GB2055786A - Portland cement clinker - Google Patents

Abstract

A Portland cement clinker comprises, as an integral set-regulating component, (a) sulphate, in the presence of alkali metal in the combined state, and/or (b) halogen in the combined state, the said set-regulating component being present in an amount sufficient to prevent premature setting of cement produced from said clinker. If components (a) and (b) are not present in sufficient quantities in the raw materials or fuel used sources of (a) and (b) may be introduced into the raw materials heated in a kiln. A cement can be formed from the clinker without the customary additions such as gypsum or anhydrite.

Description

SPECIFICATION Portland cement clinker 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 from such a clinker.

In addition to such properties as compressive strength and soundness, 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 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 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 (K2SO4). When ground with gypsum, the potassium sulphate may react with the gypsum (CaSO4.2H2O) to form syngenite (K2SO4.CaSO4.H2O) which causes air-setting whereupon the cement becomes lumpy, with poor flow characterisitics. 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 (C3A, where C represents CaO and A represents A1203) component of cement undergoes rapid reaction in contact with water. It is believed that sulphate added to the cement clinker results in the formation of a layer of ettringite on the C3A domains, which layer regulates the setting reaction. However, when there is insufficient sulphate to retard the hydration of the C3A 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 SO3 present in the form of calcium sulphate as dead-burned anhydrite in Portland cement clinker (as distinct from calcium sulphate added subsequently to the clinker) is ineffective as a set retarder, owing to 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 component, (a) sulphate, in the presence of alkali metal in the combined state, and/or (b) halogen in the combined state, the said set-regulating component being present in an amount sufficient to prevent premature setting of cement produced from said clinker.

The present invention also provides a process for the manufacture of such a clinker, which process comprises heating to partial fusion a mixture of materials containing principally lime (CaO) and silica (SiO2) with a smaller proportion of alumina (Al203) and iron oxide (we203), in the presence of (a) sulphur-bearing material and alkali metal-bearing material, and/or in the presence of (b) halogen-bearing material, under conditions such that sufficient sulphate and alkali metal and/or halogen is retained within the clinker to prevent premature setting of cement produced therefrom.

Cement can be obtained by pulverizing or grinding the clinker of the present invention and this 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 alkali metal. It will be appreciated, of course, that a reference herein to halogen or to an alkali metal normally refers to the element in a combined state, such as the halide ion or alkali metal ion, respectively. Similarly, a reference to 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.

The source of sulphur, other than that which may be derived from the main raw materials and the fuel, is preferably selected from calcium sulphate, hydrates of calcium sulphate, alkali metal sulphates, alkali 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 sodium will commonly 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.

As implied above, the use of sulphate as the integral set-regulator requires the presence within the clinker of alkali metal; however, with sufficiently high levels of sulphate, halogen (in particularfluorine) need be present only in a small amount and can in appropriate cases be dispensed with entirely. Conversely, when halogen (in particular fluorine) is used as the set-regulator at sufficiently high levels, the sulphate and alkali metal need be present only in small amounts and can also, in some cases, be dispensed with entirely.

However, it is preferred that the clinker contain both halogen and sulphate, in the presence of alkali metal.

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 is generally present in the clinker in an amount, measured as the halide, of 0.01 to 1 per cent by weight of the clinker, preferably 0.1 to 1 per cent and particularly preferably 0.15 to 0.30 per cent.

The amount of alkali metal is normally measured as the oxide. In general, the alkali metal content of the clinker (expressed as the Na2O equivalent) is from 0.1 to 3 per cent and is preferably from 0.4 to 2.0 per cent and particularly preferably 0.60 to 1.0 per cent, by weight of the clinker.

For the purposes of this invention, the minimum content of sulphate in the clinker is determined by the expression: minimum % weight of SO3 = (equivalent Na2O in clinker by weight x 1.29) + 0.2%.

This minimum requirement of SO3 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 is referred to herein, purely for convenience, as the "minimum clinker sulphate". This is preferably not less than 0.33%.

