A through solution mechanism for delayed ettringite formation in pre-existing cracks in Portland cement mortar

摘要

It has been reported that nucleation of ettringite crystals in cement pastes will preferentially occur incrack tip-zones rather than on plane solid surfaces in a cement-based matrix under conditions of supersaturation. This explanation of the phenomena is compatible with results from many case studieswhere ettringite was usually found in cracks. Recent research reported by Scrivener and Tayloralso confirmed this phenomenon. It was believed that high-temperature steam curing followed byhigh-temperature drying to form microcracks was an optimum condition for concrete deterioration due tosecondary ettringite formation. Two different arguments were discussed for interpretation of ettringite-related expansion mechanisms for Portland cement concrete; the crystal growth theory and the swelling theory. Cohen postulated that expansion results from the growth of ettringite crystals formed on the surfaces of the AI-bearing particles. The growth of these crystals results in crystallization pressure and expansion occurs. Mehta argued that expansion is attributed to the swelling of colloidal particles of ettringite. The formation of this swelling gel occurs by a through solution mechanism. Expansion could occur either by intercalation or surface adsorption effects. The phenomenon of preferred nucleation of secondary ettringite in pre-existing cracks appears to support arguments for a through-solution deposition mechanism. Resulting expansion likely occurs through crystal growth in conforming with Cohen's postulation.