AU2011245080A2

AU2011245080A2 - Low C02 cement

Info

Publication number: AU2011245080A2
Application number: AU2011245080A
Authority: AU
Inventors: Louise Margaret Keyte; Redmond Richard Lloyd
Original assignee: Boral Cement Ltd
Current assignee: Boral Cement Ltd
Priority date: 2010-04-29
Filing date: 2011-04-29
Publication date: 2013-01-24
Anticipated expiration: 2031-04-29
Also published as: WO2011134025A1; AU2011245080A1; AU2011245080B2

Abstract

A unreacted dry cement composition or a cement composition comprising greater than 20wt% and less than 65wt% portland cement or clinker, granulated blast furnace slag optionally in combination with steel slag wherein the ratio of slag to cement or clinker is in the range of 1 :2 to 4:1, 1.5 - 6 wt% activator consisting of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K) and optionally up to 10 wt% gypsum. A concrete composition comprising the cement composition, aggregate, water, and optional chemical admixtures is also claimed.

Description

1 Low C02 Cement Field of the invention This invention relates to cementitious binders and particularly to a cement composition which emits lower amounts of C02 during manufacture than conventional Portland 5 cement. Background of the invention Portland cement clinker is a hydraulic material which generally consists of at least two thirds by mass of calcium silicates (3CaO.SiO 2 and 2CaO.SiO 2 ), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The 10 ratio of CaO to SiO 2 is not less than 2.0. The magnesium content (MgO) does not exceed 5.0% by mass. Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a sintering temperature, which is about 1450 *C. The major raw material for the clinker-making is usually limestone (CaCO 3 ) mixed with a second material 15 containing clay as source of alumino-silicate. Aluminium oxide and iron oxide are present as a flux and contribute little to the strength. The production of Portland cement clinker is associated with high-CO 2 emissions. A proven way to reduce clinker content in cement and concrete is to replace clinker with supplementary cementitious materials (SCM's) like ground, granulated blast-furnace slag (slag) or fly ash (ash). Of these, slag 20 is more reactive and therefore more attractive for producing binders with high substitution of Portland cement clinker. Blast furnace slag is a nonmetallic co product produced during the production of iron. It consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. Granulated blast furnace slag (GBFS) is the result of cooling and solidifying the molten 25 slag by rapid water quenching to a glassy state, where little or no crystallization occurs. This process results in the formation of sand size (or frit-like) fragments, usually with some friable clinkerlike material. The physical structure and gradation of granulated slag depend on the chemical composition of the slag, its temperature at the time of water 2 quenching, and the method of production. When crushed or milled to very fine cement sized particles, ground granulated blast furnace slag (GGBFS) has cementitious properties, which make a suitable partial replacement for or additive to Portland cement. Replacement of Portland clinker by granulated blast furnace slag has the advantage of 5 reducing cost and carbon emissions by replacing an emissions intensive material (Portland cement) with near-zero emissions materials. The major drawback limiting the amount of slag that can be substituted into cement is the reduction in early age compressive strength as substitution of ash or slag increases. This is particularly apparent at early ages (i.e. up to 7 days). Figure 1 is a graph of mortar strength as a 10 function of the percentage replacement of Portland cement with GGBFS. Clearly the reduction in strength demonstrated is a problem. The use of activating agents to improve the activity of the oxides in slags has been reported in the prior art. These are generally not seen to favourably affect the properties of the resulting cement due to their effect of accelerating the setting time of the cement. 15 For example, in Concrete Admixtures Handbook, Chapter 3, p101, V.S. Ramachandran writes: "sodium hydroxide accelerates the hydration of C 3 S and early strengths but the later strengths are decreased". Setting time is also shortened. Ramachandran also mentions sodium carbonate: "Sodium carbonate decreases the setting time of cement by 2-4 hours... After 10-12 hours the hydration is retarded" 20 For this reason it has been found that activators, sodium hydroxide and sodium carbonate, cannot be used effectively to activate mixtures of cement (or clinker) and slag as they accelerate setting time to an unacceptable degree, and also result in low later age strength. Summary of the invention 25 In contrast to the teachings of the prior art, the applicant has found that an activator in the form of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K) does not have a major effect on the setting time of blended cement, and allows excellent strength development.

