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

A large number of laboratory ground cements were prepared from five production clinkers. Grind variables included cement SO3, source of added SO3 and specific surface area. Mortar prisms were cast as described in EN 196-1 except that the prisms were 16 x 16 x 160 mm with stainless steel pins cast in their end faces. Sets of prisms were cured for 12 h at 70 degrees 80 degrees and 90 degrees C in addition to the standard storage at 20 degrees C. After demoulding at 24 h the prisms were stored in water at 20 degrees C and lengths and weights monitored for periods up to similar to 1800 days. Expansions typical of DEF were normally only observed in prisms cured at 90 degrees C. The magnitude of the expansion increased with fineness and passed through a maximum at a cement SO3 content of similar to 4%. No expansions were obtained with a low C(3)A 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 K2SO4 dissolved in the mixing water accelerated and increased expansions greatly. Some mortars containing added alkali expanded after curing at 80 degrees C. (C) 1996 Elsevier Science Limited.