The Effect of the Partial Cement Substitution with Fly Ash on Delayed Ettringite Formation in Heat-cured Mortars

Delayed Ettringite Formation (DEF) is seen as a form of internal sulfate attack of cementitious materials, caused by early age heating to a temperature higher than 70 degrees C. In this paper, the effect of fly ash on delayed ettringite formation of heat-cured cement-based mortars was investigated. To fulfil the aim of this study, a portion of cement was replaced by class F-fly ash, with three different dosages (10, 20 and 30%). The mortars were heat-cured at early-age, and the tests of expansion, mechanical strength, dynamic elastic modulus, mercury porosity and thermogravimetric analysis were carried on these mortars along a period of 650 days. Additionally, Scanning Electron Microscopy (SEM) observations were realized. The results obtained highlighted the mitigation effects of fly ash on DEF: A replacement of the cement used with 20% to 30% of fly ash was efficient to eliminate the long-term swelling due to DEF. This positive impact was explained by the combined effects of the high Al2O3 content of fly ash and the portlandite consumption induced by the pozzolanic reactions.