To avoid problems of cement volume instability, the upper limit of SO3 in the clinker is normally 5.0% by weight. The preferred range for the content of SO3 is 2.0 to 4.0% by weight.

When the set regulator is wholly, or in part, sulphate, the presence in the clinker of an alkali metal calcium double sulphate, especially calcium potassium sulphate (calcium langbeinite), is found to be particularly advantageous.

The content of C3A phase in the clinker is related to two chemical parameters, namely the silica ratio and 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 C3A content. Since C3A is rapidly reactive with water, it would be expected that the setting of cements produced from clinkers with high C3A contents would be more difficult to control. Surprisingly, however, it has been found that the present invention still gives excellent set-control even to cements produced from clinkers with silica ratios of 2.80 and below.

Furthermore, good set control can be achieved even with clinker C3A contents higher than 5 per cent by weight.

Suitable selection and regulation of the amounts of the set-regulating components, and control of the burning by methods known as such in principle to ensure that the 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 invention but is not excluded; such an addition might prove useful to provide "fine tuning" of the setting characteristics of the cement. 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 regulatorwithin the clinker inhibits flash setting. It wili be noted that by grinding in the absence of gypsum, the problem of air-setting 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 interground 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 can be advantageously made from clinker produced by the present invention, together with a suitable extender.

The clinker according 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 m2/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 by the following Examples, in which percentages, parts and ratios are by weight, unless otherwise stated. The chemical parameters of the clinker, namely lime saturation factor (LSF), silica ratio (S/R) and alumina ratio (AIF) are defined by the following expressions: LSF = CaO - 0.7 SO3 2.8 SiO2 + 1.2 A1203 + 0.65 Fe2O3 SIR = SiO2 Al203 + Fe203 A/F = A1203 Fe203 and the C3A content can be calculated by the expression C3A = 2.65 A1203 - 1.69 Fe203 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 BS4550: Part 2: 1970, 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 BS4550; Part 3: section 3.3: 1978.

The setting times are determined by the methods described in B54550: 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 taken 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 mixture (A), ihis being a cement works raw feed, together with shale (B) and gypsum (C), the analyses of which were as follows: A B C SiO2 13.7% 55.3; 0.7% Awl203 3.8 21.2 0.19 Fe203 1.5 8.6 0.15 CaO 43.1 0.6 32.4 S(asSO3) 0.10 0.03 46.4 Loss on ignition 35.1 5.2 20.2 K20 0.93 4.8 0.02 Na2O 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 CaF2. The mixture was ground until it had a residue of 9.5% on a 90 um mesh sieve.

The resultant raw mix was mixed with water and formed into cakes which were thoroughly dried and then fired at 1400"C to produce a clinker which was found to contain 2.6% free lime and which was found to have the following oxide composition: SiO2 19.6% A1203 5.6 Fe2O3 2.3 CaO 65.1 SO3 3.6 K20 1.1 Na2O less than 0.1 F (as fluoride) 0.28 The LSF was 0.99, the S/R was 2.48 and the A/F was 2.43. The C3A content of the clinker was calculated as 10.9%. The equivalent Na2O content was 0.83% and the "minimum clinker sulphate" derived from the aforesaid Na2Ocontentwas 1.27%.

The clinker was ground, without any additions whatsoever, in a ball mill at 115 C to product a cement with a specific surface area of 296 m2/kg.

Aslump test on this cement yielded afigure of 43 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 mixed in the approximate proportions 93.7% of A, 0.2% of 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 um mesh sieve. The raw feed was formed into cakes which, after drying, were fired at 1 400"C to produce a clinker, which was found to have a free lime content of 2.8% and to have the following oxide composition: SiO2 19.9% A1203 5.5 Fe2O3 2.2 CaO 65.5 SO3 3.4 K20 1.2 Na2O less than 0.1 F 0.03 The LSF was 0.99, the S/R was 2.58 and the A/F was 2.50. The C3A content was calculated to be 10.8%.The equivalent Na2O 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 m2/kg.

