IE50009B1 - Portland cement clinker - Google Patents

Abstract

Portland cement clinker includes as a set regulator (a) a sulphate in the presence of an alkali metal in a combined state, and/or (b) halogen in a combined state, present in a sufficient amt. to prevent premature setting of the cement produced from the clinker. Pref. the cement contains 0.01-1(0.1-1) wt.% halogen, esp. F; and 0.1-3(0.4-2.0) wt.% akali(expressed as oxide), esp. K or K+Na; and 0.33-5(2-4) % sulphate. Bright set is avoided as is prodn. of syngenite (K2SO4CaSO4H2O)which produces rapid air setting.

Description

The present invention relates to a Portland cement clinker that incorporates an integral set-regulating component and to a process for the manufacture of such a clinker; the invention further relates to cement produced 5 from such a clinker.

In addition to such properties as compressive strength and soundnessj the setting time of a given cement is also a property that is of practical importance. Thus, the setting time must not be so short as to prevent proper Placement and subsequent surface-finishing of the cement.

On the other hand, if setting is very slow, an undesirable sedimentation of the concrete solids can occur, leading to the bleeding of water from the concrete; and, furthermore, the delay may be unacceptable to the user, if it impedes the construction work in hand.

It is conventional practice in the manufacture of Portland cement to intergrind a small proportion of a source of sulphate, such as gypsum or anhydrite, with the clinker in order to control the reaction between the finished cement powder and water. By this means, the setting characteristics are controlled to the degree necessary to facilitate the placing of mortar or concrete.

There are, however, problems associated with the use of sulphate, in particular gypsum, added in this manner. One is the variability of sulphate-bearing raw materials. Another is the incidence of false set reactions owing to the dehydration of gypsum at normal cement clinker grinding temperatures. The resulting hemihydrate can react with water to form crystalline 30 gypsum, the precipitation of which has an adverse effect on the rheology of the concrete mix necessitating the addition of further water in order to produce a mix of suitable workability. This extra water has an adverse effect on concrete quality and durability. Although the problem of false set can be overcome by further working, this again frequently --50009 results in adverse cement water-demand requirements, leading to a reduction in the quality of the mortar or concrete. The introduction of larger grinding units with higher grinding temperatures has necessitated the use of increasingly complex and expensive cooling systems to alleviate this problem.

A further problem can occur with clinkers containing an appreciable quantity of potassium sulphate (KgSC^).

When ground with gypsum, the potassium sulphate may react with the gypsum (CaSO^.2H20) to form syngenite (KgSOjj.CaSOjj.HgO) which causes air-setting whereupon the cement becomes lumpy, with poor flow characteristics. Lumpy cement is difficult to mix with aggregates to form a homogeneous mix, and this results in a mortar of poor quality.

A further problem is that of flash setting.

As is well known, the tricalcium aluminate (C^A , where C represents CaO and A represents AlgO^)component of cement undergoes rapid reaction in contact v?ith water.

It is believed that sulphate added to the cement clinker results in the formation of a layer of ettringite on the C^A domains, which layer regulates the setting reaction. However, when there is insufficient sulphate to retard the hydration of the C^A phase, flash setting can occur.

This is substantially irreversible, in that attempts to break up the material and re-work it result in a concrete of markedly reduced quality.

It is an object of the invention to eliminate or alleviate the foregoing problems by producing a Portland cement clinker with integral set control by providing in the clinker itself a component which performs the role hitherto played by gypsum or other sulphate added to the clinker after firing. In other words, the clinker incorporates an integral retarder component which imparts improved setting properties, of a self-regulating kind, to the cement prepared from the clinker.

It is well known to those familiar with the art that SOj present in the form of calcium sulphate as dead50009 burned anhydrite in Portland cement clinker (as distant' from calcium sjlphate added subsequently to the clinker is ineffective as a set retarder, owing tn its slow rate of dissolution in the gauging water. Also, the more readily soluble alkali metal sulphates are known to be poor retarders and furthermore promote enhanced early concrete strengths at the expense of late strengths.

The present invention now provides a Portland cement clinker comprising, as an integral set-regulating comp10 c n e n t , (a ) sulDhate in an on-sunt of 0.33 to 5 t by weigt; cf the clinker, (b) alkali metal in a combined state and in an amount (expressed as the equivalent Ma,0) of fro· 0.1 to 3ft by weight of the clinker and (c) halogen in e combined state and in an amount of 0.01 tc 1 ft by weigh of the clinker, t lie silica ratio of the clinker being not more t h => n 2 . E 0.

