Cement, Energy and Environment

heat of hydration, resulting in greater durability of concrete. For this purpose, cement exhibiting a slower rate of hydration and developing essentially calcium silicate hydrates and calcium aluminate hydrates in cement paste is able to create the desired microstructure, which cannot be created by OPC due to generation of high amounts of leachable calcium hydroxide. The present introduction and use of high- strength blended cements may contravene the above concept in a serious manner. Another important feature of making durable concrete with blended cements is the availability of proper guides to the consumers and masons. The problem will be more pronounced, when PPC and PSC will have different grades. The European countries had encountered similar problems with three strength grades in their standard specification. In order to overcome the difficulty, the French agency (ATILH) had worked out a guideline, an extract of which is given in Table 5. According to Table 5, the cements of 32.5 class should not be used for concretes with characteristic strengths higher than 35 MPa; the cements belonging to 42.5 class should not be used in concretes with characteristic strengths lower than 35 MPa, and the cements of 52.5 class is for concretes with characteristic strengths of 35 and above. The indicated quantities of cement contents in the table, corresponding to different grades of concrete, are to ensure minimum cementitious materials content in them, which is important for the durability of concrete in general. Quite contrary to this rational approach, there are hardly any guides available for different types and grades of cements in India. There are instances galore in our country for misuse of cement types and grades. The OPC 53 grade is reported to have been used for plastering applications. Now that there could be strength graded blended cements in the market, the cement consumers are likely to be confronted with choices whether a 43-grade PPC with 30% fly ash would be preferable to 53-grade PPC with 15 % fly ash or which of the types such as 53-grade OPC 0r 53-grade PPC or 53- grade PSC would be the right selection for a given application. Perhaps, it is high time that the industry and cement consumers at large wake up to formulate transparent and data-based application guides for different types and grades of cement. An important consideration that often comes in the way of promoting blended cements is their relatively higher susceptibility to 'carbonation'. It has generally been observed that the higher w/cm ratios, the higher is the extent of carbonation depth for all types of cements but the blended cements exhibit more carbonation than OPC. However, when the long-term carbonation is determined for even mortars having 28-day cube strength of 40 MPa or more, the carbonation of blended cements is almost at par with OPC mortars. In other words, for moderate-strength concretes made with PSC or PPC, the apprehension of extensive carbonation is not substantiated. Resistance to chloride diffusion is an important durability requirement for concrete and both PSC and PPC are known to strongly Table 5. Use of blended cements of different strength classes in normal concretes Strength class (EN:197) 28-day mortar strength, MPa Recommended use (Y/N) and cement content (kg/m 3 ) in different grades of concrete 20 MPa 25 MPa 30 MPa 35 MPa 40 MPa 32,5 45 Y(320) Y(360) Y(400) Y(440) N 42.5 55 N N Y(350) Y(380) Y(420) 52.5 65 N N N Y(340) Y(370) 24