3 Accordingly to one aspect of the invention, there is provided a cement composition comprising, consisting of or consisting essentially of 1.5 - 6 wt% activator consisting of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K) 5 0 - 10 wt% gypsum; greater than 20wt% and less than 65wt% portland cement or clinker; and 40-80 wt%.granulated blast furnace slag optionally in combination with steel slag wherein the ratio of slag to cement or clinker is in the range of 1:2 to 4:1, wherein the activator and cement composition does not include sodium hydroxide or sodium 10 carbonate. In another aspect of the invention there is provided an unreacted dry cement composition comprising, consisting of or consisting essentially of 1.5 - 6 wt% activator consisting of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K) 15 0 - 10 wt% gypsum; greater than 20wt% and less than 65wt% portland cement or clinker; and 40-80 wt%.granulated blast furnace slag optionally in combination with steel slag wherein the ratio of slag to cement or clinker is in the range of 1:2 to 4:1, wherein the activator and cement composition does not include sodium hydroxide or sodium 20 carbonate.

4 In the context of this aspect of the invention, unreacted dry composition is understood to relate to a combination of components in which none of the components have undergone chemical treatment although the components may have separately or in any combination have undergone heat treatment such as calcining or mechanical treatment 5 such as grinding or milling. This aspect of the invention has the benefit of providing a dry composition where the components do not require additional processing prior to or after mixing. The benefit is that the cement still has similar strength and structural characteristics as regular portland cements while producing less CO 2 during manufacture, thus producing less 10 greenhouse gases. The lower limit for portland cement addition is preferably greater than 25 wt%, and is more preferably 30 wt% and most preferably 35 wt%. The upper limit is preferably 55 wt%, more preferably 50 wt% and most preferably 45 wt%. In a preferred form or the above aspects, the ratio of slag to cement or clinker is in the 15 range of 1:1 to 4:1. Preferably the proportion of slag in the cement composition is in the range of 40 - 80 wt % of the total cement composition more preferably 40 - 70 wt% and most preferably 50 - 60 wt%. In the above ranges, the lower limit may be 50 wt% and so the slag may be present in the range of 50-80 wt% preferably 50-70 wt%. Blast furnace slag may 20 constitute all of the slag component but a proportion of the blast furnace slag may be substituted with steel slag up to a total of 30wt% of the total cement composition, preferably 0-20 wt% and most preferably 0-15 wt%. The blast furnace slag preferably has a CaO/SiO 2 of less than 1.3 and preferably in the range of 1.0 to 1.3. Generally, the lower the CaO/SiO 2 wt ratio, the lower the reactivity of the slag. The invention therefore 25 permits the use of less reactive slags than has been known previously. Further, the slag preferably has an A1 2 0 3 /SiO 2 wt ratio 0.3-0.5 and a MgO/CaO wt ratio of 0.1-0.5.

