CEMENTITIOUS COMPOSITION

Abstract: A cement composition to be combined with water comprises high alumina cement, beta anhydrite, aluminium sulphate, a source of free lime and a reactive filler, eg. blast furnace slag to form a rapid setting and hardening durable material.

Description: The invention relates to a cementitious composition based on so called high alumina cement, and in particular to a composition containing this cement and formulated so that on hydration it will set and develop strength quickly. More particularly, the invention relates to an easily placed cementitious composition which will set rapidly to develop high strength, in particular to a material requiring low solids content per cu. metre placed, which could be used for example, for filling the annulus between an underground roadway tunnel and its supporting structure. High yield grouts are of great benefit to underground operations since the transport of materials underground in operations such as coalmining, is costly and time consuming. Traditional grouts based upon Ordinary Portland cement (OPC) or blends of Ordinary Portland cement and pulverised fly ash have been used at low water:solids ratio. These set slowly and have other drawbacks. There is a need for a more rapid setting and rapid hardening dry composition which develops a thixotropic consistency within a short time of being mixed with water. Improvements can be effected in compositions based on Ordinary Portland cement by the use of additives, but the level of material consumption (generally in excess of 1 tonne per cu. metre) and environmental acceptance (Ordinary Portland cement is aggressive and highly irritant to the skin) remain. It is one object of this invention to satisfy that need. It has now been discovered that, if the high alumina cement is hydrated in the presence of some additives, especially in defined proportions, a rapid setting and hardening composition is formed and the set material has improved durability. In one aspect, the invention provides a composition for admixture with water to form a rapid setting and rapid hardening material, the composition comprising high alumina cement, beta anhydrite, at least 1% by weight of aluminium sulphate, a source of free lime in sufficient quantity to cause the formation of ettringite on hydration, and a reactive filler. The high alumina cement is any cement which falls within the definition of a high alumina cement according to B.S. 915 Part 2, i.e. a cement which contains not less than 32% by weight of alumina and has an alumina to calcium oxide ratio of between 0.85 and 1.3: 1. Analyses of suitable cements are by weight, 39.0% alumina, 38.5% calcium oxide, 16.5% iron oxide (Fe₂O₃) and 4.0% silica; and 37.7% alumina, 42% calcium oxide, 2.25% silica, balance iron oxide. The high aluminia cement can make up as little as 10% by weight and as much as 50% by weight of the composition. Ettringite is formed on hydration. Ettringite is a calcium trisulpho-aluminate having 32 molecules of water of crystallisation and has the formula 3CaO · Al₂O₃ · 3CaSO₄ · 32H₂O. It is necessary to have regard to the content of the free lime relative to the high alumina cement in order to form ettringite. Irrespective of the source of lime, it should be in the range of about 3 to about 9% by weight relative to the high alumina cement. The optimum percentage will be determined by the specific raw materials employed and the level of aluminium sulphate. Inadequate lime supply will result in low strength development, whereas excessive lime will prevent the reaction from proceeding as required. The potential free lime may be calculated by: PFL = 100 ([%CaO]+A [%CaSO₄]+B [%X]-C [%Al₂(SO₄)₃ · 14H₂P]) [%HAC] where percentage refer to the percent quantity added to the formulation: A is the proportion of lime present in the beta anhydrite employed. (In all the examples given below this was 0.0089). B is a constant for any potential reactive constituent X which may release lime. An approximation for OPC is 0.25. C is a constant of value 0.5656. This represents the stoichiometricequivalence of aluminium sulphate tetradecahydrate as lime in order to form ettringite. It is a further requirement that the aluminium sulphate content is greater than about 1% relative to the HAC, preferably from about 1% to about 12.5%. The aluminium sulphate (Al₂(SO₄)₃ 14H₂O) is required to ensure that not only will the composition set rapidly but the strength will develop rapidly. In addition, the aluminium sulphate can control the content of excess lime. It is essential that the calcium sulphate be in the form of beta-anhydrite because other forms of calcium sulphate when used alone do not give satisfactory results. Hydrated forms such as plaster of Paris or gypsum both give strengths which are too low during the initial stages of setting and gamma anhydrite hydrates very rapidly in water and gives similar results to the hydrated forms. Blast furnace slag is a preferred reactive filler and is preferably used in ground granulated form. This component may be replaced in part by other reactive fillers e.g. pulverised fly ash, but some of this slag is essential to the invention if the desired rate of set and hardening are to be achieved. Preferably the reactive filler makes up about 10% by weight of the composition if the set material is to be durable, i.e resistant to the effects of atmosphere such as carbonation. Additives may be present. For example, lithium carbonate may be present as an accelerator for the high alumina cement; thickeners or sugars may be present; Ordinary Portland cement may be the source of the calcium oxide; foaming agents; and the like. In another aspect this invention provides a method of placing a fast settable and hardenable material at a site of use comprising supplying to the site a dilute aqueous slurry of a composition of the invention and allowing or causing the placed slurry to set and harden rapidly. The dry composition may be mixed with water to form a very dilute slurry for placing. The ratio of water: dry solids may be from 1.2: 1 by weight to about 2.5: 1. The setting time and strengths may be varied according to the intended use. If the composition is to be useful in making an easily placed slurry to provide a high strength grout, then the following parameters are preferred: period for the placed slurry to become thixotropic: between about 1 and 5 minutes period for the placed thixotropic slurry to become non-pourable: between about 3 and 10 minutes water: dry solids ratio from at least 1.2: 1 to 2.5: 1 initial strength after 2 hours: at least 0.3 Newton mm⁻² 1 day strength: at least 1.5 Newtons mm⁻² 7 days strength: at least 3 Newtons mm⁻² These parameters may be varied e.g. for monolithic packing the 7 day strength will be at least 4 Newtons mm⁻². The invention is illustrated by the following examples in which parts are by weight unless otherwise specified. Example 1 A composition was made up as follows: The free lime was calculated to be about 6.7% relative to that of the high alumina cement. The composition was mixed with water in a weight ratio of 1.818: 1 water solids to provide a very dilute easily placeable slurry. The composition became thixotropic within about 4 minutes and set to a non-pourable condition in about 8 minutes. After two hours the strength was 0.60 Newtons mm⁻². After one day it was 2.2 Newtons and after 7 days 3.9 Newtons mm⁻². The resultant hardened composition was resistant to surface erosion whilst drying to a light cream/buff colour which in a mining environment is desirable because of the light reflective properties. It will be noted that, despite the high water content, the wetted composition sets rapidly and develops a high strength. Examples 2 to 20 Compositions were made up as shown in Table 1, mixed with water and tested; the details are also shown in Table 1. The results of Examples 2, 3 and 4 show the effect of reducing the content of aluminium sulphate relative to that of Example 1. A reduction of that content (Example 5) shows that there is a reduction in the setting characteristics; a further reduction (Example 6) shows even further deterioration. Example 4 is better than Example 3 despite having the same content of aluminium sulphate because of the increased free lime content. The results of Examples 5 and 6 show that initial set may be retarded slightly if a content of sugar is present, while the strength development is not affected significantly. Examples 7, 8, 9 and 10 demonstrate the effect of increasing content of beta anhydrite. If this is accompanied by an increase in the content of high alumina cement (Example 8) then a very good strength develops. Examples 11, 12, 13 and 14 show that in the absence of the ground granulated blast furnace slag while fast setting occurs, the strength development is not obtained.