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 appropriate 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 um mesh sieve. This raw feed was formed into cakes which were dried and then fired at 14500C to produce a clinker, which was found to have a free lime content of 3.0% and to have the following oxide compositions: SiO2 20.6% A1203 5.8 Fe203 2.3 CaO 66.9 SO3 1.2 K20 0.9 Na2O less than 0.1 F 0.25 The LSF was 1.00, the S/R was 2.54 and the AIF was 2.52. A C3A content of 11.4% was calculated. The equivalent Na2O content was 0.69%, from which value a "minimum clinker sulphate" content of 1.09% was calculate.

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 m2/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 limestonelshale mix (A), fuel ash (B) and gypsum (C), the analyses of which were as follows: A B C SiO2 12.3% 44.0% 0.7% A1203 3.0 29.4 0.19 Fe2O3 2.5 9.2 0.15 CaO 43.2 2.6 32.4 TotalS(asSO3) 1.2 NIL 46.4 Loss on ignition 35.4 9.45 20.2 K20 0.70 2.2 0.02 Na2O 0.31 0.4 0.02 Calcium fluoride of a general purpose reagent grade was also used.

The raw materials were mixed together in the approximate proportions 93.5% of A, 2.0% of B and 4.0% of C, together with 0.5% of the Ca F2, and then ground, the resulting mixture having a residue of 10.2% on a 90 um mesh sieve.

This raw feed was mixed with water and formed into cakes which were thoroughly dried and then fired at 1400"C to produce a clinker, the free lime content of which was found to be 2.0% and which had the following oxide composition: SiO2 19.2% Al203 5.0 Fe203 3.8 CaO 65.0 SO3 2.9 K2O 0.70 Na2O 0.40 F 0.31 The LSF was 1.01, the S/R was 2.18 and the AIF was 1.32. A C3A content of 6.8% was calculated. The equivalent Na2O content was 0.86%, from which a "minimum clinker suphate" of 1.31 % was calculated.

This clinker was ground, without any addition, in a ball mill at 1200C to produce a cement with a specific surface area of 333 m2/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 B and 0.5% of the CaF2 and then ground to give a raw feed having a residue of 10.4% on a 90 am mesh sieve. This feed was mixed with water and formed into cakes which, after thorough drying, were fired at 1400"C to produce a clinker which was found to have a free lime content of 3.2% and which had the following oxide composition: SiO2 19.9% Al203 5.2 Fe2O3 4.0 CaO 66.1 SO3 1.4 K20 0.7 Na2O 0.4 F 0.26 The LSF was 1.01, the S/R was 2.16 and the AIF was 1.30. A C3A content of 7.0% was calculated. The equivalent Na2O 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 specific surface area of 337 m2/kg.

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 90 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 SiO2 12.7% 0.7% 44.0% A1203 4.3 0.19 29.4 Fe203 1.7 0.15 9.2 CaO 43.1 32.4 2.6 Total S (as SO3) 0.75 46.4 NIL Loss on ignition 35.4 20.2 9.45 K20 0.80 0.02 2.2 Na2O 0.22 0.02 0.4 A general purpose reagent grade of 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 CaF2, and then ground to give a raw mix having a residue of 7% on a 90 cm mesh sieve.

This raw mix was mixed with water and formed into cakes which were subsequently dried and then fired at 1400"C to produce a clinker, the free lime content of which was 0.9% and which had the following oxide composition: SiO2 18.5% A1203 6.3 Fe2O3 2.5 CaO 64.6 SO3 3.5 K20 1.0 Na2O 0.26 F 0.23 The LSFwas 1.02, the S/R was 2.10 and the A/F was 2.52. The C3A content was calculated to be 12.4%. The equivalent Na2O 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 with a specific surface area of 326 m2/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 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 CaF2, and then ground to give a raw feed having a residue of 7.2% on a 9Oum mesh sieve. The raw feed was formed into cakes which, after thorough drying, were fired at 1400"C to produce a clinker having a free lime content of 0.8% and having the following oxide compositon: SiO2 19.0% A1203 6.5 Fe2O3 2.6 CaO 65.5 SO3 2.5 K20 0.95 Na2O 0.24 F 0.20 The LSF was 1.02, the S/R was 2.09 and the AIF was 2.50. The C3A content was calculated to be 12.8%. The equivalent Na2O 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 1 200C to produce a cement with a specific surface area of 330 m2/kg.

