GB2159512A

GB2159512A - Cement compositions for stowing cavities

Info

Publication number: GB2159512A
Application number: GB8513786A
Authority: GB
Inventors: Peter Alan Longman; Norman Harold Drew
Original assignee: Blue Circle Industries PLC
Current assignee: Blue Circle Industries PLC
Priority date: 1984-06-01
Filing date: 1985-05-31
Publication date: 1985-12-04
Also published as: GB8513786D0; GB2159512B

Abstract

A cementitious composition comprises (a) 20-90 wt% of an hydraulic cement containing 7-90 M% 12CaO.7Al2O3 (the balance of its hydraulic constituents being calcium silicate), (b) 5-60wt.% calcium sulfate (e.g. anhydrite), (c) 2-10 wt% another inorganic salt (e.g. sodium carbonate) and (d) 0.5-10wt% a clay (e.g. bentonite). The composition when made up with water is flowable and quick-setting and can be used for the stowing of cavities, for example in the construction of supportive packs in underground mines.

Description

SPECIFICATION Cement compositions for stowing cavities Field of the invention This invention relates to quick-setting cementitious compositions that are flowable materials and that can be pumped into cavities or moulds. The compositions are especially useful in the stowing of cavities, e.g. in the construction of supportive packs in underground mines.

Background to the invention The use of packs to support the rock strata adjacent to roadways leading to longwall coal faces is well known. It is required of such packs that they should be quick setting and should develop strength rapidly so that the wall face can be advanced at an economic rate.

One way of constructing a pack for this purpose is to use the waste rocks and other minerals which are extracted when digging the roadway as a source of aggregate in a "concrete" mix. British Patent Specification 1,362,954 describes a process in which such a pack can be constructed by concurrently injecting into a cavity constrained by a shuttering system an aqueous slurry containing a hydraulic cement and an aqueous slurry containing crushed rocks such that the two slurries mix and the resultant mixture sets, thereby permitting the shuttering system to be moved forward with the coal face.However, as the waste materials employed often contain coal and other carbonaceous materials which retard the setting of commonly available cements, even when mixed with accelerators, it is necessary to use special quick setting, early strength cements in order consistently to obtain the desired quick setting and early strength development.

British Patent Specifications 1,387,075 and 1,497,670 disclose a quick setting, early strength cement suitable for use in the process described in British Patent Specification 1,362,954. This cement comprises 7-90%, preferably 15-30%, by weight of the hydraulic constituents, of the calcium aluminate phase 12CaO.7AI2O3 together with the other hydraulic phases commonly found in cements the bulk of which are calcium silicates. The cement may also comprise calcium sulphate and small additions of a retarder, such as citric acid, and of an accelerator, such as potassium sulphate.

Another method of constructing suitable packs adjacent to roadways leading to longwall coal faces is described in British Patent Specification 2,058,037A, in which an aqueous slurry containing a cement, or cement mixture, is concurrently injected into the cavity with a second aqueous slurry containing a clay and an inorganic salt which accelerates the setting and hardening of the cement. Suitable cement mixtures are described which comprise ordinary Portland cement, a calcium aluminate, calcium sulphate and a carboxylic or hydroxycarboxylic acid or a salt of such an acid.A typical cement slurry is said to comprise a cement mixture and water mixed in a ratio of 0.5:1 to 2:1 by weight where the cement mixture contains 66.65% ordinary Portland cement, 18.4% High alumina cement, 14.7% anhydrous calcium sulphate and 0.25% citric acid, whereas a typical clay slurry comprises bentonite and sodium carbonate such that the bentonite is present in an amount of between 0.25% and 10% by weight of the water and the sodium carbonate is present in an amount equivalent to between 1% and 10% by weight of the cement in the cement slurry. When concurrently injected into a cavity the final mix consists of the cement mixture, bentonite and water substantially in the proportions of 14%, 1% and 85% by volume.This method has the advantage that relatively cheap pumps can be used to convey the cement-based and clay-based slurries compared with the robust pumping system required to convey the slurry prepared from crushed waste materials described in British Patent 1,362,954. On the other hand, the waste materials obtained when constructing the roadways still need to be removed and disposed of in some quay. In general terms the cost of materials used in constructing a pack as described in British Patent Sperifica-tion 2,058,037A is broadly similar to the cost of materials used in constructing a pack from waste mi3te- rials as described in British Patent Specification 1,362,954.

