Cement Energy and Environment

like fly ash, ground granulated slag and si lica fume are also permitted , although thorough and uniform blending with the cement in the concrete mixer has to be ensured. Whether the mineral admixtures are blended with cement clinkers in the cement plants, as in PPC or PSC, or mixed with cement and other ingredients in the concrete mixer, the performance is the same. Use of Silica fume was considered possible with only OPC. However use with blended cements like PPC and PSC, or in ternary blends of OPC, silica fume and fly ash or slag, comprise efficient cementitious binders. The mechanisms of improved performance are the following: • Synergic effect, • Improving aggregate matrix interfacial zone • Reduction in porosity of the composite. Investigations by Kumar and Kaushik have clearly shown the improvements in the transition zones at aggregate-matrix interface due to incorporation of silica fume and fly ash in OPC concrete. One such micrograph is reproduced in Figure 2. Ternary blends of cement, silica fume and mineral admixtures are now being adopted for high performance concrete in many projects. One of the early applications in India was for M60 concrete for pile caps in Bandra - Worli Sea link project. Such innovative approach to materials selection is a positive contribution to quality Ternary blends enhance durability, workability and/or strength. Mix proportions for M60 grade self-compacting concrete (SCC) in a recent bridge construction 1s depicted in Figure 3 above. Figure 2. Densified transition zone in OPC + r-Iy ash l MS n11x (Kumar and Kaushil~) Ft e4g;~ 777kg Waia- 158kg Qu<Mltnies per m3of co ncrete CcmCJ11470kg FA 100Y.g SF33 ~Q C Aoc 3J ITIT' 842 kq I)Ccmmt . Fly Ash OSF OC.A:Jg I . FineAgj 1 j CWcta- i ,__ __ ---- ------ -- Figure 3 Mtx Design forM 60 Self Compacting Concrete. 28 days comp strength - 74 4 MPa Admixtures Chemical admixtures are being increasingly used. Some specifications tend to limit the amount of superplasticisers, which is uncalled for. Deviations of setting time and air entrainment envisaged in IS:91 03 provide sufficient safeguard against overdose. Assessment of performance and determination of optimum dosage of the superplasticisers are important quality tnputs. Compatibility with the cement systems is an important consideration. At higher dosages of superplasticisers in concrete mixes of low w/c ratios, unexpected behavior is sometimes experienced. Such phenomena are usually referred to as 'cement - superplasticisers incompatibility'. This may happen even when individually the cement, superplastciser, and other cementiti ous materials like fly ash, slag or silica fumes, satisfy the requirements of the respective Standard Specifications. It is heartening that this subject has been discussed in this forum last evening. Reinforcement Steel for reinforcement and stress relieved wires and strands for Prestressing are available in 7