5 Preferably the cement composition has a fineness index of 350 to 550 m 2 /kg and preferably the cement has Bogue values of C3S, 28 to 75%; C2S, 0.5 to 44.2%: C3A, 2.5 to 12%: and C4AF, 8.5 to 19.3%. Hence the use of the activator in accordance with the invention is particularly suited to 5 low reactivity slags. This is contrary to the teachings of the prior art which found the use of activating agents accelerated the setting time to an unacceptable degree, and also result in low later age strength. Further, the slag preferably has an A1 2 0 3 /SiO 2 wt ratio 0.3-0.5 and a MgO/CaO wt ratio of 0.1-0.5 10 Accordingly steel slag may be present in an amount of 0 - 30wt% and when steel slag is present blast furnace slag makes up 20 to 80wt% of the cement composition more preferably 30 to 70wt% In a preferred form of the invention, the activator is sodium sulphate. The preferred amount of activator is 2-6wt% and most preferably 3-5 wt%. 15 While the addition of gypsum is optional, it is preferably that it is present in an amount of 1-6 wt%, more preferably greater than zero to 5wt% and most preferably 2-5wt%. . Concrete is a construction material composed of cement (commonly Portland cement) as well as other cementitious or pozzolanic materials such as fly ash and slag, aggregate (generally a coarse aggregate made of crushed rocks such as limestone, or 20 granite, plus a fine aggregate such as sand), water, and chemical admixtures. The applicant has found that the addition of sodium sulphate in the above weight percentages based on the binder content (ie slag, portland cement or clinker and gypsum) to a concrete mixture comprising or consisting essentially of granulated blast furnace slag optionally with steel slag and portland cement wherein the ratio of slag to 25 cement is in the range of 1:2 to 4:1, optionally gypsum from 0 to lOwt%, aggregate, 6 water and optional chemical admixtures has the same effect on early age strength and drying shrinkage. Therefore according to another aspect of the invention there is provided a concrete mixture comprising a cement binder which is the cement mixture as described above. 5 Broadly, the cement binder comprises, consists essentially of or consists of a cement binder of anyone of claims 1 to 13 comprising, 40-80 wt% blast furnace slag optionally in combination with steel slag as the slag component and portland cement binder wherein the ratio of slag to cement is in the range of 1:2 to 4:1, 10 0 - 10 wt% gypsum; greater than 25wt% and less than 65wt% Portland cement; an activator present in the amount of 1.5-6wt% of the weight of the cement and slag binder, the activator consisting of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K), and 15 aggregate, water, and optional chemical admixtures. Typically the level of aggregate is in the range of 65 - 85wt % of the total dry mixture. The aggregate may be a mixture of coarse and fine aggregate. The chemical admixture may be components such as water reducers, superplasticisers, air entraining agents, accelerators, retarders, etc at the manufacturers recommended dose. 20 The preferred amount of activator is 2-6wt% and most preferably 3-5 wt% The activator is preferably sodium sulphate.

7 While the addition of gypsum is optional, it is preferably that it is present in an amount of 1-6 wt%, more preferably greater than zero to Swt% and most preferably 2-5wt%. Brief description of the drawings / figures Figure 1 is a graph of mortar strength as a function of the percentage replacement of 5 Portland cement with blast furnace slag Figure 2 is a graph of comparative data between high slag blended cements, and the same cements with 5% sodium sulphate activator; Figure 3 is a graph of mortar compressive strength as a function of sodium sulphate addition; 10 Figure 4 is a graph of the compressive strength of a sodium sulphate activated slag/portland cement mixture with varying amounts of gypsum; Figure 5 is a graph comparing the concrete strength over time for slag substituted cements according to the invention and comparative examples; and Figure 6 is graph comparing drying shrinkage for slag substituted cements according to 15 the invention and comparative examples. Figure 7 is a graph comparing the results of a mortar bar sulphate expansion test for mortar made with Type SL cement and a sodium sulphate activated slag/Portland cement mixture contain 70% slag and 30% Portland cement. Detailed description of the embodiments 20 As shown in figure 1, the greater the substitution of portland cement clinker with blast furnace slag optionally with steel slag, the greater the reduction in early age compressive strength. This demonstrates the problem of simply substituting slag for cement particularly when using low reactivity slag where the CaO/SiO 2 of less than 1.3 and preferably in the range of 1.0 to 1.3. Figure 1 shows that early strength decreases 25 almost linearly as slag content is increased.