More recently, however, alternative cement compositions and modified clay slurries have been disclosed which offer a number of advantages over the compositions described in British Patent Specification 2,058,037A. In particular, British Patent Specification 2,123,808A describes a cement composition based on a mixture of High Alumina cement, beta anhydrite and calcium oxide and/or calcium hydroxide.

According to this invention the cement composition will usually contain High Alumina cement, and beta anhydrite in a ratio of from 3:7 to 4:1 by weight, whereas the calcium oxide and/or calcium hydroxide added will typically range from 3 to 5% by weight. Small additions of gypsum or an alkali metal sulphate such as potassium sulphate can also be made. By mixing the cement composition with a bentonite slurry containing an accelerator of High Alumina cement, such as lithium carbonate, such that the quantity of bentonite is between 10% and 25% by weight of the beta anhydrite and the quantity of lithium carbonate is between 0.05 and 0.2% by weight of the High Alumina cement, it is stated that very high water contents can be tolerated in the final mix and that as a consequence the costs of filling a cavity can be substantially reduced.

One major disadvantage of the various processes so far described is that there is no common preferred cement composition. Thus, neither of the cement mixtures described in British Patent Specifications 2,058,037A and 2,123,808A is ideally suited for binding crushed rocks or waste materials as in the process described in British Patent Specification 1,362,954. Conversely, the cements that are described in British Patent Specifications 1,387,075 and 1,497,670, which can be successfully used in the latter process, yield comparatively poor packs in the process described in British Patent Specification 2,058,037A when they are used in place of the cement compositions described in the latter specification or in British Patent Specification 2,123,808A.A consequence of this situation is that mines that have more than one system for constructing packs installed on their coal faces must have facilities for handling separately at least two cement compositions and possibly also two clay-based compositions. The clay-based compositions are less of a problem since only relatively small quantities of materials are used and these are handled normally in bags. With the cement compositions, however, a common or dual-purpose cement would be clearly desirable since in the case of bulk handling, which is a cheaper and easier method of transporting cement, only one storage silo and conveying system would be required and the risk of mixing two different cements accidentally would be removed.

Accordingly, an object of the present invention is to provide a cement composition that can be used as the cement slurry in the pack process described in British Patent Specification 2,058,037A as well as for binding crushed rocks in the process described in British Patent Specification 1,362,954. A further object of the present invention is to achieve high water contents in the final cavity-filling mix similar to those obtained according to British Patent Specification 2,123,808A, thereby ensuring that the material costs are competitive.

Summary of the invention The present invention provides a cementitious composition comprising (a) from 20 to 90%, preferably from 55 to 80%, of an hydraulic cement that contains from 7 to 90%, preferably 10 to 30%, by weight of its hydraulic constituents, of 12CaO.7AI203 the majority by weight of the balance of its hydraulic constituents beinq one or more calcium silicates; (b) from 5 to 60%, preferably from 10 to 40%, of calcium sulfate; (c) from 2 to 10%, preferably from 4 to 6%, of further inorganic salt (d) from 0.5 to 10%, preferably from 4 to 6%, e.g. 4-5%, of a clay; and, optionally, (e) an extender in an amount, for example, of up to 20%, the stated amounts of components (a)-(e) being relative to the total composition on a dry weight basis.

In use, the composition will be made up with (f) water, preferably in an amount of from 55 to 75% by weight of the total aqueous composition (generally equivalent to from 74 to 93% of water when expressed on a volume basis). Such aqueous cementitious compositions may typically comprise 5 to 35%, preferably 15 to 30%, by weight of component (a), from 2 to 15%, preferably 4 to 10%, by weight of component (b), from 0.5 to 5%, preferably 1 to 3%, by weight of component (c) and from 0.1 to 5%, preferably 1 to 3%, e.g. 1-2%, by weight of component (d).

Compositions according to this invention are quick-setting and flowable, and are suitable as filling media. Accordingly, the present invention also provides a method of stowing or filling a cavity with a filling medium, wherein the said medium is an aqueous cementitious composition as broadly defined in the preceding paragraph.

