The gradual shift of the Indian construction industry is from an OPC dominated usage to blended cement/blended concrete dominated scenario, says SA Khadilkar.

Over the last decade, the cement and concrete industry has been gradually shifting towards high performance concrete durable and sustainable construction.

Construction practices in the country have accepted the blended cements or blended concrete as means of achieving durable concrete structures, which are more resistant to environmental deterioration.

There are many premium brands of different cement groups in the country. These include fly ash/slag-based cements competing the early strength and faster setting USPs of OPC 43/53, which are gradually replacing the OPCs especially in the IHB segment. Some of these high strength low water demand blended cement products are also making its growth in the RMC segment.

In 2015, the Bureau of Indian Standards (BIS) formulated a new standard for composite cement (fly ash slag based), IS 16415:2015, permitting simultaneous use of fly ash and granulated blast furnace slag as mineral additives for manufacture. The composite cements (PCC) permits Portland cement clinker/ordinary Portland Cement (35-65 per cent), fly ash (15-35 per cent) and granulated slag (20-50 per cent), this opens a new avenue for improved performance of concrete and meeting the objectives of sustainable construction.

PCC would produce a cement/concrete product with the combined beneficial effects of granulated slag and fly ash resisting more effectively, environmental deterioration of concrete.

Presently, substantial experimentation data is available on composite cements with different combinations of fly ash and slag, along with its comparative performance with PPC (fly ash based) and PSC (granulated slag based) in terms of the cement mortar properties, fresh and hardened concrete properties and concrete performance in terms of resistance to aggressive environments and environmental deterioration.

The gradual shift of the Indian construction industry is from an OPC dominated usage to blended cement/blended concrete dominated scenario. One of the contributing factors being that it has been very well observed that the concrete structures made with blended cement concrete have demonstrated durability and have resisted deterioration by environmental factors and have been observed to be less prone to deterioration. The fly ash-based cements (PPC) and the granulated slag based cements (PSC) have been able to establish their superiority over OPC in terms of durability and resistance to environmental deterioration of the resultant structures.

The properties of these mineral components, i.e., fly ash and slag, assist the concretes to resist the environmental deterioration. The effect of these mineral components on fresh and hardened concrete properties and their influence on properties determining durability of concrete could be summarised as per the given table.

In fly ash – slag composite cements, the two mineral components, fly ash and granulated slag would compliment each other, making the composite cements to be a much more durable option for concrete.

Comparative studies with OPC, PPC, PSC and PCC have indicated that composite cements show much improved resistance to environmental deterioration. Some of the important observations reported in literature from the comparative studies are briefly discussed.

In the hydration of composite cements, the mineral components exhibit higher extent of reactions with the hydrated lime (calcium hydroxide), liberated from the hydration of the OPC component.

These increased reactions of the mineral components would result in formation of higher extent of hydrates in hydrated composite cement paste, which would result in a more compacted concretes with reduced connected porosity/reduced permeability. The compacted concrete microstructure have been assessed by morphological studies with SEM.

The neat cement hydration studies on these cements at different age of hydration, indicate that composite cement hydrated pastes have lower free hydrated lime up to 28 days and onwards. It also indicates higher ettringite formation up to three days and higher tobermerrite gel formation indicated by the higher extent of reaction of mineral components in case of composite cement pastes. This, of course, would be function of the clinker factor of the composite cement (i.e the total % of fly ash and slag and their ratio), PPC and PSC.

The hydrated cement paste micro-structure of the different cements at 28 days of hydration is illustrated.

In OPC hydration, as the hydration proceeds, the growth of CH crystals (portlandite) progresses. (Fig: 4a) Where as, in PPC at 28 days, the paste microstructure becomes relatively more compact and denser as compared to the OPC of same age (Fig. 4b) and in PSC as the hydration proceeds a highly dense and compact paste is formed (Fig. 4c) due to the high degree of networking of fibrous and thin plate like CSH intermixed with Aft, Afm and CH. Whereas in composite cements, much more denser microstructure is observed with higher formation of CSH (Fig. 4d).

The improvement in compacted microstructure is observed to be in the order PCC >PSC>PPC>OPC. This is substantiated by the performance of M20 concretes (28 days) made from these cements in water permeability test (DIN – 1048 Part V),rapid chloride penetration test (ASTM C 1202 – 07).