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

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

Example 8 The raw materials employed were chalk (A), china clay (B), sand (C), iron oxide (D), calcium fluoride (E), gypsum (F) and potassium sulphate (G),the analyses of which were as follows: A B C D E F G Si02 2.0 49.9 98.8 1.13 - 0.1 A1203 0.3 34.1 0.2 0.62 - 0.09 Fe2O3 0.09 0.8 0.1 96.86 - 0.05 CaO 54.4 0.2 0.6 0.53 71.8 32.4 Loss on 42.9 10.5 0.5 0.75 - 20.2 ignition S03 0.06 0.01 0.04 0.02 - 46.4 45.94 K20 0.04 3.4 0.03 0.02 - 0.02 54.06 F - - - - 48.7 - These raw materials were mixed together in the following approximate proportions: A 75.45% B 2.74 C 11.65 D 1.13 E 0.33 F 6.35 G 2.35 The mixture of raw materials was ground to give a raw feed having a residue of 4.8% on a 9Oum mesh sieve.As in the preceding Examples, the raw feed was mixed with water and pressed into cakes which were thoroughly dried before being fired. The firing temperature in this Example was 1400"C. A clinker was produced having a free lime content of 2.40% and having the following oxide composition: SiO2 22.3% A1203 1.8 Fe2O3 1.9 CaO 69.0 SO3 2.9 K20 1.2 F 0.14 The LSF was 1.02, the S/R was 6.0 and the A/F was 0.95. The C3A content was calculated to be 1.6. The equivalent Na2O content was 0.79% from which a "minimum clinker sulphate" of 1.22% was calculated.

The clinker was ground, without any additions, in a ball mill at 115"C to form a cement having an SSA of 444 m2/kg.

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

The cement was found to have an initial setting time of 145 minutes and a final setting time of 195 minutes.

Example 9 The raw materials described in Example 8 were mixed in the following approximate proportions: A 75.43% B 4.82 C 10.48 D 0.36 E 0.33 F 6.36 G 2.22 The mixture of raw materials was ground to give a raw feed having a residue of 5.9% on a 90 um mesh sieve. The raw feed was pressed into cakes which were thoroughly dried and then fired at 1400"C to give a clinker, which was found to have a free lime content of 1.62% and the following oxide composition: SiO2 22.6% A1203 2.8 Fe2O3 0.8 CaO 68.8 SO3 2.7 K20 1.3 F 0.16 The LSF was 1.00, the S/R was 6.3 and the AIF was 3.5. A C3A content of 6.07% was calculated. The equivalent Na2O was 0.86%, from which a "minimum clinker sulphate" of 1.30% was calculated.

The clinker was ground, without any additions, in a ball mill at 11 50C to give a cement, the SSA of which was found to be 448 m2/kg. A slump test was performed on this cement, which yielded a result of 58 mm.

The cement was found to have an initial setting time of 140 minutes and a final setting time of 170 minutes.

Example 10 The raw materials described in Example 8 were mixed together in the following approximate proportions: A 73.94% B 9.60 C 7.00 D 0.75 E 0.33 F 6.38 G 1.97 The mixture of raw materials was ground to give a raw feed having a residue of 5.2% on a 90 um mesh sieve. The raw feed was formed into cakes which were thoroughly dried and then fired at 1400"C to give a clinker, the free lime content of which was found to be 1.8% and which had the following oxide composition: SiO2 20.4% A1203 5.2 Fe2O3 1.4 CaO 67.3 SO3 2.8 K20 1.3 F 0.17 The LSFwas found to be 1.02, the S/R was 3.1 and the A/F was 3.71.The C3A content was calculated to be 11.4%. The equivalent Na2O was 0.86%, from which a "minimum clinker sulphate" of 1.30% was calculated.