Description of preferred embodiments Component (a) of the cementitious composition of this invention comprises a ground clinker that is rich in 12CaO.7Al2O3 (usually abbreviated to C,2A7). Such a clinker is described in detail in British Patent Specification No. 1,387,075 (the teaching of which is incorporated herein by reference) and may be produced by fusing or sintering, generally at a temperature above 1250"C, a mixture of siliceous material (e.g. a kaolinitic clay such as china clay, ball clay or fire clay), calcareous material (e.g. chalk or limestone) and aluminous material (e.g. bauxite) proportioned to yield a product rich in C1A7 and poor in CaO.Al2O3 (abbreviated to CA) or 3CaO. Al2O3 (abbreviated to C3A).When the mixture is fired under conditions to provide an uncombined lime content of less than 5%, a C,2A7-rich clinker is obtained, the major calcium silicate phase of which is dicalcium silicate (2CaO.SiO2, abbreviated to C2S) with a calcium alumino ferrite (4CaO.Al2O3.Fe2O3, abbreviated to C4AF), magnesia, alkali metal sulfate and alkali metal calcium double sulfate also being present in minor amounts.

As described in British Patent Specification No. 1,497,670 (the teaching of which is also incorporated herein by reference), the C,2A7-rich clinker may be underlimed, an underlimed clinker being one in which the lime limitation factor (LLF) is less than unity, where CaO LLF 1.87 SiO2 + 0.94AI203 + 0.65 Fe2O3 the chemical symbols representinq the amounts by weight of the represented substances that are present, the underlimed condition being at least counterbalanced in the cement by the presence of a substance effective to increase the basicity of the cement.

The said substance effective to increase the basicity of the cement may be a metal oxide, e.g. MgO or FeO, present in the C,2A7-rich clinker in solid solution in substitution for CaO in the lattices of major phases. Alternatively, the said substance effective to increase the basicity may be a source of calcium ions, e.g. calcium chloride, calcined lime or calcium hydroxide, added to the cement.

The hydraulic cement constituting component (a)of the present composition may contain a mixture of the C12A7-rich clinker with, for example, Portland cement, especially ordinary Portland cement. Such a cement mixture may be formed conveniently by intergrinding the Cl2A7-rich clinker and ordinary Portland cement clinker, usually together with a source of sulfate and a small quantity of a retarder known as such, e.g. a sugar, a lignosulfonate or, especially, citric acid. When the C12A7-rich clinker is underlimed, as described above, the substance effective to increase the basicity of the cement may be a source of calcium ions provided in the form of uncombined lime in the Portland cement clinker.

The amount of calcium sulfate present as component (b) of the present composition is calculated as the an hydros material. The preferred form of calcium sulfate is anhydrite, which can be either natural anhydrite or a synthetic anhydrite such as the by-product from an acid-producinq plant. Although the latter is preferred, account must be taken of the effect that any trace impurities in the synthetic anhydrite, such as fluorine or phosphorus, may have on the setting and hydration of the cement.

As indicated above, sulfate may be used in the preparation of the hydraulic cement component. Any calcium sulfate introduced in this manner is regarded as part of component (b), rather than component (a) or (c), for the purpose of determining the amounts of the components.

Inorganic salts suitable as component (c) include known accelerators of cement such as halides, nitrites, nitrates, borates, hydroxides, bicarbonates, carbonates and sulfates (other than, of course, the calcium sulfate present as component (b)), including the alkali metal (e.g. sodium or potassium), alkaline earth metal (e.g calcium or magnesium) and aluminium salts. Preferred salts are alkali metal carbonates, alkali metal sulfates and calcium chloride. Potassium carbonate and, especially, sodium carbonate are particularly preferred.

Salts, such as calcium chloride or potassium sulfate, may be used in the preparation of the hydraulic cement component. Such salts may or may not be regarded as part of component (c), rather than component (a), depending, for example, upon the extent to which they affect the setting properties of the cementitious composition. Thus, alkali metal sulfates introduced in the cement component (a) will usually be regarded as part of component (c) for the purpose of calculating the amount thereof, whereas the calcium chloride usually need not be. Salts which are fixed in the cement clinker matrix are, of course, regarded as part of component (a).

The clay employed as component (d) can be, for example, a hectorite, bentonite, smectite or kaolinite type clay, of which bentonite (e.g. the calcium form or, especially, sodium-activated bentonite) is preferred.

The extender (e), if such is employed, is usually slag, although other latently hydraulic materials such as pulverised fly ash or pozzolana may be used.

It has been stated above that component (a) may be a mixture of materials. It will be understood that each of components (b), (c), (d) and (e) may also be constituted by a mixture of suitable substances.

In view of the rapid setting characteristics of the present compositions, it is preferred to form the composition at the site where it is to be used, in order to avoid the danger of blockage of the supply means.

