The effect of cement composition and fineness on expansion associated with delayed ettringite formation

Abstract

A large number of laboratory ground cements were prepared from five production clinkers. Grind variables included cement SO 3, source of added SO 3 and specific surface area. Mortar prisms were cast as described in EN 196-1 except that the prisms were 16 × 16 × 160 mm with stainless steel pins cast in their end faces. Sets of prisms were cured for 12 h at 70 °, 80 ° and 90 °C in addition to the standard storage at 20 °C. After demoulding at 24 h the prisms were stored in water at 20 °C and lengths and weights monitored for periods up to ~ 1800 days. Expansions typical of DEF were normally only observed in prisms cured at 90 °C. The magnitude of the expansion increased with fineness and passed through a maximum at a cement SO 3 content of ~ 4%. No expansions were obtained with a low C 3A sulfate resisting clinker but there was little difference in the behaviour of the other clinkers. Natural gypsum, natural anhydrite and desulfurisation gypsum gave similar expansions. Adding KOH or K 2SO 4 dissolved in the mixing water accelerated and increased expansions greatly. Some mortars containing added alkali expanded after curing at 80 °C.