The clinker was ground, without any additions, in a ball mill at 11 50C to give a cement, the SSA of which was found to be 457 m2lkg.

This cement, when subjected to a slump test, gave a result of 45 mm.

The cement was found to have an initial setting time of 120 minutes and a final setting time of 185 minutes.

Example 71 The raw materials employed were a chalk/clay mix (A), sand (B), commercial ground chalk (C), gypsum (D), potassium sulphate of general purpose reagent grade (E) and calcium fluoride of general purpose reagent grade (F), the analyses of which were as follows (in%): A B C D E F SiO2 12.7 97.8 2.1 0.1 A1203 3.3 1.3 0.2 0.09 - Fe2O3 1.6 0.1 0.07 0.05 - CaO 44.9 - 54.6 32.4 - 71.8 Loss on 35.6 0.3 42.8 20.2 - ignition S03 0.29 - 0.07 46.4 45.94 K20 0.73 0.72 0.02 0.02 54.06 Na2O 0.16 0.11 0.02 - - F - - - - - 48.7 These raw materials were mixed together in the following approximate proportions: A 15.05% B 13.66 C 62.21 D 6.55 E 2.00 F 0.50 The mixture of raw materials was ground to give a raw feed having a residue of 5.8% on a 90 um mesh sieve.The raw feed was formed into cakes which were thoroughly dried and then fired at 1400"C to give a clinker having a free lime content of 0.8% and having the following oxide composition: SiO2 26.9% A1203 1.5 Fe2O3 0.57 CaO 67.8 SO3 0.91 K20 0.42 Na2O 0.10 F 0.14 The LSF was 0.87, the S/R was 13.0 and the A/F was 2.63. The C3A content was calculated to be 3.0%. The equivalent Na2O content was 0.38%, from which a "minimum clinker sulphate" of 0.69% was calculated.

The clinker was ground, without any additions, in a ball mill at 11 50C to give a cement having an SSA of 450 m2/kg.

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

The cement was found to have an initial setting time of 215 minutes and a final setting time of 260 minutes.

Example 12 The raw materials described in Example 11 were mixed in the following approximate proportions: A 45.79% B 7.10 C 38.07 D 6.55 E 2.00 F 0.50 The mixture of raw materials was ground to give a raw feed having a residue of 6.9% on a 90 um mesh sieve. The raw feed was formed into cakes which were thoroughly dried and fired at 1350"C to give a clinker having a free lime content of 1.5% and having the following oxide composition: SiO2 22.2% A1203 2.7 Fe2O3 1.2 CaO 67.2 SO3 3.6 K20 1.5 Na2O 0.11 F 0.25 The LSF was 0.98, the S/R was 5.7 and the A/F was 2.25. The C3A content was calculated to be 5.1%. The equivalent Na2O was 1.10%, from which a "minimum clinker sulphate" of 1.62% was calculated.

The clinker was ground, without any additions, in a ball mill at 115 C give a cement, the SSA of which was found to be 453 m2/kg.

A slump test performed on this cement yielded a result of 54 mm.

The cement was found to have an initial setting time of 215 minutes and a final setting time of 270 minutes.

Example 13 The chalk/clay mix (A), sand (B) and calcium fluoride (F) described in Example 11 were mixed in the following approximate proportions: A 97.1% B 2.2 F 0.7 The resultant mixture was ground in order to provide a raw feed having a residue of7.3% on a 90 um mesh sieve. The feed was formed into cakes which, after drying, were fired at 1 3750C to a free lime content of 1.3%.