Accordingly, the components of the composition are, in general, conveyed to the cavity to be filled in at least two separate supply lines. The components can be distributed between the supply lines in various ways. For example, it is possible to convey the solid components pneumatically as a dry powder through one supply pipe, the water being conveyed through a second pipe. Alternatively, component (a) and one or two of components (b), (c) and (d) are conveyed in one line, as a slurry or dry powder, the remaining component(s) being conveyed, as a slurry or dry powder, through a second line. Although slurries have the advantage that the raising of dust is avoided or minimized, the inorganic salt, component (c), will usually be conveyed with the cement component (a) only in the form of a dry powder, in order to avoid premature setting of the mixture.It is preferred, however, that in one line the hydraulic cement, component (a), is conveyed, either pneumatically as a dry powder or as a slurry, and a mixture of the other components (b), (c) and (d) is conveyed in a second line, also either as a dry powder or as a slurry. In these embodiments, optional components, such as an extender (e), can be conveyed through either line, as convenient.

With slurry conveying, it is preferable to incorporate all the water than can be tolerated in the final mix in the slurries so that their pumpability time is maximised. With slurries it is also possible to achieve the requisite degree of mixing by concurrently injecting them into the cavity. With dry powder conveying, however, a mixing device may be necessary and a third feed pipe for feeding water will be required in order to achieve the requisite mixing and medium composition.

Normally, the above-mentioned mixture of components (b), (c) and (d) will comprise, on a dry-weight basis, 50-80%, e.g. 55-80%, calcium sulfate (especially anhydrite), 10-28% inorganic salt (especially sodium carbonate) and 10-28%, e.g. 10-20%, clay (especially bentonite), the exact proportions being adjusted to yield the desired properties. In general, the calcium sulfate content will be such that the weight ratio of calcium sulfate to calcium aluminate phases in the final composition in the cavity will be from 1:1 to 3.5:1. If too little calcium sulfate is added, although setting will be quick the optimum early strengths will not be obtained. If too much calcium sulfate is added the setting will be delayed. The inorganic salt content should be such that sufficient is present to accelerate the hydration of the cement.

Typically in the final composition or filling medium this will be around 8% by weight of the cement com ponent. The clay addition should be such that sufficient is present to act as a suspension agent in the final medium and in the mixture of calcium sulfate, clay and inorganic salt if this is pumped as a slurry.

In addition to achieving the desired proportions of materials in the mixture, however, we have also found that the setting and strenqth development of the cavity filling medium can be influenced and controlled by the fineness of the mixture employed and especially that of the calcium sulfate. Accordingly, the aforementioned mixture of calcium sulfate, inorganic salt, and clay is preferably prepared either by pregrinding or classifying the calcium sulfate before blending with the other materials or by intergrinding and classifying all the materials together such that, in either case, the calcium sulfate has a fineness of 65-75% passing a 45 micron British Standard (B.S.) sieve.

By controlling both the composition and the fineness of the aforementioned mixture of calcium sulfate, clay and inorganic salt it is possible to use this mixture with the cements described in British Patent Specifications 1,387,075 and 1,497,670 to construct a pack or fill a cavity, especially by the method described in principle in British Patent Specification 2,058,037A, with a medium in which the water content can be in excess of 90% by volume. As a consequence, use of costly materials in the construction of packs is minimised; the handling time of the materials, both cement and mixture, in the form of slurries can be maximised by the high water addition permitted; and the properties of the final filling medium are not severely affected by a fluctuation in water content.Of course, the present invention also permits the use of the cement described in British Patent Specifications 1,387,075 and 1,497,670 to be used to construct packs or fill cavities in underground mines by the method described in principle in British Patent Specification 1,362,954.

It is not, however, envisaged that this invention should be restricted just to the above-described applications, for it could find use wherever it is desired to fill a cavity, encapsulate a material, or carry out a construction with a quick setting, early strength developing cementitious mixtures The following examples serve to illustrate the invention: Example Z A clinker rich in C12A7 was prepared as follows::- A limestone whose principal constituents were SiO2 1.05%, Al2O3 0.79%, Fe2O3 0.09% and CaO 54.29%, a fireclay whose principal constituents were SiO2 44.80%, Al2O3 31.45%, FQQ 2.31% and CaO 1.66% and a calcined bauxite whose principal constituents were SiOz 6.91%, Al2O3 81.49%, Fe203 1.82% and CaO 1.73% were blended to form a raw feed in the approximate proportions 68.9% limestone, 26.2% fireclay and 4.9% bauxite, and ground in a ball mill to a residue of 8.0% on a BS 170 (90 ,a) sieve, the raw feed being adjusted to give a carbonate percentage of 68.4 + 0.2 and a silica ratio of 0.90 i 0.05.

