Secondary cementitious materials in cement and concrete

Introduction

In order to achieve sustainable industrial development in the future, the rate at which secondary cementitious materials (SCMs) are utilised in concrete and cement must be accelerated. In order to achieve this, it is imperative to identify and then remove the barriers that stand in the way of an increase in their use. It is the following points that need to be investigated:

Variable chemical composition

The chemical composition of flyash or slag is determined by the raw materials used and the conditions under which they are processed. These factors can vary, not only from one plant to another, but also within the same plant. It is natural to expect large variations in chemical composition between different batches of flyash and slag. It is, however, generally accepted that the pozzolanic and cementitious properties of these materials are governed less by the chemistry and more by mineralogy and particle size.¹

Industrial flyash and slag happens to differ widely in both mineralogy and particle size, and this has a great effect on their reactivity. A given application might call for a relatively reactive material, whereas another may not require such high reactivity. Thus, instead of rejecting materials inappropriate to one application, an innovative system of matchmaking could be implemented in order to find a suitable use within the cement and construction industry for every type of fly ash and slag produced.²

On some occasions, the reactivity of a fly ash or slag may need to be altered. Fine grinding and thermal curing are two widely used methods of accelerating both pozzolanic and cementitious reactions. On the other hand, there may be occasions when it is necessary to retard the reactivity of a more highly reactive material before it is put to use. This can be achieved by partial prehydration. For instance, in the construction of a roller-compacted concrete dam in Greece, a very reactive, high-calcium flyash with 42% total CaO (15% free CaO, some C3A and calcium sulphate) is being used after grinding and prehydration.³

Ultimately, variability between sources of supply need not be an obstacle in the way of increasing the use of flyash and slag in concrete. One real bottleneck, though, is the lack of customer confidence in the uniformity of quality within a single source.

For years, the cement and concrete industries have practiced blending inhomogeneous batches of materials to obtain end products with acceptable uniform quality. With just a little cooperation, the producers and potential customers of these by products can work together to overcome the issue.

Specifications and codes

In most countries, the standards for Portland cements, slag, pozzolans and blended cements are ‘prescriptive’ in the sense that they tend to specify limits on certain chemical constituents. With blended Portland cements specifically, these prescriptive standards tend to specify maximum permissible amounts of a blending material. This forms a serious obstacle to large scale, innovative users of secondary cementitious materials.

From the study of existing world standards on pozzolanic and cementitious materials, it can be concluded that separate standards covering them are not necessary. Indeed, many requirements in these standards are obsolete and can therefore be deleted. However, for the purposes of assurance, a few parameters can be held onto.

In the existing Indian standard specification for PPC the permissible limit maximum is 35% by mass. Research has shown, however, that this standard makes no sense in light of sheer level of variability between batches of SCM.³ If performance-based codes were adopted in place of this ‘prescriptive’ measure, the levels of flyash and slag utilisation in cement and concrete will be increased.

Impact on eventual concrete properties

The water-reducing properties of fly ash and ground granulated blast furnace slag are well established: it has been observed that their use has a positive influence on the rheological properties of fresh concrete, specifically on its pumpability. Two key properties that should be looked into with regard to hardened concrete though, are its strength and durability.

For example, it is wrong to assume that the durability of reinforced concrete containing flyash or slag would be less than plain Portland cement concrete when exposed to sea water.⁴ A lot of examples drawn from both the theoretical sphere and from field experience reveal that SCMs of a good quality, when used in the correct proportions and followed by proper concrete curing, can actually have a positive effect on concrete properties. Studies conducted around the world have concluded that the use of SCMs can offer attributes such as reduced permeability and reduced expansive stresses. In combination, these attributes minimise the tendency towards cracking. It has also been proven that proper usage of SCMs will protect concrete structures from alkali aggregate reactions and sulphate attack.

Many problems with the premature deterioration of concrete structures arise from accelerated construction speeds, increased water content in concrete mixtures and production of PPC with high early strengths to enable lower cement content in concrete.⁴

Conclusion

Based on the above, the following conclusions can be drawn:

• The abolishment of ‘prescriptive’ codes and replacing them with ‘performance-based codes’ will enable an increase in the rate of SCM utilisation in cement and concrete.
• Cementitious properties of SCMs are governed by the mineralogy and particle size than chemistry.
• Reactivity of fly ash and slag may have to be altered by fine grinding and thermal curing for accelerating pozzolanic and cementitious reactions.
• Matching batches of flyash and slag to applications suited to their properties will enable a greater uptake of the materials and result in less waste material going unused.
• Communication between producers (of SCMs) and potential customers would ensure a greater deal of confidence in the materials.
• Rather than simply lowering the cement content of concrete, using more SCMs and proper curing cycles could help to avoid premature deterioration of concrete structures.

References

1. MALHOTRA, V.M. and MEHTA, P.K., Pozzolanic and cementitious materials, Gordon and Breach Publishers, Philadelphia, (1996), pp.191.
2. BHANUMATI DAS, N. AND KALIDAS, N., ‘Imperatives on mass scale flyash utilisation in India’. Cement Industry, (1995), pp.133 – 134.
3. MALHOTRA, V.M., ‘CANMET Investigation dealing with high volume fly ash concrete’. Advances in concrete technology, Natural resources, (1994) pp.445 – 482.  
4. MEHTA, P.K., Bringing the concrete industry into a new era of sustainable development, Cement Manufacturers Association, New Delhi, (1998), pp.49 – 67. 5. PRAVEEN KUMAR and S.K.KAUSHIK, ‘Some trends in the use of concrete: Indian scenario’. The Indian Concrete Journal, (2003), pp.1503 – 2003.

Written by Dr S.B. Hegde.
Edited by Jack Davidson.

Read the article online at: https://www.worldcement.com/asia-pacific-rim/12112012/reliance_cement_secondary_cementitious_materials_india_736/