RepRint of papeR published in published in inteRnational Cement Review oCtobeR 2011

Cement additivesmodeRn Cement additive ConCepts

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OCTOBER 2011 INTERNATIONAL CEMENTE REVIEW

The production of cement affects the ecological balance in several ways. Firstly, natural

resources are used as raw materials and after that, every tonne of clinker emits approximately 830kg of CO2 due to the chemical reaction and the required thermal energy. Furthermore, a significant amount of electrical energy is consumed during the whole process. This environmental impact is one reason why the cement industry seeks to reduce the clinker ratio during the manufacturing process.

Moreover, the purpose of a production unit in general is to be profitable. Hence, the industry is constantly searching for solutions to replace traditional inputs with economic alternatives (which is applicable to both fuel and clinker). Furthermore, a reduced clinker ratio increases capacity at existing plants and, therefore, is able to provide more cement to the market.

Influences on cement strengthWhile the reduction of the clinker ratio and increases in clinker substitution support the above factors, the strength of cement

will increasingly reduce the more clinker replacements that are introduced (see Figure 1). Slag offers the highest clinker replacement potential due to its relatively small adverse effect on strength. However, availability of and transport costs need to be considered. From this perspective, limestone is an interesting alternative if the plant’s quarry offers the desired limestone quality. On the other hand, limestone usually leads to the highest loss of early and final cement strength.

Beside the type and amount of clinker replacements available, there are other factors which influence cement quality. One which is directly related to

cement composition as well as to the resulting production rate is cement fineness. The clinker itself, with its specific composition and microstructure, is among the most important influences on the product’s properties. The grinding system has an influence on the particle size distribution as well as on the grinding temperature and, with this, influences strength development. Finally, cement additives – such as grinding aids and quality improvers – directly or indirectly affect cement strength.

Case study: chemically-accelerated strength increaseAdditives, which chemically accelerate cement hydration and are often called quality improvers, have been applied for decades to increase strength and enable producers to reduce the amount of clinker in blended cements. Figure 2 shows results from the production of a CEM III/A 42.5 N with a low clinker factor of 0.3. Two different types of strength enhancers had been tested at their usual dosage. The gained strength increase of 2-2.7MPa (two days) and 2.4-4.7MPa (28 days) would allow for a clinker replacement in the range of 4-5 per cent.

The strength enhancements which can be achieved in general depend on the local conditions and are also related to the type, amount and fineness of the applied clinker replacements. As a rule of thumb, for specially-designed products, one-day strength can be chemically accelerated up to 5MPa, while after 28 days up to 7MPa more strength can be reached.

Physical effects of cement additivesWe have seen the effect of chemically-accelerating cement additives. More commonly known is the widespread use

IModern additive conceptsby Jorg M Schrabback, Sika Services AG, Switzerland/Germany

Cement manufacturers have to cope with the challenge of producing cement economically while taking care of sustainability issues such as reducing energy consumption and CO2 emissions. Reducing the clinker ratio using modern cement additive concepts is one way to help achieve these goals.

Cement manufacturers have to economically produce cement while

taking care of sustainability issues

Figure 1: early and final strength decrease in relation to the type and amount of clinker replacement

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of grinding aids which physically improve grinding and separating efficiency resulting in significant increases in production.

These are usually added in small doses to the mill feed or directly into the mill. The resulting benefit can be related to the dosage as well as to the chosen technology. The highest production rates can be achieved while using the newest grinding aid technology (see Figure 3).

Case study: ball millThe SikaGrind product, which was selected for a ball mill application by one particular customer, increased production by 20 per cent on an already high production rate of 160tph, while ensuring the targeted Specific Surface Area (SSA) fineness parameters and sieve residue as well as the strength demands.

Case study: VRMThe second case study from a Sika customer was for a vertical roller mill (VRM) application. The local SikaGrind

product resulted in a production increase of eight per cent at unchanged fineness parameters and strength development. Consequently, specific energy consumption dropped to 26.7kWh/t of cement. Even more important in this specific case was the target of reduced mill vibrations.

Physical strength enhancementWhen the highest strength results are needed to reduce the clinker content, this physical effect of an increased production rate with cement additives can be used to achieve the needed fineness.

Higher fineness leads to improved strength development, but also reduced production. At this stage, the potential production increase can be used to grind cement finer while maintaining a constant production rate. While the traditional grinding aids, for example, would allow for 200cm²/g higher specific surfaces according to Blaine, new Sika technology can achieve 300cm²/g more fineness at a constant production rate. Consequently this can be used to further reduce the clinker factor.

Case study: physically- and chemically-accelerated strength increaseAs an example, Sika tested a CEM II/A/S 42.5 R in plant application with the aim of increasing fineness to enhance strength. The particle size distribution slightly shifted towards the finer particles, which resulted in 250cm²/g higher specific surface according to Blaine and a slightly lower sieve residue of 32µm. Consequently, the strength values rose by, on average, approximately 4MPa at all ages (Figure 4).

Due to the grinding aid technology, the production rate increased slightly despite the dosage applied being 10 per cent lower and the fineness being higher (see Table 1).

Together, both actions lead to a maximised strength increase (Figure 5) and a minimised clinker ratio. Modern cement

Figure 2: strength enhancement with chemically active cement additives

Figure 3: production increase with physically active cement additives

Figure 4: strength increase using chemical as well as physical action of cement additives

INTERNATIONAL CEMENT REVIEW OCTOBER 2011

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additive concepts use the full potential to approach the customers’ targets.

SummaryCement manufacturers have to face the challenge of economically producing quality cement in parallel with considering sustainability.

The main issue in helping reduce the clinker ratio is to achieve increased strength development.

There are many factors which influence strength development including locally-available materials, cement formulation (including the chosen fineness) and the available grinding systems.

Chemically active

strength enhancers lead to improved strengths at all ages. Grinding aids increase production of grinding systems like ball mills and vertical roller mills and allow the increase of fineness at a constant production rate. Therefore, they can additionally be used to increase the strength.

A minimised clinker ratio can be achieved while using all available benefits of cement additives application. Thus reduced clinker ratios with modern cement additive concepts can add value to the finished cement.

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Table 1: production and cement values

0.061% 0.054% Comments Standard GA New GA/QI TechnologyBlaine (cm2/g) 5230 5480 HigherSieve residue 32µm (%) 2.1 1.4 LowerProduction rate (tph) 30.5 31.6 Slightly higherPowder flowability acc. Imse (%) 36 45 HigherWater demand (%) 34 34 SameSpread a0 (mm) 175 173 SameSet times (min) 145/205 125/180 Shorter

Air content (%) 2.7 2.9 Same

Figure 5: modern cement additive concepts

OCTOBER 2011 INTERNATIONAL CEMENTE REVIEW