Reaction Mechanisms for the Formation of C-S-H Gel During the Early Hydration of Cement

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Reaction Mechanisms for the Formation of C-S-H Gel During the Early Hydration of CementAbstractWhen cement and water are mixed the resulting paste typically stays fluid for several hours before it sets and hardens. This period of time, the induction period, is technically extremely important because it allows enough time for the concrete to be mixed, transported, and poured into place. The induction period ends abruptly and is followed by a period of accelerating reaction that leads to setting and hardening. The versatility and widespread use of Portland cement concrete results, to a large extent, from the ability to control these hydration processes by the use of chemical admixtures, for example to either to extend the period of workability or shorten the time to set. Despite the fundamental importance of the concrete hardening process to the construction industry, and the fact that these chemical admixtures are of significant industrial importance, the fundamental chemical mechanisms underlying the cement hydration process are not well understood. This problem has been debated in the literature for more than 100 years without resolution. In recent years, modern analytical techniques have provided new information about the reaction products formed by the hydration of cement. These new findings led to the development of a quantitative nanometer-level model of cement paste by one of the present proposers, which is receiving increasing acceptance. Also, recently under NSF support new information about the structure and solubility of the main reaction product in cement has been obtained. This knowledge has provided new insight about the mechanisms that control the rate that cement paste and concrete hardens, and the promise of a much deeper understanding of the reactions between cement and water. It also opens the door not only to controlling the rate of hydration of cement paste, but also to controlling the microstructure and thus the properties of the final concrete. This will have a significant impact on the industry that supplies chemical admixtures for concrete, and on understanding the overall effects of adding a wide variety of materials to concrete.