the action of gravity. Self-leveling concretes constitute a special class of self-leveling concretes (self-leveling concretes) corresponding to horizontal applications, such as cladding, paving or flooring.

These concretes contain particulate matter (which may be up to about 20mm in size) which is encapsulated in a cement composition containing a hydraulic binder, such as Portland cement, and at least one adjuvant, preferably a liquefier or superplasticizer, capable of imparting to the mixture an increased fluidity approximating that of a liquid. This explains the self-leveling (self-leveling) or self-leveling (self-leveling) properties of the concrete.

These concretes have the ability to be moulded, coated and advantageously densified. They are very valuable in the building and construction field, thanks to their appearance and the ability to "finish" uneven surfaces, to place themselves in iron frames and to coat, for example, ground thermal pipelines, without the need to apply external vibrations, which are commonly used in conventional building techniques. These compositions are also generally valuable in terms of their ease of use, since it is possible to pump fresh concrete to a height of several hundred meters, which prevents the various components from being handled near the site of application, and to omit the vibrating operation, thus optimizing the cost and organization of the construction site.

However, such fluidity properties of self-leveling concrete should be obtained without causing surface condensation or settling problems of particulate matter.

The stability of these concretes is generally achieved by incorporating into their formulation solid mineral components known as fillers, as mentioned in the standard NF EN 206-1. These fillers are siliceous or calcareous inert products in the form of small-sized particles, preferably of a size less than or equal to that of the cement, i.e. generally between 0.10 and 100 μm. The filler improves the compactness of the concrete by filling in the spaces between the cement particles. In particular, they are used for self-levelling concretes having a low mechanical strength, i.e. for example from about 25 to 30MPa in 28 days. Their proportion in the concrete can be up to 5 to 10% by weight of the total concrete.

The incorporation of fillers in self-leveling concrete formulations is particularly necessary in the case of concretes belonging to the lowest class of compression resistance. In fact, in self-leveling concrete where the water/cement ratio is high, the solid volume fraction of the concrete is smaller and the risk of segregation and surface condensation is more pronounced.

In general, to increase stability in terms of segregation and/or surface condensation, self-leveling concrete generally contains, in addition to the mineral additions (fillers) and liquefiers or superplasticizers indicated above, agents having different chemical properties which make it possible to control the rheology of the composition, generally known as "thickeners" or "stabilizers".

The purpose of the "thickening" agent is to improve the stability of the self-levelling concrete with respect to segregation and/or surface condensation; they are usually macromolecules, in particular cellulose derivatives such as cellulose ethers, or other polysaccharides, or other synthetic polymers such as polyethylene oxide, or polyvinyl alcohol. They have the advantage of modifying the rheological properties of the cement composition (in particular by increasing the viscosity). However, they are molecules with very high viscosity in aqueous solution (more than 70 to 100Pa · s in 2 wt% aqueous solution).

Furthermore, in order to improve the workability of the concrete, in particular to increase its spreadability, the ratio can be increased by super-plasticizing the formulation. However, higher proportions of superplasticizers can produce undesirable effects, such as non-uniform distribution, segregation and/or surface condensation of particulate matter in the mixed concrete. Thus, self-leveling concrete may exhibit stability or cohesion problems during its use and/or immediately after its installation.

These superplasticizers cause this particular mechanism of action (deflocculation) which increases the spreadability, and may be the reason for the instability of the concrete in its fresh state. In particular, the reduction in the shear threshold produced by the superplasticizer leads to a reduction in the reversible elastic deformation zone of the cement paste (here, water, cement, filler units). This phenomenon limits the ability of the cement slurry to maintain the particulate in itself under the force of gravity (segregation at rest). This drawback is more pronounced when the size of the particulate matter is increased compared to mortars containing sand with a particle size of less than or equal to 6 mm: especially in concrete containing particulate matter which may be up to 20mm in size.

With this reduction in the flow threshold, a high proportion of superplasticizer can lead to a reduction in the viscosity of the cement paste and therefore of the concrete. This reduction in viscosity exposes the cement to phenomena of instability and dynamic segregation when the concrete is flowing (particularly during its pumping or installation), which are not necessarily observed at rest.