The permeability of concrete is related to durability of concrete in terms of its resistance against progressive deterioration under exposure to severe climatic conditions and leaching due to prolonged seepage of water. The permeability is measured in terms of depth of penetration of hardened concrete cube after 28 days of curing at certain water pressure by Water permeability test (DIN – 1048 Part V. The test results are graphically depicted in Fig. 5 a. The chloride permeability of the M20 Concrete at 28 days as per rapid chloride penetration test (ASTM C 1202 – 07), show decrease in chloride permeability in the order PCC

The performance of OPC, PPC, PSC and PCC M20 concretes would definitely be a function of the clinker factor of these cements, i.e., % fly ash and % slag contents. The durable aspects of concretes observed in the sulphate resistance tests of the resultant concrete are in the order PCC>PSC>PPC>OPC with PCC showing the highest sulphate resistant property.

Conclusion
1.The results discussed without any doubts illustrate that the composite cements with fly ash and slag together outperform OPC and have an edge over PPC and PSC in terms of fresh and hardened concrete properties as well as properties that relate to resistance to environmental deterioration such as to water permeability, chloride permeability (RCPT), sulphate resistance, etc.

2.The properties of fly ash and slag would complement each other in the composite cements (PCC), thus making the PCC "A Performing Cement" in concrete and would enhance the longevity of the concrete structures.

3.In a tropical country like India having abundant cement replacement materials viz fly ash and slag, a composite cement would gradually establish itself as a preferred durable cement.

4.Manufacturing of PCC, by many cement groups in India, could also result in an increased production capacity by at least 15 to 20 per cent, without much of capital investments, with substantial reduction in CO2/tonne of cement produced.

5.The composite cements are expected to perform much better in aggressive environments (having higher sulphate and chloride concentrations) i.e in both agricultural land mass as well as the coastal climatic condition of the cement markets.

6.The fly ashes available in country are low lime class ? F, with relatively lower contents of amorphous alumino-silicate glassy phases. Use of comparatively reactive calcium alumino-silicate glass contributed by the granulated slag would enhance the hydraulic potential of the combined blending components in composite cements.

7.On the manufacturing side of composite cement, some of the important aspects are:

The fly ash component of the composite cement should preferably be fly ash from pulverised fuel fired super thermal plants produced with higher overboard temperatures. These fly ash particles are high temp sintered particles and help reduce the water demand of the resultant composite cement, which on one hand contributes to achieving higher % fly ash in composite cement and also helps to improve the concrete performance of the PCC.

Use of low temperature / CPP Fly ash from AFBC/CFBC boilers tend to increase the water demand and necessitates use of cement additives for reducing water demand of the resultant PCC.

It has been observed that it is feasible to achieve lowest possible clinker factor in PCC with fly ash being around 30 per cent and the granulated slag being around 25 per cent, as indicated it is more a function of fly ash characteristics and the type of cement grinding circuit available at the manufacturing site.

If PCC is manufactured in the inter-grinding mode the OPC and the fly ash component should not get over-ground, very fine grinding of Clinker and fly ash may result in increased water demand. Optimisation of the distribution of the mineral components in size fractions of the PCC is important from the angles of concrete performance of the resultant PCC.

In separate grinding and blending mode the component to be ground separately needs to be judiciously selected so as to achieve a PCC of improved concrete performance.

The OPC component continues to be the activator for especially the granulated slag component of PPC. The fly ash characteristics and its distribution below, say 20 microns but not very high fractions below 3 -5 microns, would help regulate on one hand the pozzolanic reactivity of the fly ash and on the other determine the water demand of the resultant PCC.

Manufacturing PCC seems to be more commercially attractive for grinding units located where good fly ash sources are in the vicinity and there is adequate availability of granulated slag. It would be more commercially attractive to manufacture PCC than PSC, the product portfolio could gradually change to higher volumes of PCC then PPC and if required PSC.

PCC manufactured should have market acceptable quality with improved performance characteristics in concrete.

Popularisation of composite cement would help the country to achieve the objectives of sustainability in years to come.

ABOUT THE AUTHOR: SA Khadilkar is a consultant and advisor. He is a former director of Q&PD ACC Ltd. He can be contacted at: khadilkarshreesh@gmail.com.