Claims: 1. A dry cementitious composition for admixture with water to form a rapid setting and rapid hardening material, the composition comprising high alumina cement, beta anhydrite and a source of lime characterised in that sufficient source of lime is present so that about 3 to about 9% of free lime is available and in that about 1% to about 13% of aluminium sulphate is present, both calculated with respect to the weight of the high alumina cement, and in that a reactive filler is present. 2. A composition according to Claim 1 characterised in that the aluminium sulphate is present in a concentration of about 1% to about 12.5%, relative to the high alumina cement. 3. A composition according to Claim 1 or 2, characterised in that the content of free lime is calculated according to the formula PFL = 100 ([%CaO] + A [%CaSO₄] + B [%X] - ([%Al₂(SO₄)₃ · 14H₂O])/[%HAC], where PFL stands for potential free lime, HAC stands for high alumina cement, A is the proportion of lime present in the beta anhydrite employed, B is a constant for any potential reactive constituent X which may release lime and C is a constant of value 0.5656 representing the stoichiometrie equivalence of aluminium tetradecahydrate as lime in order to form ettringite and where all percentages refer to the percent quantity added to the formulation. 4. A composition according to any preceding Claim characterised in that the content of the reactive filler is at least 10% by weight of the composition. 5. A composition according to any preceding Claim characterised in that the reactive filler is blast furnace slag. 6. A composition according to Claim 5 characterised in that pulverised fly ash is present with the blast furnace slag. 7. A composition according to any preceding Claim characterised in that additives such as accelerators, thickeners and/or foaming agents are present. 8. A slurry comprising a dry cementitious composition and water characterised in that the dry cementitious composition is according to any of Claims 1 to 7. 9. A method of placing a fast settable material at a site of use by supplying a dilute aqueous slurry of a cementitious composition to the site and allowing or causing the slurry to set characterised in that the slurry comprises water and a dry composition according to any of Claims 1 to 8 in a weight ratio of from about 1.2: 1 to about 2.5: 1. water: solids.

Inventors: VILES, ROBERT FRANKLYN
Applicants: FOSROC INTERNATIONAL LIMITED
Application year: 1988
Main classification: C04B28
IPC codes: C04B28