These two phenomena of non-uniformity and segregation, both at rest and in a dynamic state, compromise the normal implementation of self-levelling concrete.

To prevent these problems, two types of measures are provided in the prior art:

1) increasing the volume fraction of solids in the cement slurry.

This is achieved by using a cement with a higher content than recommended for the required mechanical strength, or by introducing fine mineral additions with or without hydraulicity (e.g. calcium-containing fillers, fly ash, etc.). This increase in the fraction of solids has the effect of increasing the compactness of the mixture by excluding segregation at rest; thus increasing the viscosity of the resulting mixture. This makes it possible to reduce the risk of segregation and/or instability of the concrete in the fresh state.

2) Specific auxiliary agents for directly activating the concrete rheology are introduced.

They are adjuvants belonging to a group of very different chemical families, known under the name of stabilizing (or cohesion) agents, thickening agents or viscosifying agents. These definitions should be considered purely customary, since the effect of most of these adjuvants on the rheology of the concrete is clearly more complex and diversified than a single increase in viscosity.

In practical use, the introduction of specific adjuvants capable of activating the rheology of the concrete is always complementary to the introduction of the filler.

The proportion of filler is generally between 30 and 45% by weight with respect to the cement, which corresponds to a content of between 5 and 10% by weight of the total concrete that can be achieved, the introduction of which constitutes a significant cost (supply, transport, handling, etc.).

Furthermore, the metering of adjuvants in self-leveling concrete formulations is often difficult due to the effect on the rheological properties of the concrete. Typically these adjuvants have an effect on both shear threshold and viscosity. The relative significance of the changes in shear threshold and viscosity are different according to their measures: low concentrations of these adjuvants may have a significant effect on the shear threshold with a minor effect on viscosity; these effects may be reversed when higher doses are used.

It is therefore possible to achieve the aim of increasing the viscosity of the concrete (to prevent its instability during installation) at the expense of a higher shear threshold and therefore of a reduction in the spreadability.

During the course of recent studies on many rheology modifying adjuvants which activate the shear threshold and viscosity, it was sufficient to discover, surprisingly, the combination of two specific agents which makes it possible to impart to the cement composition, and therefore also to the concrete, surprising properties, in particular making it possible to reduce the content of filler in said concrete and even to eliminate the presence of filler.

Disclosure of Invention

To this end, the invention relates to a cement composition for self-levelling concrete comprising cement, a superplasticiser, and a mixture of rheology modifying adjuvants, characterised in that the mixture comprises a combination of at least a first agent and at least a second viscosifying agent:

-said first agent essentially increases the shear threshold of the concrete in the fluid state and is selected from the group consisting of natural polysaccharide ethers, hydroxyalkyl guar ethers, hydroxyethyl cellulose or hydroxypropyl guar hydrophobically modified by the introduction of hydrophobic side-chain graft moieties, and

-the second viscosifying agent primarily increases the viscosity of the concrete in the fluid state, selected from hydroxyalkyl celluloses and polyethylene oxides, the hydroxyalkyl celluloses being non-hydrophobically modified, having a degree of polymerization of from about 500 to about 4500, having a viscosity of less than 50 Pa.s in a 2 wt% aqueous solution,

which limits or prevents the dynamic segregation of particles in the concrete during its construction and makes it possible to reduce the filler content in the concrete to values lower than 20% by weight with respect to the cement.

Each of these two agents has a specific, selective and independent effect.

An "agent that increases the shear threshold" is defined herein as an agent for modifying the rheology of a fluid concrete, which activates mainly said shear threshold without modifying other rheological parameters of the concrete, in particular its specific viscosity.

"tackifying agent" is defined herein as an agent whose primary effect of addition to the concrete is to increase the specific viscosity of the concrete in the fluid state.

"concrete in fluid state" is defined herein as concrete obtained immediately after mixing, i.e. after water has been added to the hydraulic binder (cement) and mixed, but before the setting of said cement has started.