The raw feed was sintered in a rotary coal fired kiln at 13000C to give a free lime content of 1.15% As a result of ash absorption during sintering, the final clinker had an analysis as follows:- SiO2 19.24%, Al2O3 19.24%, Fe2O3 1.73%, CaO 56.63%, Mn203 0.05%, P2Os 0.06%, TiO2 0.79%, MgO 0.80%, SO, 0.37%, K2O 0.66% and Na2O 0.12%.

The lime saturation factor of the clinker was 0.725, the silica ratio 0.92, the alumina ratio 11.12 and the free lime content 1.15%.

The potential phase analysis of the clinker was C,2A7 35.19%, C2S 55.16%, C4AF 5.26%, magnesia, free lime and other minor phases to 100%.

Cements were prepared by intergrinding 42.7% by weight of the C,2A7 rich clinker, prepared as described above, together with 57.3% of the Portland cement clinker specified below, white gypsum and citric acid to a surface area of 450 m2/kg measured by the air permeability method according to BSS12 (1971) so as to provide a potential phase composition taking into account only the clinkers and as calculated from the oxide analyses of the two clinkers of 15.0% Cl2A7, 33.9% C3S, 32.7% C2S, 7.3 /O C4AF and 5.7% C3A with magnesia, uncombined lime and other phases to 100%. The quantity of gypsum added was such as to give a total SO3 content, attributable both to the added gypsum and to the alkali sulphates present in the clinker of 2.0% by weight as determined by analysis.

The Portland cement clinker used in this and in the cements of all the following examples had an analysis as follows: SiO2 21.15%, Awl203 5.42%, Fe2O3 2.93%, Mn2O3 0.06%, P205 0.05%, TiO2 0.35%, CaO 67.39%, MgO 0.89%, SO3 0.19%, K2O 0.64%, Na2O 0.30%.

It had a lime saturation factor of 1.02, a silica ratio of 2.33, an alumina ratio of 18.5, a free lime content of 1.80% and a potential phase analysis as follows: C4AF 8.9%, C3A 10.0%, C2S 15.9% and C3S 59.2%.

A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the intergrind described above with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The mixtures of anhydrite, sodium carbonate and bentonite, employed were prepared by grinding the components together in the proportions indicated in Table 1 to a fineness such that between 65 and 75% can pass a 45 micron B.S. sieve when determined using an air-jet sieving technique.

In all cases the water contents of the slurries and the proportion of cement slurry to the slurry containing the mixture of anhydrite, sodium carbonate and bentonite were adjusted to give the following components in the cavity filling medium: Component % by weight % by volume Cement 19.5 Anhydrite 4.9 Sodium carbonate 1.5 Bentonite 1.2 Water 72.9 90% The compressive strengths of 100mm cubes of the resultant mixes were determined over periods between 2 hours and 28 days and are given in Table 1.

Example 2 A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the same cement described in Example 1 with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The latter were prepared in the same way as the mixtures of these materials described in Example 1 but the proportions were adjusted as shown in Table 2.

In all cases the water contents of the slurries and the proportion of cement slurry to the slurry containing the mixture of anhydrite, sodium carbonate and bentonite were adjusted to give the following components in the cavity filling medium.

Component % by weight % by volume Cement 15.8 Anhydrite 8.6 Sodium carbonate 1.5 Bentonite 1.2 Water 72.9 90% The compressive strengths of 100mm cubes of the resultant mixes were determined as in Example 1 and are given in Table 2.

Example 3 A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the same cement described in Example 1 with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The latter were prepared in the same way as the mixtures of these materials described in Example 1 but the proportions were adjusted as shown in Table 3.

Component % by weight % by volume Cement 21.6 Anhydrite 11.7 Sodium carbonate 2.1 Bentonite 1.6 Water 63.0 85 The compressive strengths of 100mm cubes of the resultant mixes were determined as in Example 1 and are given in Table 3.

Example 4 A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the same cement described in Example 1 with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The latter were prepared in the same way as the mixture of these materials described in Example 1 but the proportions were adjusted as shown in Table 4.