8 The invention achieves acceptable strength performance while also achieving high levels of clinker reduction. Figure 2 demonstrates the benefit of our invention, i.e. even at high substitution levels, very good early strength can be achieved This is achieved by adding an activator to the Portland cement (or clinker)/blast furnace 5 slag with optional steel slag mixture. The activator added in an amount of 1.5 - 5wt% is preferably a sulphate of an alkali metal such as Na or K and most preferably sodium sulphate. By the addition of the activator, substitutions of portland cement clinker can be achieved from 2:1 to 1:4 Portland cement to slag, preferably 1:1 to 1:4. The preferred addition of sodium sulphate is 2-5wt%. 10 Gypsum which is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 4 -2H 2 0 may also be added up to a level of 10 wt%. Preferably the proportion of slag in the cement composition is in the range of 50 - 80 wt % of the total cement composition more preferably 50 - 70 wt%. Blast furnace slag may constitute all of the slag component but a proportion of the blast furnace slag may be 15 substituted with steel slag up to a total of 30wt% of the total cement composition. Accordingly steel slag may be present in an amount of 0 - 30wt% and when steel slag is present blast furnace slag makes up 20 to 80wt% of the cement composition more preferably 30 to 70wt% Examples 20 To illustrate the effectiveness of the invention, sodium sulphate was added as an activating agent to a number of portland cement and slag mixtures and the results tabulated in Table 1.

9 Slaq composition: Typical composition of granulated blast-furnace slag used by the applicant in the examples is as follows wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Si0 2 A1 2 0 3 CaO MgO SO 3 P K 2 0 FeO MnO TiO 2 min 30.0 12.0 35.0 3.5 0.3 0.0 0.2 0.1 0.05 0.5 max. 38.0 17.0 45.0 1.0 1.5 1.2 0.6 2.0 0.7 1.5 5 The CaOISiO2 weight ratio is lower than that taught in some of the prior art and generally speaking, the lower the Ca/Si ratio, the less reactive the slag. Preferably the CaO/SiO 2 for GBFS is less than 1.3 Ratios of some of these components are: CaO/SiO2 Al 2

O

3 /SiO 2 MgO/CaO wt ratio wt ratio wt ratio 1.0-1.3 0.3-0.5 0.10-0.5 10 Portland cement clinker composition: Typical composition of portland cement clinker used by the applicant in the examples is as follows 10 CaG S10 2 A1 2 0 3 Fe 2 0 3

SO

3 MgO K 2 0 TiO 2 Na 2 0 wt% wt% wt% wt% wt% wt% wt% wt% wt% Min 60.0 19.0 4.0 2.0 0.1 0.5 0.1 0.2 Max 70.0 25.0 6.0 6.0 1.5 2.0 1.0 0.5 Clinker and cement are often characterised by Bogue ratio, which is a theoretical calculation of mineralogy. The information on Bogue ratios is described in PCA's Control of Portland Cement Quality by Clyde Moore and his original work 5 ' "Chemical Control of Portland Cement Clinker," Ceramic Bulletin, Vol. 61, No. 4, 1982, pages 511 to 515. Silica ratio = SR or SM = Si0 2 /Al 2 0 3 + Fe 2 03 Alumina- iron ratio = AR + A1 2 0 3 /Fe 2

O

3 Lime factor = C - (1.65A + 0.315F)/S 10 Lime saturation factor = LSF = 100(CaO + 0.75Mg)/(2.85SiO 2 ) + 1.18Al 2 0 3 +0.65 Fe 2 03 For MgO below 2% Bogue Equations for Potential Composition