Throughout this document, "modified" is defined as grafting of a side chain graft moiety, such as a functional group or a hydrophobic side chain, onto the polymer backbone; advantageously, the grafting moiety is C₄-C₃₀Alkyl chains (i.e. may beA chain which may or may not be branched and which contains from 4 to 30 carbon atoms), and "hydrophobically modified" is defined as the presence of such a hydrophobic grafted moiety.

These agents have very high water solubility. However, it is sufficiently surprising that the aqueous solution containing the second agent, more particularly the hydroxyalkyl cellulose described above, has a low viscosity, i.e. a viscosity of less than about 50Pa · s (preferably less than 10Pa · s or preferably also less than 5Pa · s) at 25 ℃ in water at a concentration of 2 wt%, which is non-hydrophobically modified, has a degree of polymerization of more than 5000, and is more effective in limiting or preventing kinetic segregation of particulate matter than hydroxyalkyl cellulose having a viscosity of more than about 70 or 100Pa · s in the same concentration in aqueous solution.

Advantageously, the second agent is a non-hydrophobically modified hydroxyalkyl cellulose having a degree of polymerization of from about 500 to about 4000.

Preferably, the polyethylene oxide has a molecular weight of 1,000,000 to 5,000,000, preferably 2,500,000 to 4,000,000, more preferably 3,000,000 to 3,500,000.

Obviously, a combined synergistic effect on the shear threshold and the viscosity of the concrete in the fluid state is observed, in particular when the composition advantageously contains at most 1% by weight, preferably at most 0.1% by weight, of the first agent and at most 5% by weight, preferably at most 2% by weight and even more preferably at most 1% by weight, of the second agent, of the cement.

The invention also relates to any self-levelling concrete comprising the above cement composition, and filler content in an amount of less than or equal to 20% by weight of the cement, preferably comprising filler content in an amount of less than or equal to 10% by weight of the cement, and even more preferably less than 5% by weight of the cement, very advantageously without the addition of any filler.

In fact, sufficiently surprisingly, this mixture of adjuvants of the invention makes it possible to make the presence of fillers unnecessary in the concrete, in particular in self-levelling concrete, without reducing the level of performance in terms of spreadability, screen stability, surface water condensation or dynamic segregation compared to concrete containing fillers, in particular concrete having the same water/cement ratio, i.e. the same strength grade.

This reduction in the filler content, and even the absence of such added filler, offers great advantages in the concrete field by eliminating a significant part of the volume, weight and handling aspects (transportation, storage and other logistical aspects) and therefore leads to considerable simplification of the concrete construction at reduced cost.

The invention also relates to the use of a mixture comprising:

-at least a first agent which increases mainly the shear threshold of the concrete in the fluid state, selected from natural polysaccharide ethers, hydroxyalkyl guar ethers, hydroxyethyl cellulose or hydroxypropyl guar hydrophobically modified by the introduction of hydrophobic side-chain grafting moieties, and

-at least a second viscosifying agent which primarily increases the viscosity of the concrete lost in the fluid state, selected from hydroxyalkyl celluloses which are not hydrophobically modified, have a degree of polymerization of from about 500 to about 4500, have a viscosity of less than 50Pa s in a 2 wt% aqueous solution, and polyethylene oxide,

it is used to render the presence of filler particles unnecessary in self-levelling concrete without reducing the performance level with respect to spreadability, surface water-binding properties and kinetic segregation of the particulate matter.

Detailed Description

The invention will now be illustrated by the following non-limiting examples.

The sole figure shows a schematic view of the apparatus for the "chute test".

Starting Components

-cement: although the invention is applicable to any type of cement, the cement tested was Portland cement,

-the filler is a calcium-containing filler, preferably in ground form, having a particle size of 1 to 100 μm,

-a cohesive agent: for the reference example, it has an important role, in addition to the role of the filler, to limit the segregation of the concrete at rest; the reagent used in the present invention is a product sold by AXIM under the trade name COLLAXIM L10,

-particles, preferably of siliceous or siliceous-calcareous type, characterized by having two or more particle size classes and always

D_max≤20mm，

-superplasticizer: it is a copolymer type of acrylic carboxylic acid with acrylic esters, such as the product sold under the name Cimfluid duo 1001 by Axim.