The present invention also provides a process for the manufacture of a Portland cement clinker, which pror-- .1 comprises heating to partial fusion a mixture of mat20 crisis containing principally lime (CaO) and silica (5i0_) with a smaller proportion of alumina (Ai.O,) £ c J and iron oxice (fe?0^), in the presence of (A) sulphur bearing material, (0) alkali metal-bearing material, and (C; halogen-bearing material, under conditions sues that there is retained within the clinker (a) sulphate in an amount of 0.33 to by weight of the clinker, (b) alkali metal in combined state arid in an amount (expressed as the equivalent Αθ^Ο) of from 0.1 to 3% by weight of the clinker and (c) halogen in a combined state and in an amount of 0.01 to 1% by weight of the clinker, the silica ratio of the clinker being not more than 2.80.

Cement can be obtained by pulverizing cr grinding the clinker of the present invention and thio can be effected by conventional methods and using conventional apparatus, however, in order that the advantages of the present invention may be fully realised, it is preferred that the clinker be ground in the substantial absence of added sulphate such as gypsum or anhydrite.

At least a proportion of the elements required to form the set-regulating component in the clinker may already be present in the cement raw materials or may be introduced into the kiln by the fuel used to fire the kiln. however, the process will normally involve the introduction into the kiln of one or more additives which include halogen and/or sulphur and^cr alkali meta’ It will be appreciated, of course, that a reference herein to halogen or tc an alkali metal normally refers to the element in a combined state, such aa the halide ion or alkali metal ion, respectively. Similarly, a reference tc sulphur will include sulphur in combined form, usually as sulphate. The additives may be introduced into the kiln either separately or together by incorporation in the raw cement feed or by some other method such as by insufflation. lhe source of sulphur, other than that which may be derived from the main raw materials ana the fuel, is preferably selected from calcium sulphate, hydrates of calcium sulphate, alkali metal sulphates, alksli metal calcium double sulphates and mixtures of any of these. Particularly preferred sources are gypsum, potassium sulphate and cement-works precipitator dust rich in potassium and sulphur.

The preferred sources of alkali metal compounds are the main raw materials or the fuel or the additions of alkali metal sulphate or alkali metal calcium double sulphate referred to above. The preferred alkali metal is potassium, although sodiui:· will common) y also be present.

The preferred halogen is fluorine. The source of fluorine, other than that which may be in the main raw materials, is preferably calcium fluoride. The halogen and sulphur introduced into the kiln exert a combined mineralising action which permits combination of the raw materials in the kiln to occur at reduced burning temperatures, thus facilitating, particularly in kilns of low thermal efficiency, the desired retention of sulphate and alkali metal in the clinker at the levels required for the set-regulating action.

The halogen, in particular fluorine, also has a retarding effect which assists the set control.

The halogen employed as the set-regulating agent ie preferably present in the clinker in an amount, measured as the halide, of 0.1 to 1 per cent by weight of the clinker, especially 0.15 to 0.30 per cent.

The amount of alkali metal is normally measured as 2C the oxide. Tbe alkali metal content of the clinker (expressed as the h'a^O equivalent) is preferably from0.4 to 2.0 per cent and particular!) preferably 0.60 to 1.0 per cent, by weight of the clinker.

Tor the purposes of this invention, the minimum content of sulphate in the clinker is determined by the expression: minimum % weight of SO^i (equivalent hfljO in clinker by weight x 1.29) + 0.2co.

This minimum requirement of SOj is sufficient for all of the alkali metal oxide to occur as the sulphate. together with a small proportion of calcium sulphate.

This calcium sulphate and potassium sulphate will in turn combine to form the double sulphate, known as calcium langeinite.

The figure calculated in any given case from the preceding expression ic referred to herein, purely for convenience, ao the minimum clinker sulphate. This is nof less than 0.33¾.

To avoid problems of cement volume instability, the upper limit of SOj in the clinker is 5.0¾ by weight.

The.preferred range for the content of SD^ is 2.0 to 4.0¾.by weight.

The presence in the clinker of an alkali metal calcium double sulphate, especially calcium potassium sulphate (calcium Isngbeinite), is found to be particularly advantageous.