C

3 S = 4.071 C - 7.6S - 6.718a - 1.43F 15 C 2 S= -3.071C + 8.6S + 5.068A + 1.079F

C

3 A = 2.65A - 1.692F

C

4 AF = 3.043F For the current clinker used by the applicants, the Bogue values varied depending on the supply as follows: 11 CAS C 2 S CaA C 4 AF Mean 62-68 10-14 4.00- 9.00 10.5 16.0 Min 28.0 0.5 2.5 8.5 Max 75.7 44.2 12.00 19.3 Table 1 Effect of 5% addition of sodium sulphate to cement/slag mortars. Cement 50% slag 50% slag 60% slag 70% slag Portland g 450 225 225 180 135 cement Slag g - 225 225 270 315 (GGBFS) Standard g 1350 1350 1350 1350 1350 Sand Sodium g - - 22.5 22.5 22.5 sulphate Water g 220 220 205 205 205 Flow % 110 113 108 111 113 3-day MPa 38.8 23.9 43.9 41.6 34.2 7-day MPa 50.3 38.9 53.9 51.9 41.9 28-day MPa 62.7 57.8 64.6 62.1 52.9 12 A comparison of high slag content blended cements, and the same cements with 5% sodium sulphate activator, is shown in Figure 2. Clearly then, sodium sulphate addition provides a very effective mechanism to improve early strength. This activating agent has the benefit of being cheap, readily available 5 and is non-toxic and non-hazardous. In addition, the setting time of the cement is not compromised. There is also a modest water-reducing effect provided by the sodium sulphate. This is demonstrated in the water addition necessary to achieve standard flow, demonstrated in Table 1. The applicant also conducted tests with other activating agents as a comparison to the 10 invention. In comparison to other activating agents containing alkali cations, such as sodium hydroxide or sodium carbonate, these were not able to provide strength improvement as well as that of sodium sulphate. In some cases they greatly reduced strength. In addition, they resulted in overly rapid setting times whereas sodium sulphate addition does not have a detrimental effect on setting time. 15 Other activating agents containing sulphate were also trialled. Calcium sulphate (typically gypsum) is added to cement and often to slag when it is milled. A type of cement containing extra calcium sulphate as an activating agent is well known, and is included in European standards. This type of cement, usually known as "supersulphated cement" has poor early age strength performance compared to cement with sodium 20 sulphate addition. Potassium sulphate can be added in place of sodium sulphate, with roughly similar performance. However sodium sulphate is preferred as it is cheaper than potassium sulphate. Example of use of steel slag composition 25 Steel slag is a waste from steel production, and is typically difficult to use for a variety of reasons. It has lower value than that of blast-furnace slag as it cannot be used solely to substitute clinker in the way blast-furnace slag can.

13 In addition to the use of GGBFS, the applicant has also trialed ground steel slag (GSS) having a composition as follows. wt% wt% wt% wt% wtOwt% wt% wt% wt% wt% SiO 2 A1 2 0 3 CaO MgO S P K 2 0 FeO MnO TiO 2 11.9 2.9 36.1 9.2 0.9 0.6 0.02 25.6 4.0 1.17 The following results of 3-day, 7-day and 28 day mortar strength tests shown in Table 2 5 of GSS and GGBFS mixtures were produced. Steel slag performance: Table 2: Sample 1 2 3 4 Portland g 135 90 90 90 Cement GGBFS g 315 360 315 270 GSS g - - 45 90 Standard g 1350 1350 1350 1350 Sand Na 2

SO

4 g 22.5 22.5 22.5 22.5 Water g 215 210 205 200 14 Flow % 114 110 110 108 Test results 3 days MPa 31.4 23.1 30.7 31.4 7 days MPa 36.4 30.2 36.8 37.5 28 days MPa 46.9 44.7 46.4 46.3 Columns 1 and 2 show a decrease in compressive strength as Portland cement content is reduced from 30 to 20%. Column 3 shows that replacing Portland cement with ground steel slag (GSS in the table) results in no significant loss in strength at 3, 7 and 28 days, 5 compared to column 1. It has the same cement content as column 2, but better performance. This further reduces the carbon footprint of the cement, and reduces the cost, as steel slag is less expensive than clinker and has no CO 2 penalty. Column 4 shows that some of the ground granulated blast-furnace slag (GGBFS in the table) can also be replaced with ground steel slag with no loss of performance. This 10 further reduces cost, as steel slag is generally cheaper than granulated blast-furnace slag. Limits of the addition: The amount of sodium sulphate necessary to achieve activation is not fixed - there is an increasing benefit with increasing addition up to about 5 wt%. This is shown in Figure 15 3 which shows mortar compression strength as a function of sodium sulphate addition. It is clear that the region of interest is greater than about 2% and less than about 6% with the area of most interest between 3-5 wt%. There is little benefit in increasing sodium sulphate content above about 6%.