In the following examples, the water/cement ratio implemented has been specified, since it is important for the final mechanical strength of the concrete.

After mixing (addition of water), the volume of the cement slurry (except for the particulate matter) was the same in order to better compare the tested compositions, the corresponding reference compositions (containing fillers), and compositions containing single or multiple different agents than those of the present invention (comparative examples).

Tests carried out

-spreadability: spreadability was measured using an Abrams cone (according to EN 12350-2 standard).

-chute test: this is a test developed in the laboratory in an attempt to demonstrate kinetic isolation and thus compare the effects of different adjuvants. A schematic of the equipment necessary for carrying out such tests is shown in the sole figure. The test consists in pouring a bucket 1(2 litres) of the concrete to be tested, immediately after mixing with water, into a tube 2 of diameter 15cm, length 110cm, inclined at 30 degrees to the horizontal. The outlet of the pipe 2 is arranged 20cm above the centre of the decking 3. This test makes it possible to obtain both quantitative and qualitative information: the flow time (i.e. the time interval necessary for the leading edge of the concrete to pass through the distance between the reference points a and B, which are 80cm apart), and the appearance of the concrete during and after its spreading (in the form of a "slab") on the spreading plate 3.

(this aspect is evaluated in terms of slab shape and concrete uniformity, based on visual criteria).

If the spread slab has an asymmetric shape, or if slurry/particulate separation is observed, it is considered that the concrete is affected by dynamic segregation.

-sieve stability index: the test was carried out according to the instructions of the draft of the EN 12350-11 standard;

-surface condensation: surface condensation was assessed visually.

-the rheology modifying agents tested were as follows:

a first reagent: agents that mainly increase the shear threshold of fluid concrete:

the NEXTON compound marketed by AQUALON-HERCULES company is a modified natural polysaccharide whose main chain is cellulose (β -D-glucose), which is etherified by a strong base and ethylene oxide to obtain hydroxyethyl cellulose (HEC) and the introduction of alkyl side chains, which results in the formation of hydrophobically modified hydroxyethyl cellulose (HMHEC); the number of substituents (MNS) per β -D-glucoside unit is about 2.5.

-ESACOL MX 144, marketed by the company LAMBERTI, is a polysaccharide with a guar (repeating unit consisting of β -D-mannose and α -B-galactose) backbone, which has been etherified with propylene oxide, which results in the formation of hydroxypropyl guar (HPG) with a number of substituents per unit greater than 2.5. The product has been hydrophobically modified by the introduction of side chains.

As comparative examples ESACOL HS26 and ESACOL HS30, marketed by the company LAMBERTI, were tested, which are molecules of the hydroxypropyl guar or hydroxyalkyl guar type, but not hydrophobically modified. These molecules impart considerable changes in the viscosity and shear threshold parameters of the cement composition and thus have no major effect on the shear threshold.

A second reagent: agent which increases the viscosity mainly:

-a modified natural polysaccharide having a cellulose backbone (β -D-glucose), which is etherified with a strong base and ethylene oxide to obtain Hydroxyethylcellulose (HEC) having a molar number of substituents per unit of β -D-glucose (MS) of 2.5 and a variable degree of Polymerization (PD).

