STUDY ON HIGH EARLY AGE STRENGTHS OF POZZOLANIC CEMENTITIOUS COMPOSITES

Abstract

Cement is used for all types of constructions, be it a small residential building or a multistoreyed skyscraper, a dam, a highway, a bridge etc. Use of cement has been indispensable for development and its production has increased many-fold in last few decades. The total volume of global production of cement was 1.39 billion tonnes in 1995, which has registered 200% growth in less than three decades and has reached to 4.1 billion tonnes in 2022. This trend is likely to continue due to high demand of cement for infrastructural projects throughout the world. India is the second highest producer of cement in the world. Its overall cement production capacity for 2020-21 was placed at 537 million tonnes. Experts predict that cement demand in India will increase up to 800 million tonnes by 2030. It is also predicted that in case of high demand, it may reach up to 1.4 billion tonnes by 2050. Thus, it is quite evident that for infrastructural development, our dependence on cement is not going to diminish in near future. Unfortunately cement is not a green material, its manufacturing involves blending of various raw materials in required proportion and burning them at a high temperature. Limestone is used for calcium requirement, while clay, mudstone or shale as the source of silica and alumina. Starting from querying of raw materials then crushing, blending, burning, grinding of clinkers and finally packing of cement, the entire cycle of cement production is highly energy intensive and requires 2.8 GJ of energy for producing one tonne of cement. Production of one tonne cement consumes about 1.5 tonne of calcium carbonate (i.e. limestone), which is around 80-90% of total raw material, while remaining 10-15% is clayey raw material. On the other hand, cement manufacture accounts for 7% of world energy sector emissions, including process emissions. 40% of CO2 emissions from a cement plant are generated by combustion, while 60 percent emissions are due to calcification. The burning of fuels for maintaining high temperature, required for fusion is responsible for combustion-generated CO2 emissions. Decomposition of raw materials such as limestone at about 900°C liberates CO2 during their calcification. In recent decades, carbon dioxide emissions from cement manufacturing in India have seen a significant increase. In 2021, the recorded figures peaked at 149 million metric tons. It also releases huge amounts of air pollutants like PM (particulate matter), NOX, CO, SO2 and some volatile organic compounds. Another important concern of the construction industry in particular and entire globe as a whole is durability of concrete. Durability refers to the capacity to render its service over an extended period without notable deterioration. A durable material contributes to environmental conservation by preserving resources, minimizing waste and lessening the environmental consequences associated with repairs and replacements. In the context of concrete, durability is characterized by its resistance to weathering, chemical attacks and abrasion etc., all while retaining its intended engineering properties. Recognizing the significance of ensuring the durability of concrete structures, the Indian Standard (IS: 456-2000) has included a complete clause covering different aspects of durability of concrete. As we know, concrete is a composite material formed by combining fine and coarse aggregates, bonded with cement paste, which gain strength over time. It holds the distinction of being the second-most-utilized substance globally, following water and stands as the most extensively employed building material. It is so widely used because of its strength, durability, sustainability and versatility. Concrete structures have to withstand normal to the most severe exposure conditions. Sometimes they are so located that their repair is very difficult. Hence, their durability is very important. Additional concerns that have contributed to above mentioned pressures of environmental pollution and resource depletion on cement industry, are associated with rise in the occurrences of concrete structures, facing significant durability challenges. An effective solution to the above mentioned problem due to ever increasing use of Ordinary Portland Cement (OPC) lies in partial replacement of cement by pozzolans. Pozzolans encompass a broad range of siliceous and aluminous materials that may or may not have cementations properties. Yet, finely divided and in the presence of water, these materials undergo a chemical reaction with calcium hydroxide [Ca(OH)2] at ambient temperatures, yielding compounds that exhibit cementitious characteristics. Fortunately, a number of pozzolans are available in nature and also as industrial byproducts such as burnt clay, volcanic ash, fly ash, metakaolin etc. Thus, in order to reduce the harmful effects of OPC production, Portland Pozzolana Cement (PPC) may be used as an alternative. As per Indian Standard codes, the overall pozzolana content in Portland Pozzolana Cement (PPC) is carefully regulated, so that it remains between 10 percent and 35 percent by mass of the Portland Pozzolana Cement. The Indian Standard Codes [IS: 269 - 2015 and IS: 1489 (Part 1 and Part 2) 2015] have prescribed compressive strengths of PPCs that are significantly lesser than the OPC strengths. This difference in the strengths of OPC and PPC is definitely a hindrance in the use of pozzolanic cements as a replacement to OPC. During hydration of OPC, alite i.e. tricalcium silicate (C3S) and belite i.e. dicalcium silicate (C2S) are the two co mpounds, which contr ibute ma inly to the strength o f hydrated cement products. They react with water and transform into calcium silicate hydrate (C3S2H3) and hydrated lime [Ca(OH)2]. C2S hydrates slowly and influences gain in later age strength. The released lime from hydration of C3S and C2S reacts with pozzolanic material, if used, mostly of siliceous nature like fly ash, silica fume, ground granulated blast furnace slag etc., which makes additional calcium silicate hydrate at later stage and improves later age strength. But, addition of pozzolanic material having high alumina content may give advantage of increased initial strength and enhanced durability by consuming the free calcium hydroxide made available by the hydration of C3S in the beginning Incorporation of pozzolanic admixtures of siliceous nature results in slow gain of strength and supports later age strength gain of the cementitious composites whereas, aluminous pozzolan support early gain of strength. Hence, judicious blending of aluminous pozzolans and siliceous pozzolan has proved an effective solution in present study to enhance the compressive strength of PPC equivalent to OPC. The present research program involves the examination of effects of various blends of OPC and pozzolans on development of early age strengths of concrete and also study on their durability, so that suitable blend(s) of pozzolanic materials could be recommended to enhance the early age strengths of pozzolanic cementitious composites at par with OPC products of durable nature. In order to suggest various blends of different pozzolans of optimum proportions, for high early age strength development of acceptable durability, at par with OPC products, detailed experimental study has been carried out on 11 blends of OPC with different pozzolans with regard to development of 7 days and 28 days compressive strength, split tensile strength and flexural strength. Further, the durability aspect on these blended products and OPC product was also conducted under accelerated environmental effect by subjecting the specimens to alternate wetting and drying cycles in 10% solution of sodium sulphate. Residual strengths and weight loss of these products were measured after 150, 300 and 500 cycles. The study suggests that certain blends of OPC and pozzolans give comparable performance to pure OPC. This study has been attempted to provide alternatives to pure OPC composites by using blends of siliceous and aluminous pozzolans with OPC. These pozzolans are abundantly available either as industrial waste or natural materials. Use of industrial wastes reduces their negative environmental impact. The availability of these materials at low cost proved to be an economic alternative to the highly processed material like OPC. Findings of the research are a step towards solution of the problems due to ever increasing consumption of cement, which is a significant threat to environment because of high consumption of natural resources and energy along with generation of enormous amount of pollution.