Component % by weight % by volume Cement 26.6 Anhydrite 6.7 Sodium carbonate 2.1 Bentonite 1.6 Water 63.0 85 The compressive strengths of 100mm cubes of the resultant mixes were determined as in Example 1 and are given in Table 4.

Example 5 A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the same cement described in Example 1 with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The latter were prepared in the same way as the mixtures of these materials described in Example 1 but the proportions were adjusted as shown in Table 5.

Component % by weight % by volume Cement 20.7 Anhydrite 3.7 Sodium carbonate 1.5 Bentonite 1.2 Water 72.9 90% The compressive strengths of 100mm cubes of the resultant mixes were determined as in Example 1 and are given in Table 5.

Example 6 A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the same cement described in Example 1 with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The latter were prepared in the same way as the mixtures of these materials described in Example 1 but the proportions were adjusted as shown in Table 6.

Component % by weight % by volume Cement 28.3 Anhydrite 5.0 Sodium carbonate 2.1 Bentonite 1.6 Water 63.0 85 The compressive strengths of 100mm cubes of the resultant mixes were determined as in Example 1 and are given in Table 6.

Example 7 A series of compositions simulating those that could be used to fill cavities were produced by mixing a cement slurry prepared from the same cement described in Example 1 with a second slurry prepared from an anhydrite, sodium carbonate and bentonite mixture.

The latter were prepared in the same way as the mixtures of these materials described in Example 1 but the proportions were adjusted as shown in Table 7.

Component % by weight % by volume Cement 27.8 Anhydrite 4.8 Sodium carbonate 2.2 Bentonite 2.2 Water 63.0 85 The compressive strengths of 100mm cubes of the resultant mixes were determined as in Example 1 and are given in Table 7.

This exemplifies a preferred class of compositions according to this invention wherein the inorganic salt component and the clay component are present in the same amount (in percent of the composition on a dry weight basis). It has been found that the reproducibility or consistency of the properties of such cement compositions can be particularly good, especially when the inorganic salt is sodium carbonate and the clay is bentonite.

TABLE 1 Mix Designation 1 2 3 4 Composition ofAnhydrite % % % Mixture by weight by weight by weight by weight Natural Anhydrite 64 - - Synthetic Anhydrite A - 64 - Synthetic Anhydrite B - - 64 Synthetic Anhydrite C - - - 64 Sodium Carbonate 21 21 21 21 Bentonite 15 15 15 15 Compressive Strengths of Cavity Fllllng Medium psi psi psi psi 2 hours 50 80 50 35 1 day 110 90 65 7 days 370 130 195 28 days 460 200 250 135 TABLE 2 Mix Designation 5 6 7 Composition ofAnhydrite % Mixture by weight by weight by weight Natural Anhydrite 76 Synthetic Anhydrite A - 76 Synthetic Anhydrite B - - 76 Sodium Carbonate 14 14 14 Bentonite 10 10 10 Compressive Strengths of Cavity Filling Medium psi psi psi 2 hours 20 50 1 day 45 65 10 7 days 60 100 30 28 days 65 130 60 TABLE 3 Mix Designation 8 9 10 Composition ofAnhydrite % % Mixture by weight by weight by weight Natural Anhydrite 76 Synthetic Anhydrite A - 76 Synthetic Anhydrite B - - 76 Sodium Carbonate 14 14 14 Bentonite 10 10 10 Compressive Strengths of Cavity Fllllng Medium psi psi psi 2 hours 75 60 20 1 day 130 85 30 7 days 215 120 60 28 days 475 200 80 TABLE 4 Mix Designation 11 12 13 14 Composition ofAnhydrite % % Mixture by weight by weight by weight by weight Natural Anhydrite 64 - Synthetic Anhydrite A - 64 Synthetic Anhydrite B - - 64 Synthetic Anhydrite C - - - 64 Sodium Carbonate 21 21 21 21 Bentonite 15 15 15 15 Compressive Strengths of Cavity Filling Medium psi psi psi psi 2 hours 140 105 130 140 1 day 300 280 290 7 days 660 650 645 28 days 895 795 810 675 TABLE 5 Mix Designation 15 Composition ofAnhydrite Mixture by weight Natural Anhydrite Synthetic Anhydrite A Synthetic Anhydrite B Synthetic Anhydrite C 60 Sodium Carbonate 23 Bentonite 17 Compressive Strengths of Cavity Filling Medium psi 2 hours 43 1 day 65 7 days 175 28 days 295 TABLE 6 Mix Designation 16 Composition ofAnhydrite Mixture by weight Natural Anhydrite Synthetic Anhydrite A Synthetic Anhydrite B Synthetic Anhydrite C 60 Sodium Carbonate 23 Bentonite 17 Compressive Strengths of Cavity Filling Medium psi 2 hours 106 1 day 215 7 days 610 28 days 810 TABLE 7 Mix Designation 17 Composition of Anhydrite % Mixture by weight Natural Anhydrite Synthetic Anhydrite A Synthetic Anhydrite B 52 Synthetic Anhydrite C Sodium Carbonate 24 Bentonite 24 Compressive Strengths of Cavity Filling Medium psi 2 hours 65 1 day 150 7 days 470 28 days 650 It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope and spirit of the invention.