In the examples of the present invention, NATROSOL 250GXR, which is HEC with a degree of polymerization of about 1000, was used, and NATROSOL 250HXR, which is HEC with a degree of polymerization of about 3700, was used. These water-soluble products have a low viscosity, including at concentrations up to 2% by weight in water:

natrosol 250 GXR: 0.5 wt% in water to 8.5.10^-3Pa·s

1 wt% in water is 0.035 Pa.s

2 wt% in water-0.26 pas

Natrosol 250 HXR: 1 wt% in water 2 pas

2 wt% in water-30 pas

Polyethylene oxide (molecular weight 3,000,000 to 3,500,000) sold under the name ALKOXE-130 by the company MEISEI CHEMICAL WORKS LTD, soluble in water;

as a comparative example, NATROSOL 250HHXR was tested, which has a higher degree of polymerization, i.e. a degree of polymerization between about 4,800 and 5,000. The compound imparts a considerable change in the viscosity and shear threshold parameters of the cement composition and therefore does not show a major effect on viscosity. Furthermore, the viscosity of NATROSOL 250HHXR in aqueous solution increases significantly with its concentration, i.e. 7 pas at 1 wt%, 50 pas at 1 wt% and 300 pas at 2 wt% in water.

The "NATROSOL" compound is sold by AQUALON-HERCULES.

The above-mentioned viscosities are measured on a rheometer with a mandatory restriction (AR 1000 from TA Instruments) with a flat cone geometry with a 2 ° opening.

In the embodiment shown, after the introduction of water into the cement mixture, the agents for modifying the rheology (first and second agents) and the superplasticizer are added. As a variant, it is possible to start with a "premix", i.e. with a cement composition comprising cement, superplasticizer, first and second agents; the particulate matter and water are then added thereto on site.

In all the following examples, the contents of cement, water, filler and particulate matter are expressed in Kg/m³(ii) a The amounts of superplasticizer, cohesion agent and first and second agents, all expressed as wt% of cement.

In the present invention, tests considered acceptable are:

for spreadability t_oValues greater than or equal to 630mm, where for self-levelling concrete, the desired spreadability is generally greater than or equal to 600mm,

the sieve stability will be less than 30%,

it should be observed that there is no surface condensation and no kinetic surface condensation

And the time in the chute is from 2 to 10 seconds, preferably from 2 to 7 seconds, and even more preferably from 4 to 6 seconds.

Example 1 (for comparison) -example 2 (for comparison) -example 3 (see Comparative) -example 4 (invention)

In the first step, the comparative example was carried out using only one reagent. Comparative example 1 used only the agent that increases the shear threshold, and comparative example 2 used only the agent that increases the viscosity.

Table 1 shows the results of these two comparative examples 1 and 2 and example 4 comprising a combination of two agents, and is compared to a composition containing filler in the presence of a cohesion agent. This last composition, i.e. "example 3 (reference)" in all tables, was formulated according to the prior art.

It has been noted that in comparative example 1, which contains only the agent that increases the shear threshold, the time in the chute is less than 1 second, reflecting insufficient viscosity and a sieve stability of more than 50%. In addition, surface condensation and kinetic segregation were noted.

Kinetic segregation was also observed at the end of pouring the concrete according to comparative example 2 containing only the viscosity-increasing agent.

Comparable screen stability values were observed for reference example 3 with additive and example 4 according to the invention without filler, comprising a combination of two agents.

TABLE 1

	Example 1 (comparative)	Example 2 (comparative)	Example 3 (reference)	Example 4
					CEM II 32.5R cement	376	376	300	376
Water of implementation	227	227	180	227
					Filler material	0	0	192	0
Particulate matter	1,654	1,654	1,654	1,654
					Superplasticizer	1.0％	1.0％	2.0％	1.0％
Cohesive agent			0.05％
					First reagent (threshold) ESACOL MX 144	0.02％			0.025％
Second reagent (tackifying) NATROSOL 250GXR		0.05％		0.05％
					Implemented water/cement	0.6	0.6	0.6	0.6
Volume of slurry	370L	370L	370L	370L
					t0Spreading property of time	650mm	695mm	720mm	680mm
Time in chute	＜1s	6s	4.5s	6.5s
					Screen stability	＞50％	＞50％	27％	23％
Surface condensed water	Appear	Is not present	Is not present	Is not present
					Kinetic isolation	Appear	At the end of pouring	Is not present	Is not present

Examples 5, 6 and 7

Different concentrations of agents that increase the shear threshold and viscosifying agents are shown in table 2. Different concentrations of these two agents have been tested in examples 5, 6 and 7.

