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This paper presents a mechanism to explain the ability of Ca(OH)2 and CaSO4 to prevent flash set in portland cement pastes and the ability of CaSO4 and other salts to accelerate the rates at which calcium silicates develop strengths. The mechanism is developed on the basis of solid state reactions in which ions from the liquid phase of a cement paste are chemisorbed by ions in the surfaces of the crystals of the cement minerals and then diffuse into the crystals to produce the reaction products responsible for the setting and hardening of the cement paste. The abilities of organic compounds to retard the rates of these reactions, cause dispersion of the particles, reduce water requirements, and act as grinding aids are explained in terms of adsorbton of molecules and negative ions.
Director, Research Laboratories, Uinversal Atlas Cement Division of United States Steel Corp., Buffinnton, Ind.