Cement, Energy and Environment

7 From the table it is evident that with 50% slag and 50% cement in about 300 kg/m 3 of total cementitious material the strength properties of concrete were much higher than the control concrete without slag, when the w/c-m ratios and super- plasticizer dosage were comparable.The 28-day strength of concrete with 65% slag and 35% cement in total cementitious material of 428kg/m 3 exceeded all other results, when the w/c-m ratio was 0.27, although the dose of super-plasticizer had to be increased. The low w/c-m ratio is the key to the achievement of high strength with less cement in concrete and it requires judicious use of SCMs and super-plasticizers. BENEFITS OF DURABLE STRUCTURES Construction is broadly divided into two categories: new build and refurbishment. This is easily appreciated from the life-cycle concept of a building as depicted in Fig.11. During the time span extending from the completion of construction to achieving the design life of the structure all concrete buildings are exposed to the rigours of environment and they undergo certain extent of fatigue due to various modes of use, which result in degradation of structure. The degradation is rectified by planned maintenance in phases. For every built structure there is a design performance level and a minimum performance level. The maintenance activity aims at restoring the design performance level, which may also increase with increased expectations with time. However, despite maintenance, there is a point in the life-cycle of a building, when the performance touches or even goes below the minimum performance level, which sets the time for deciding if the structure should undergo major refurbishment or it should be demolished and rebuilt. Refurbishment and new build decisions are seen along with the energy used in the built environment (Fig.12). It is obvious that the refurbishment activity is much less energy consuming than new build measure. It is therefore highly desirable that the built structures should achieve the designed life in order to save energy and to protect the environment. Fig.11. Schematic representation of the life-cycle of a building Fig. 12. Energy used in the built environment It is interesting to note that this preference for refurbishment over new build is indirectly reflected in the recent trend of consumption of cement in some of the European countries, particularly Spain and Italy. The drastic fall in the reduction of cement use is primarily due to high emphasis on refurbishment than in the new construction. It is also pertinent to note that the new construction demands larger use of reinforced concrete, for which there is always an apprehension of carbonation and concrete deterioration. Apart from the special measures taken to protect the reinforcing steel, there is a preference for cement that will be resistant to the natural process of carbonation. In this context, often the blended cements lose out in the market. One does not know for sure how much of reinforced concrete is used globally. Some surveys indicate that it is only 25% of the total concrete made. If it is so, environmentally it would be more prudent to market cement meant for reinforced concrete relatively smaller application demand separately from the common cement with high SCMs for larger market