TABLE 2

	Example 3 (reference)	Example 5	Example 6	Example 7
					CEM II 32.5R cement	300	376	376	376
Water of implementation	180	227	227	227
					Filler material	192	0	0	0
Particulate matter	1,654	1,654	1,656	1,654
					Superplasticizer	2.0％	0.6％	0.8％	1.0％
Cohesive agent	0.05％
					First reagent (threshold) ESACOL MX 144NEXTON		0.015％	0.02％	0.02％
Second reagent (tackifying) NATROSOL 250GXRNATROSOL 250HXR		0.3％	0.5％	0.05％
					Implemented water/cement	0.6	0.6	0.6	0.6
Volume of slurry	370L	370L	370L	370L
					t0Spreading property of time	720mm	650mm	665mm	630mm
Time in chute	4.5s	4s	5s	2.12s
					Screen stability	27％	16％	14％	12％
Surface condensed water	Is not present	Is not present	Is not present	Is not present
					Kinetic isolation	Is not present	Is not present	Is not present	Is not present

It is worth noting that the performance levels are comparable regardless of their concentration, wherein these concentrations are preferably lower than 0.05 wt% for the first agent and lower than or equal to 0.5 wt% for the second agent. It is thus possible to obtain filler-free self-levelling concrete by combining these two agents, which has the same level of properties as filler-containing self-levelling concrete.

Comparative examples 8, 9 and 10

Table 3 shows other comparative examples in which compositions containing different molecules were tested by varying one of the two agents, i.e. the agent that increases the shear threshold or the agent that increases the viscosity.

TABLE 3

	Example 3 (reference)	Example 8 (comparative)	Example 9 (comparative)	Example 10 (comparative)
					CEM II 32.5R cement	300	376	376	376
Water of implementation	180	227	227	227
					Filler material	192	0	0	0
Particulate matter	1,654	1,654	1,654	1,654
					Superplasticizer	2.0％	1.0％	1.0％	1.0％
Cohesive agent	0.05％
					First reagent (threshold) ESACOL HS30ESACOL MX 144ESACOL HS26		0.05％	0.02％	0.02％
Second reagent (viscosifying) NATROSOL 250GXRNATROSOL 250HHXR		0.05％	0.06％	0.045％
					Implemented water/cement	0.6	0.6	0.6	0.6
Volume of slurry	370L	370L	370L	370L
					t0Spreading property of time	720mm	610mm	560mm	680mm
Time in chute	4.5s	2.8s	12.5s	2.3s
					Screen stability	27％	35％	21％	38％
Surface condensed water	Is not present	Is not present	Is not present	Appear
					Kinetic isolation	Is not present	Appear	Is not present	Appear

When only one reagent according to the invention is present, no performance levels are listed with respect to the kinetic resolution which occurs in particular in comparative examples 8 and 10 or with respect to the chute time in comparative example 9.

Example 11 (reference containing filler) and example 12

Portland cement, CEM II45.5R cement, which differs mainly in its strength grade, was tested. The results are shown in table 4.

TABLE 4

	Example 11 (reference)	Example 12
			CEM II45.5R cement	340	391
Water of implementation	187	215
			Filler material	120	0
Particulate matter	1,665	1,665
			Superplasticizer	3.30％	2.750％
First reagent (threshold) ESACOL MX 144		0.056％
			Second agent (tackifying))NATROSOL 250GXR		0.095％
Implemented water/cement	0.55	0.55
			Volume of slurry	374L	374L
t0Spreading property of time	665mm	680mm
			Time in chute	2.30s	2.67s
Screen stability	12％	17％
			Surface condensed water	Is not present	Is not present
Kinetic isolation	Is not present	Is not present

The mixture of the two agents of the invention is also effective for such cements. The performance levels obtained are comparable to those of the concrete containing the filler (cf. example 11).

Example 13

In table 5, example 13 shows the use of polyethylene oxide as the second agent, i.e. the agent that increases viscosity. It is quite advantageous for such concrete and it is possible to obtain a level of performance which is comparable to that of reference 3, i.e. concrete with filler.

