THE EFFECT OF METAKAOLIN ON INTERNAL SULPHATE ATTACK OF THE HEAT-CURED MORTARS

Delayed ettringite formation (DEF) is a type of internal sulphate attack caused by heat-induced decomposition and/or prevention of normal ettringite formed during the initial hydration of cement at elevated temperature (above about 70 degrees C) and its re-crystallization in the hardened matrix. DEF is a physico-chemical phenomenon inducing an expansion of the cement paste that could lead to cracking of concrete. In this paper, the effect of metakaolin on DEF of the heat-cured mortars was investigated. To fulfil the aim of this study, a portion of cement was replaced by metakaolin, with three different dosages (10, 20 and 30%). The mortars were heat-cured at early-age, and the tests of expansion, strength, dynamic elastic modulus and thermogravimetric analysis were carried on these mortars a period of 650 days. Additionally, scanning electron microscopy (SEM) observations were realized. The test results obtained highlighted the mitigation effects of metakaolin on DEF.