15 Calcium sulphate (typically gypsum (CaSO 4 .2H 2 0), but potentially bassanite (CaSO 4

.%YH

2 0) and/or anhydrite (CaSO 4 )) is added to activated slag cement during milling. This has a modest impact on strength development. The effect of calcium sulphate at 0, 5 and 10 percent levels is shown in Figure 4. 5 Concrete results: Testing these cements in concrete (as opposed to mortars, presented previously) is important, and demonstrates some additional attractive properties of these binders. The strength of activated slag concretes is comparable to Portland cement concrete, and varies according to the Portland cement content, as shown in Figure 5. Concrete 10 made with similar replacement of cement by slag but without activator shows much poorer early age strength performance. Drying shrinkage is an important parameter for concrete performance in the field. When tested according to Australian standard AS1012.13, the drying shrinkage of activated slag concrete is significantly lower than that of low-shrinkage (i.e. "shrinkage limited" or 15 "Type SL") cement, as shown in Figure 6. This is an attractive property. Other indicators of durability show that activated slag cement according to the invention performs very well. Some durability indicators for 32 MPa concrete produced from Type SL cement and from a sodium sulphate activated slag/Portland cement mixture containing 70% slag 20 and 30% Portland cement are presented in Table 3.

16 Table 3 Cement Type SL 70% slag + 5% sodium sulphate

RCPT

1 Coulombs 5600 670 RTAsorptivity mm 17.0 11.3 RCPT = Rapid chloride permeability test, ASTM C1202 The inclusion of sodium sulphate in a Portland cement mixture containing 70% slag with 30% Portland cement shows a clear difference on the expansion characteristics over 5 time when exposed to a sulphate solution, as shown in Figure 7. The lower expansion characteristic, or "microstrain", is also an attractive property. Manufacture: The activated slag cement according to invention may be produced according to current cement-making practice i.e. add slag, clinker, gypsum and activator to mills, e.g. ball 10 mills, and mill to a produce a cement. Alternatively the ingredients may be milled individually or in any combination, and then blended in the correct proportions to achieve an activated slag cement. The activated slag cement or individual ingredients are milled to a fineness index in the range of 350 to 550 m 2 /kg, preferably 400 to 500 and most preferable around 450 m 2 /kg 15 fineness index, measured using a Blaine air permeability apparatus. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A cement composition comprising: 1.5 - 6 wt% activator consisting of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K) 5 0 - 10 wt% gypsum; greater than 20wt% and less than 65wt% portland cement or clinker;

