Characterization of Calcium-Silicate-Hydrate Products in Expansive and Nonexpansive Heat-Cured Mortars-Electron Microscopy Study

Abstract

Mortars or concretes that have been cured at high temperature may suffer from expansion during subsequent moist storage at room temperature. This phenomenon is usually referred to as Delayed Ettringite Formation (DEF) because ettringite deposits have been observed in expansive structures. However no correlation has been established between the amount of ettringite detected in expansive heated mortars and the degree of expansion. Ettringite is prevalent in old structures regardless of the occurrence of deterioration. This study is aimed at investigating the expansion mechanism and in particular the role of ettringite relating to expansion. The results indicate that there is no direct cause between the ettringite detected by x-ray diffraction and expansion. Instead, evidence is presented for the possible implication of the calcium-silicate-hydrate (C-S-H) gel in the expansion mechanism. An important compositional difference was found between the outer C-S-H of expansive and non-expansive heated mortars that is the presence of ettringite formed in the outer C-S-H from calcium monosulfoaluminate, present within the outer C-S-H, and released sulfate from the C-S-H. Such ettringite can cause expansion because its formation takes place in situ in the outer C-S-H. The paste expands and detaches from the non-expansive components such as aggregates forming gaps at the paste/aggregate interfaces.