The content of C^A phase in the clinker is related to two chemical parameters, namely the silica ratio anc the alumina ratio (the calculation of which ratios is described hereinafter). In general, at a given alumina ratio, a decrease in silica ratio implies an increase in C^A content. Since C^A is rapidly reactive with, water, it would be expected that the setting of cement' produced from clinkers with high C^A contents would be more difficult to control. Surprisingly, however, it has been found that the cements produced from the present clinkers still have excellent set control, even though the clinkers have silica ratios of 2.SO and below.

Furthermore, good set control can be achieved even with clinker CjA contents higher than 5 per cent by wei ght.

Suitable selection and regulation of the amounts o' the set-regulating components, and control of the burning by methods known as such in principle to ensure that tng required amounts of these components are retained in the clinker produced, particularly in the form of readily soluble sulphate, enables a cement to be obtained from the clinker which has regulated setting properties, thereby obviating the need for addition of sulphate, such as gypsum, to the clinker during grinding. The addition of further sulphate can be rendered unnecessary by the i η ι c r, t ι o r, but is not excluded; such an addition might prove useful to provide fine tuning of the setting characteristics of the cer.r.t.

In the absence of gypsum or the like, the grinding can be effected hot without adverse effects on the cement quality, thereby obviating the need for expensive cooling equipment.

Thus, by enabling the clinker to be ground without additional gypsum, the problem of false set can be avoided. Furthermore, the presence of an integral regulator within the clinker inhibits flash setting. It will be noted that by grinding in the absence of gypsum, the problem of airsetting due to syngenite formation is obviated. Furthermore, even if some gypsum were to be added at the grinding stage, when the readily soluble calcium langbeinite is formed during the clinker-producing process, this reduces the amount of potassium sulphate that would otherwise be available to form syngenite.

An additional advantage of the invention is that low grade gypsums and by-product gypsum normally considered to be unsuitable for use as retarders when ground with Portland cement clinker can nevertheless be utilized as the source of sulphate in the present invention.

A further advantage is that materials (extenders) such as slag, fly ash or pozzolana can be intergrour.d with clinker according to the present invention in a possibly hot grinding process to produce an extended cement without any adverse effect on cement properties; such extenders can also be ground separately and then blended with the separately ground clinker. Thus, a wide range of cements of the extended Portland type ean be advantageously made from clinker produced by the present invention, together with a suitable extender.

The clinker p.cccrding to the present invention can be ground to form a cement of any desired fineness.

However, the specific surface of the cement will not normally be less than 225 m /kg.

In general, the amount of set-regulating component is selected such that the initial setting time is not less than 45 minutes (in accordance with BS (British Standard) 12:1978, which document is incorporated herein by reference), preferably not less than 60 minutes. The final setting time should not normally be more than 10 hours. Initial setting times for typical production cements are commonly from 90 to 220 minutes, whereas final setting times for such cements commonly range from 150-300 minutes.

The present invention is illustrated ty the following 500 09 ίο by weight, unless otherwise stated. The chemical parameters of the clinker, namely lime saturation factor (LSF), silica ratio (S/R) and alumina ratio (A/F) are defined by the following expressions: LSF = CaO - 0.7 SOj 2.8 Si02 + 1.2 A12O3 + 0.65 FegO^ S/R = Si02 A120j + Fe-jO-j A/F = A12°5 and the C^A content can be calculated by the expression C?A = 2.65 A12O3 - 1.69 Fe2C3 wherein each chemical symbol represents the percentage by weight of the identified substance present in the composition under consideration. The percentages of the oxides are as determined by the methods described in BS455O: Part 2: 197θ, which document is incorporated herein by reference.

Free lime contents are determined by the hot ethylene glycol extraction method.

Specific surface area (SSA) is as determined by the method described in BS455O; Part 3·’ section 3.3· 1978.

The setting times srs determined by the methods described in BS455O: Part 3: sections 3.5 and 3·6: 1978.

Slump tests were carried out in accordance with BS1881: Part 2: 1970· The slump test figure was taker. to be an indication of the workability of the composition in question, a figure of 40 mm being regarded as acceptable and 50 mm being regarded as good.