Claims

1. A cementitious composition comprising (a) from 20 to 90% of an hydraulic cement containing from 7 to 90%, by weight of its hydraulic constituents, of 12CaO.7AI2Os, the majority by weight of the balance of its hydraulic constituents being one or more calcium silicates; (b) from 5 to 60% of calcium sulfate; (c) from 2 to 10% of inorganic salt and (d) from 0.5 to 10% of clay, the amounts of the components (a) to (d) being relative to the total composition on a dry weight basis.

2. A composition according to claim 1, characterised in that it comprises (a) from 55 to 80% of the hydraulic cement, (b) from 10 to 40% of calcium sulfate, (c) from 4 to 6% of inorganic salt and (d) from 4 to 6% of clay, the amounts of the components (a) to (d) being relative to the total composition on a dry weight basis.

3. A composition according to claim 1 or 2, characterised in that the hydraulic cement contains from 10 to 30%, by weight of its hydraulic constituents, of 12CaO.7AI203.

4. A composition according to claim 1, 2 or 3, characterised in that the hydraulic cement comprises a ground clinker rich in 12CaO.7Al2O3 and containing dicalcium silicate as its major calcium silicate phase and containing a calcium alumino ferrite, magnesia, alkali metal sulfate, alkali metal calcium double sulfate and uncombined lime as minor phases.

5. A composition according to any one of claims 1 to 4, characterised in that component (a) also comprises ordinary Portland cement.

6. A composition according to any one of claims 1 to 5, characterised in that component (b) is anhydrite.

7. A composition according to claim 6, characterised in that the anhydrite is synthetic anhydrite.

8. A composition according to any one of claims 1 to 7, characterised in that the inorganic salt component is selected from alkali metal sulfates, alkali metal carbonates, calcium chloride and mixtures of two or more of these.

9. A composition according to claim 8, characterised in that the inorganic salt is sodium carbonate.

10. A composition according to any one of claims 1 to 9, characterised in that the clay is selected from clays of the hectorite, bentonite, smectite and kaolinite types, or is a mixture of two or more of such clays.

11. A composition according to any one of claims 1 to 10, characterised in that it also comprises up to 20% on a dry weight basis, of an extender selected from slag, fly ash and pozzolana.

12. A composition according to any one of claims 1 to 11, characterised in that component (c) and component (d) are present in the same amounts (in percent by weight).

13. A composition according to any one of claims 1 to 12, characterised in that it contains water.

14. A composition according to claim 13, characterised in that it contains water in an amount of from 55 to 75% by weight of the total aqueous composition.

15. A method of stowing or filling a cavity with a filling medium, characterised in that the filling medium is a cementitious composition according to claim 13 or 14.

16. A method according to claim 15, characterised in that the composition is made at the site of the cavity to be stowed or filled.

17. A method according to claim 15 or 16, characterised in that the cavity is at least partly defined by shuttering and the stowing of the cavity results in the construction of a supportive pack.

18. A method according to claim 15, 16 or 17, characterised in that the hydraulic cement is supplied as a dry powder or as an aqueous slurry through first supply means and the calcium sulfate, inorganic salt and clay components of the cementitious composition are supplied as a dry powder or as an aqueous slurry through second supply means.

19. A method according to claim 15, 16 or 17, characterised in that the hydraulic cement, calcium sulfate, inorganic salt and the clay are supplied as a dry powder through first supply means and the water is supplied through second supply means.

20. A mixture suitable for use in the method of claim 18, characterised in that it comprises from 50 to 80% calcium sulfate, 10 to 28% inorganic salt and 10 to 28% clay, the percentages being on a dry weight basis.