Cement, Energy and Environment

exhibit this property, although the performance of PSC overtakes that of PPC in most circumstances. It is illustrated in Fig.6. Chloride is transported into concrete by the medium of water through capillary absorption and/or diffusion mechanisms. Hence w/cm ratios are as critical for this property as for carbonation as discussed above. For basic understanding, it can be stated that permeability of hardened cement paste generally decreases with the progress of hydration, and the degree of this reduction in permeability has no direct relation to the amount of gel or hydrates produced and is correlated with the sites where the gel formation occurs. Permeability decreases, when the gel is formed at such a site as to block properly the capillary pores. Calcium hydroxide, formed during the hydration of OPC, precipitates in pores but does not have the blocking effect. On the other hand, calcium silicate hydrates and calcium aluminate hydrates, formed as a result of pozzolanic reactions in blended cements hydration, tend to block capillary pores, thereby decreasing permeability of hardened cement paste. Concluding Remarks The Indian cement industry with the current production capacity of over 500 Mt per annum is poised to grow to about 600 Mt per annum by 2025. With such a large production base, the industry has its legitimate concerns about sustainability and GHG emissions. The industry has already established itself as a very responsible enterprise from the above perspectives. The clinker factor has been reduced to 0.71 and the blended cements production has increased to about 72% of the total production of 280 Mt in 2017. In producing the blended cements the entire quantity of GGBS, which is estimated at about 10-11 Mt, and around 26% of about 197 Mt of fly ash generated in the year, have been consumed. With the available quantity of GGBS and with increasing production of cement, the proportion of PSC in the total cement production in the country has stagnated below 8% during the last several years. The growth in the production of fly ash based PPC has been substantial as it touched 65% in 2017. Since the average substitution of clinker/OPC by GGBS is estimated to have reached a level of 50% and that by fly ash has touched the present permitted limit of 35%, there Fig.6. Illustrative comparison of chloride diffusion in concrete prepared with OPC, PPC and PSC 25