Example 1 The raw materials used were a limestone/shale/sand S0009 mixture (A), this being a cement works raw feed, together with shale (B) and gypsum (C), the analyses of which were as follows : A B c ·— «— —* s 10 2 13.7% 55.3% 0.7% ai2o3 3.8 21.2 0.19 Fe?0, 1.5 8.6 0.15 fc y CaO 43.1 0.6 32.4 S(as SO3) 0.10 O.Og 46.4 Loss on ignition 35.1 5.2 20.2 k2o 0.93 4.8 0.02 I’a,,0 0.15 0.54 0.02 A general-purpose reagent grade of calcium fluoride (CaF2) was also used.

The raw materials were mixed together in the following approximate proportions: 93.2% of A, 0.2% of B, 6,1% of C, together with 0.5% of the CaF~. The mixture was ground until it had a residue of S.5% on a 90 pm mesh sieve.

The resultant raw mix was mixed with water and formed into cakes which were thoroughly dried and then fired at l400°C to produce a clinker which was found to contain 2.6% free lime and which was found to have the following oxide composition: Si02 19.6% A12O5 5.6Fe2°3 2.3 CaO 65.1 so3 3.6 K?0 1.1 Na20 less than 0.1 F (3.S fluoride) 0.28 The LSF was 0.99, the S/R was 2.48 and the A/K vras 2.43. The C^A content of the clinker was calculated as 10.9%. The equivalent Na20 content was 0.83% and the minimum clinker sulphate derived from the aforesaid NagO content was 1.27%.

The clinker was ground, without any additions whatsoever, in a ball mill at 115°C to produce a cement p with a specific surface area of 296 m /kg_ A slump test on this cement yielded a figure of mm.

When tested for setting times, this cement was found to have an initial setting time of 230 minutes and a final setting time of 290 minutes.

Example 2 The raw materials described in Example 1 were mixe in the approximate proportions 93.7% of A, 0.2% cf B and 6.1% of C and then ground in order to produce a raw feed having a residue of 9.2% on a 90 pm mesh sieve. The raw feed was formed into cakes which, after drying, were fired at l400°C tc produce a cl inker, which was found to have a free lime content of 2.8% and to have the following oxide composition: SiOg 19.9% AlgO 5.5 ^2°3 2.2 CaO 65.5 SO^ 3.4 KgO 1.2 Na20 less than 0.1 F 0.03 The LSF was 0.99, the S/R was 2.58 and the A/F was 2.5Ο. T'he C^A content was calculated to be 10.8%.

The equivalent NagO content was 0.89%, from which a minimum clinker sulphate of 1.35% was calculated.

The clinker was ground, without any addition, in a ball mill at 115°C to produce a cement, the specific surface area of which was determined to be 331 m /kg.

J.; The slump test yielded a figure of 48 mm.

When tested for setting times, the cement was found to have an initial setting time of 50 minutes and a final setting time of 65 minutes.

Example 3 The raw materials described in Example 1 were mixed in the approximate proportions 97.3? of A, 0.2% of B and 2% of C, together with 0.5? of calcium fluoride, and then ground to give a raw feed having a residue of 9.4? on a 90 pm mesh sieve. This raw feed was formed into cakes which were dried and then fired at 145O°C to produce a clinker, which was found to have a free lime content of 3.0? and to have the following oxide composition: SiOg 20.6? AlgO3 5.8Fe2°3 2.3 CaO 66.9 SO^ 1.2 KgO 0.9 NagO less than 0.1 F 0.25 The LSP was 1.00, the S/R was 2.54 and the A/F was 2.52. A C,A content of 11.4? was calculated.

The equivalent NagO content was 0.69?, from which value a minimum clinker sulphate content of 1.09? was calculated.

The clinker was ground, without any addition, in a ball mill at 115°C to produce a cement with a specific surface area of 337 m /kg.

A slump test carried out on this cement yielded a figure of 52 mm.

This cement was found to have an initial setting time of 50 minutes and a final setting time of 60 minutes. Example' 4 The raw materials used were a limestone/shale mix (A), fuel ash (B) and gypsum (C), the analyses of which were as follows: ' δ ' £ SiO2 12.3% 44.0% 0.7% AlgOj 3.0 29.4 0.19Pe2°3 2.5 9.2 0.15 CaO 43.2 2.6 32.4 Total S (as SO^) 1.2 NIL 46.4 Loss on ignition 35.4 9.45 20.2 k2o 0.70 2.2 0.02 NagO 0.31 0.4 0.02 Calcium fluoride of a general purpose reagent gr. was also used.