40-80 wt% granulated blast furnace slag optionally in combination with steel slag wherein the ratio of slag to cement or clinker is in the range of 1:2 to 4:1; wherein the activator and cement composition does not include sodium hydroxide 10 or sodium carbonate. 2. The cement composition of claim 1 wherein the content of granulated blast furnace slag optionally in combination with steel slag is present slag is 50-80 wt%. 3. The cement composition of claim 1 wherein the content of granulated blast furnace slag optionally in combination with steel slag is present slag is 40 - 70 wt%. 15 4. The cement composition of one of claims 1-3 wherein a proportion of the blast furnace slag is substituted with steel slag up to a total of 30wt% of the total cement composition. 5. The cement composition of claim 1 wherein steel slag constitutes 0-20 wt% of the total cement composition. 18 6. The cement composition of any one of claims 1-5 wherein the CaO/SiO 2 wt ratio of the slag is less than 1.3. 7. The cement composition of claim 5 wherein the CaO/SiO 2 wt ratio of the slag is in the range of 1.0-1.3. 5 8. The cement composition of claim 1 wherein the portland cement addition is greater than 25 wt%. 9. The cement composition of claim 1 or 8 wherein the upper limit of portland cement addition is 55 wt%. 10. The cement composition of claim 1 wherein the portland cement addition is 30 10 50wt%. 11. The cement composition of claim 10 wherein the ratio of slag to cement or clinker is 1:1 to 4:1. 12. The cement composition of any one of claims 1 to 12 wherein the cement composition has a fineness index of 350 to 550 m 2 /kg. 15 13. The cement composition of any one of claim claims 1 to 13 wherein the cement has Bogue values of C35, 28 to 75%, C25, 0.5 to 44.2%, C3A 2.5 to 12%; and C4F, 8.5 to 19.3% 14. An unreacted dry cement composition comprising: 1.5 - 6 wt% activator consisting of a sulphate of an alkali metal selected from the 20 group of sodium (Na) and potassium (K); 0 - 10 wt% gypsum; 19 greater than 20wt% and less than 65wt% portland cement; and 40-80 wt% granulated blast furnace slag optionally in combination with steel slag and portland cement or clinker wherein the ratio of slag to cement or clinker is in the range of 1:2 to 4:1, 5 wherein the activator and cement composition does not include sodium hydroxide or sodium carbonate. 15. The cement composition of claim 14 wherein the content of granulated blast furnace slag optionally in combination with steel slag is present slag is 50-80 wt%. 16. The cement composition of claim 14 wherein the content of granulated blast 10 furnace slag optionally in combination with steel slag is present slag is 40-70 wt%. 17. The cement composition of claims 14 or 16 wherein a proportion of the blast furnace slag is substituted with steel slag up to a total of 30wt% of the total cement composition. 18. The cement composition of claim 14 or 15 wherein steel slag constitutes greater 15 than zero to 20 wt% of the total cement composition. 19. The cement composition of any one of claims 14 to 18 wherein the CaO/SiO 2 wt ratio of the slag is less than 1.3. 20. The cement composition of claim 18 wherein the CaO/SiO 2 wt ratio of the slag is in the range of 1.0-1.3. 20 21. The cement composition of claim 19 wherein the portland cement addition is greater than 25 wt%. 20 22. The cement composition of claim 14 or 18 wherein the upper limit of portland cement addition is 55 wt%. 23. The cement composition of claim 15 wherein the portland cement addition is 30 50wt%. 5 24. The cement composition of claim 23 wherein the ratio of slag to cement or clinker is 1:1 to 4:1. 25. The cement composition of claim 14 or 22 wherein the activator is present in an amount of 2-6wt%. 26. The cement composition of claim 14 wherein the amount of activator is 3-5 wt%. 10 27. The cement composition of claim 14 wherein the activator is sodium sulphate. 28. The cement composition of claim 14 wherein gypsum is present in an amount of 1-6 wt%. 29. The cement composition of claim 14 wherein gypsum is present in an amount greater than zero to Swt%. 15 30. The cement composition of claim 14 wherein gypsum is present in an amount of 2-5wt%. 31. A concrete mixture comprising: a cement binder comprising, 21 40-80 wt% blast furnace slag optionally in combination with steel slag as the slag component and portland cement binder wherein the ratio of slag to cement is in the range of 1:2 to 4:1, 0 - 10 wt% gypsum; 5 greater than 25wt% and less than 65wt% Portland cement; an activator present in the amount of 1.5-6wt% of the weight of the cement and slag binder, the activator consisting of a sulphate of an alkali metal selected from the group of sodium (Na) and potassium (K), wherein the activator and cement binder do no include sodium hydroxide or sodium 10 carbonate; and aggregate, water and optional chemical admixtures. 32. The concrete mixture of claim 31 wherein the level of aggregate is in the range of

65-85wt % of the total dry mixture. 33. The concrete mixture of claim 31 wherein the activator is present in an amount of 15 2-6wt%. 34. The concrete mixture of claim 31 wherein the amount of activator is 3-5 wt%. 35. The concrete mixture of claim 31 wherein the activator is sodium sulphate. 36. The concrete mixture of claim 31 wherein gypsum is present in an amount of 1 6wt%. 20 37. The concrete mixture of claim 31 wherein gypsum is present in an amount greater than zero to 5wt%. 22 38. The concrete mixture of claim 31 wherein gypsum is present in an amount of 2 5wt%. 39 The concrete mixture of any one of claims 31 to 38 wherein the CaO/SiO 2 wt ratio of the slag is less than 1.3. 5 40. The concrete mixture of any one of claims 31 to 38 wherein the CaO/SiO 2 wt ratio of the slag is in the range of 1.0-1.3. 41. The concrete mixture of claim 31 wherein the portland cement addition is 30 50wt%. 42 The concrete mixture of claim 41 wherein the ratio of slag to cement or clinker is 10 1:1 to 4:1.