TABLE 5

	Example 13	Example 3 (reference ratio)
			CEM II 32.5R cement	376	300

	Example 13	Example 3 (reference ratio)
			Water of implementation	227	180
Filler material	0	192
			Particulate matter	1,654	1,654
Superplasticizer	1.0％	2.0％
			Cohesive agent		0.05％
First reagent (threshold) ESACOL MX 144	0.02％
			Second agent (tackifying) polyethylene oxide	0.5％
Implemented water/cement	0.6	0.6
			Volume of slurry	370L	370L
t0Spreading property of time	715mm	720mm
			Time in chute	3.7s	4.5s
Screen stability	22％	27％
			Surface condensed water	Is not present	Is not present
Kinetic isolation	Is not present	Is not present

Thus, from all these results it can be seen that the synergistic effect obtained by using in combination a first agent increasing the shear threshold and a second agent increasing the viscosity at a very low content with respect to the cement makes it possible to completely eliminate the need for the presence of fillers in self-levelling concrete.

In all of these results, the mechanical strength at 28 days was about 28 to 30MPa, as expected.

Claims (8)

1. A cement composition for self-levelling concrete comprising cement, a superplasticiser, and a mixture of a rheology modifying adjuvant, characterised in that the mixture comprises a combination of at least a first agent and at least a second viscosifying agent:

-said first agent essentially increases the shear threshold of the concrete in the fluid state and is selected from the group consisting of natural polysaccharide ethers, hydroxyalkyl guar ethers, hydroxyethyl cellulose or hydroxypropyl guar hydrophobically modified by the introduction of hydrophobic side-chain graft moieties, and

-the second viscosity increasing agent substantially increases the viscosity of the concrete in fluid state, selected from hydroxyalkyl celluloses and polyethylene oxides, the hydroxyalkyl celluloses being non-hydrophobically modified, having a degree of polymerization of from about 500 to about 4500, having a viscosity of less than 50 Pa-s in a 2 wt% aqueous solution,

which limits or prevents the dynamic segregation of particles in the concrete during its construction and makes it possible to reduce the filler content in the concrete to values lower than 20% by weight with respect to the cement.

2. The cementitious composition for self-leveling concrete of claim 1, wherein the grafted portion of the first agent is C₄-C₃₀An alkyl chain.

3. The cement composition for self-leveling concrete according to one of claims 1 or 2, wherein the polyethylene oxide has a molecular weight of 1,000,000 to 5,000,000.

4. The cement composition for self-leveling concrete according to claim 3, wherein the molecular weight of the polyethylene oxide is 2,500,000 to 4,000,000.

5. A composition according to any one of the preceding claims, wherein it contains said first agent in an amount of at most 1%, preferably at most 0.1% by weight of the cement, and a second agent in an amount of at most 5%, preferably at most 2% by weight of the cement.

6. Self-levelling concrete, wherein it contains a cement composition according to any one of claims 1 to 5, and filler content in an amount of less than or equal to 20% by weight of the cement.

7. The self-leveling concrete of claim 6, wherein it contains filler content in an amount of less than 10% by weight of cement, preferably less than 5% by weight of cement.

8. Use of a mixture comprising:

-at least a first agent which increases mainly the shear threshold of the concrete in the fluid state, selected from natural polysaccharide ethers, hydroxyalkyl guar ethers, hydroxyethyl cellulose or hydroxypropyl guar hydrophobically modified by the introduction of hydrophobic side-chain grafting moieties, and

-at least a second viscosity increasing agent which increases mainly the viscosity of the concrete in fluid state, selected from hydroxyalkyl celluloses and polyethylene oxides, which hydroxyalkyl celluloses are not hydrophobically modified, have a degree of polymerization of from about 500 to about 4500, have a viscosity of less than 50Pa s in a 2 wt% aqueous solution,

it is used to render the presence of filler particles unnecessary in self-levelling concrete without reducing the performance level with respect to spreadability, surface water-binding properties and kinetic segregation of the particulate matter.