The raw materials were mixed together in the approximate proportions 93.5% of A,2,0% of E and 4.0% C, together with 0.5% of the CaFg, and then ground, the resulting mixture having a residue of 10.2% on a 90 pm mesh sieve.

This raw feed was mixed with water and formed into calces which were thoroughly dried and then fired at l400°C to produce a clinker, the free lime content of vihich was found to be 2.0% and vihich had the following oxide composition: Si02 19.2%A12°3 5.0Pe2°3 3.8 CaO 65.0S°3 2.9 KgO 0.70 Na?0 0.40 F 0.31 The LSF was 1.01, the S/R was 2.18 and the A/F was 1.32. A CjA content of 6.8% was calculated. The equivalent NagO content was 0.86%, from which a minimum clinker sulphate of 1.31% was calculated.

This clinker was ground, without any addition, in a ball mill at l20°c to produce a cement with a specific surface area of 333 m /kg.

A slump test performed on this cement yielded a result of 49 mm.

The cement, when tested, was found to have an initial setting time of 180 minutes and a final setting time of 230 minutes.

Example 5 The raw materials described in Example 4 (with the exception of the gypsum) were mixed in the proportions 97-5% of A, 2.0^ of D and 0.5^ of the CaF., and then ground to give a raw feed having a residue of 10.4^ on a 90 um mesh sieve. This feed was nixed with water and formed into cakes which, after thorough drying, were fired at l400°C to produce a clinker which was found to have a free lime content of 3.2£ and which had the following oxide composition: Si0? CaO 19.9# .2 4.0 66.1 1.4 0.7 0.4 0.26 The LSF was 1.01, the S/R was 2.16 and the A/F was I.30. A C^A content of 7-0? was calculated. The equivalent Ka20 content was 0.86%, from which a minimum clinker sulphate of 1.31? was calculated.

This clinker was ground, without any additions, in a ball mill at 120°C to produce a cement having a 2 specific surface area of 337 m /kg.

A slump test performed on this cement yielded a result of 45 mm.

When tested for setting times, this cement was found to have an initial setting time of 60 minutes and a final setting time of 9C- minutes.

Example 6 As raw materials, a chalk/clay mix (A), gypsum (B) and fuel ash (C) were used, the analyses of which materials were as follows : 'A B C SiOg 12.7%A12°3 4.3 Feo0, 1.7 2 y CaO 43.1 Total 5(as SO.,) 0.75 Loss on ignition 35.4 KgO 0.80 Na-0 0.22 A general purpose reagent grade of 0.7% 44.0% 0.19 29.4 0.15 9.2 32.4 2.6 46.4 Sil 20.2 9.45 0.02 2.2 0.02 C.4 calcium fluoride was also used.

These raw materials were mixed together in the approximate proportions 92.65% of A, 6.0% of B, 1.0% of C and 0.35% of the CaFg, and then ground to give a raw mix having a residue of 7% on a 90 WJi mesh sieve.

This raw mix was mixed with water and formed into cakes which were subsequently dried and then fired at l400°C to produce a clinker, the free lime content of which was 0.9% and which had the following oxide composition: SiOg 18.5% ai2o3 6.3Fe2°3 2.5 CaO 64.6 so5 3.5 k2o 1.0 Na 0 0.26 F 0.23 .02, the S/R was 2.10 and the A/F was 2.52. The C^A content was calculated to be 12.4%.

L7 The equivalent Ma2C content was 0.92%, from which a minimum clinker sulphate of 1.39? was calculated.

This clinker was ground, without any addition whatsoever, in a ball mill at 120°C to produce a cement 2 with a specific surface area of 326 m /kg.

A slump test performed on this cement yielded a result of 48 mm.

The cement, when tested, was found to have an initial setting time of 200 minutes and a final setting time of 230 minutes.

Example 7 The raw materials described in Example 6 were mixed in the approximate proportions 94.65? of A, 4.0? of B, 1.0? of C and 0.35? of the CaFg, and then ground to give a raw feed having a residue of 7.2? on a 90pm mesh sieve. The raw feed was formed into cakes which, after thorough drying, were fired at l400°C to produce a clinker having a free lime content of 0.8? and having the following oxide composition: Si02 19.0? ai2o3 6.5Fe2°3 2.6 CaO 65.5 so3 2.5 k2o 0.95 Na?0 0.24 F 0.20 was 1.02, the S/R was 2.09 was 2.50. The C_A content was calculated to be 12.8?.