The resultant clinker had the following oxide composition: SiO2 22.2% At203 5.0 Fe203 2.4 - CaO 66.9 SO3 0.1 K20 0.65 Na2O 0.13 F 0.49 The LSF was 0.96, the S/R was 3.0 and the A/F was 2.08. The C3A content was calculated to be 9.2%. The equivalent Na2O content was 0.56.% This is an example of a clinker wherein the integral set-regulating component is fluorine alone.

Accordingly, the "minimum clinker sulphate" was of no relevance.

The clinker was ground in a ball mill at 1200C, without any additions, to give a cement, the SSA of which was determined to be 345 m2/kg.

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

The cement was found to have an initial setting time of 105 minutes and a final setting time of 280 minutes.

Claims (26)

1. A Portland cement clinker, characterised in that it comprises, as an integral set-regulating component, (a) sulphate, in the presence of alkali metal in a combined state, and/or (b) halogen in a combined state, the said set-regulating component being present in an amount sufficient to prevent premature setting of cement produced from said clinker.

2. A clinker according to claim 1, characterised in that it contains halogen in an amount of 0.01 to 1% by weight of the clinker.

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

4. A clinker according to claim 1,2 or 3, characterised in that the clinker comprises fluorine as the said halogen.

5. A clinker according to any of claims 1 to 4, characterised in that it contains alkali metal in an amount (expressed as the equivalent Na2O) of from 0.1 to 3% by weight of the clinker.

6. A clinker according to claim 5, characterised in that it contains alkali metal in an amount (expressed as the equivalent Na2O) of from 0.4 to 2.0% by weight of the clinker.

7. A clinker according to any of claims 1 to 6, characterised in that the alkali metal is substantially potassium or a mixture of potassium and sodium.

8. A clinker according to any of claims 1 to 7, characterised in that it contains sulphate in an amount of 0.33 to 5% by weight of the clinker.

9. A clinker according to claim 8, characterised in that it contains sulphate in an amount of from 2.0 to 4.0 per cent by weight of the clinker.

10. A clinker according to claim 9, characterised in that it contains fluorine in an amount of 0.15 to 0.30% by weight of the clinker and contains alkali metal in an amount (expressed as the equivalent Na2O) of from 0.60 to 1.0% by weight of the clinker.

11. A clinker according to any of claims 1 to 10, characterised in that it contains calcium langbeinite.

12. A clinker according to any of claims 1 to 11, characterised in that it has a silica ratio of not more than 2.80.

13. A clinker according to any of claims 1 to 12, characterised in that it has a tricalcium aluminate content greater than 5%.

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

15. A process for the manufacture of a Portland cement clinker, which process comprises heating to partial fusion a mixture of materials containing principally lime and 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 and alkali metal-bearing material, and/or in the presence of (b) halogen-bearing material, under conditions such that sufficient sulphate and alkali metal and/or sufficient halogen is retained within the clinker to act as an integral set-regulating component that prevents premature setting of cement produced therefrom.

16. A process according to claim 15, characterised in that the sulphur-bearing material comprises calcium sulphate, a hydrate of calcium sulphate, an alkali metal sulphate, an alkali metal calcium double sulphate or a mixture of any of these.

17. A process according to claim 15, or 16, characterised in that calcium fluoride is employed as a halogen-bearing material.

18. A process according to claim 15, 16 or 17, when applied to the manufacture of a clinker according to any one of claims 2 to 14.

19. A Portland cement clinker whenever produced by a process according to any one of claims 15 to 18.

20. A method of producing cement by grinding Portland cement clinker, characterised in that the clinker is according to any one of claims 1 to 14 and 19.

21. A method according to claim 20, characterised in that the grinding is effected in the substantial absence of added sulphate.

22. A process according to claim 20 or 21, characterised in that the grinding is effected hot.

23. Cement whenever produced by a method according to claim 20, 21 or 23.

24. Cement according to claim 23, 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.

25. Cement according to claim 23 or 24 characterised in that it contains an extender.

26. Cement according to claim 25, characterised in that the extender is selected from slag, fly ash and pozzolana.