The equivalent Ka20 content was 0.87?, from which a minimum clinker sulphate of 1.32? was calculated.

This clinker was ground without any addition in a ball mill at 120°C to produce a cement with a specific surface area of 330 m /kg.

A slump test performed on this cement yielded a result of 48 mm.

This cement was found to have an initial setting time of 195 minutes and a final setting time of 225 minutes.

Claims (21)

1. A Portland cement clinker, choractcrired in that it comprises, as an integral set-regulating component, (a) sulphate in an amount of 0.33 to 5% by weight of the clinker, (b) alkali metal in a combined state and 5 in 5n amount (expressed as the equivalent Ka^O) of from 0.1 'to 3% by weight of the clinker and (c) halogen in a combined state and in an amount of 0.01 to 1% by weight of the clinker, the silica ratio of the clinker being riot mere than 2.6C. 1P

2. A clinker according tc claim 1, characterised ir that it contains halogen in an amount of 0.1 to 1S by weight of the clinker.

3 . A clinker according tc claim 1 or 2, characterised in that the clinkei comprises fluorine ae the said ha 1 15 ogen.

4. A clinker according to claim 1, 2 or 3, characterised in that it contains alkali metal in sn amount (expressed as the equivalent \s 7 0) af fro- 0.4 to 2.0% by weight of the clinker. 20

5. A clinker according tc any one of claims 1 to 4. characterised in that the alkali metal is substantial!· potassium or a mixture of potassium and sodium,.

6. A clinker according to any one cf claims 1 to 5. characterised in that it contains sulphate in an amour·. 25 of from 2.0 to 4.0 per cent by weight of the clinker.

7. A clinker according to claim 6, characterised 1-, that it contains fluorine in an amount of 0.15 to 0.3Cby weight of the clinker and contains alkali metal in an amount (expressed as the equivalent KagO) of from 30 0.60 to 1.05i by weight of the clinker.

8. A clinker according to any one of claims 1 to 7. characterised in that it contains calcium langbeinite.

9. A clinker according lo any one of claims 1 to 8, characterised in that it has a tricalcium aluminate content greater than 5b. ID.

A Portland cement clinker having an integral 5 set-regulatιng component, being a clinker substantially as hereinbefore described in any one of the specific Examples.

11. A process for the manufacture of a Portland cemen; clinker, which process comprises heating to partial fus13 ion a mixture of materials containing principally lime anc silica together with a smaller proportion of alumina and iron oxide, characterised in that the heating is effected in the presence of (A) sulphur-bearing material, (B) alkali metal-bearing material, and (C) halogen15 bearing material, under conditions such that there is retained within the clinker (a) sulphate in an amount of 0.5? to 5b by weight of the clinker, (t) alkali metal in combined state and in an amount (expressed as the equivalent ^.a^C) of from 0.1 to 35, by weight of the

2. C clinker and (cl hslcoen in a combined state and in an amount of 0.01 to 1b by weight cf the clinker, the silica ratio of the clinker being net more than 2. SC.

12. A process according to claim 11, characterised ir that the sulphur-bearing material comprises calcium 25 sulphate, a hydrate of calcium sulphate, an alkali metal sulphate, an alkali metal calcium double sulphate or a mixture of ariy of these.

13. A process according to claim 11 or 12, characterise in that calcium fluoride is employed as a halogen-bearing 30 material.

14. A process according to claim 11, 12 or 13, when applied to the manufacture of a clinker according to any one of claims 2 to 10.

15. A Portland cement clinker whenever produced by a process according to any one of claims 11 to 14.

16. A method of producing cement by grinding Portland cement clinker, characterised in that the clinker is

3. 5 according to any one of claims T to 10 and 15.

17. -A method according to claim 16, characterised in that tbe grinding is effected' in the substantial absence of added sulphate. 1B.

Cement whenever produced by a method according to 10 claim 16 or 17.

15. Cement according to claim 16, characterised in that it has an initial setting time of not less than 45 minutes and a final setting time of not more than 10 hours. 15

20. Cement according to claim 18 or 19 .characterises in that it contains an extender.

21. Cement according to claim 20, characterised in that the extencer is selected from slag, fly ash and poz2olar,a .