Abstract
The objective of this paper is to assess the risk of delayed ettringite formation (DEF) in concrete made with fly ash cements. To achieve this goal, optical and electron microscopic observations were performed. Cements were prepared by adding K2SO4 to plain OPC to elevate equivalent SO3 to 2 or 4%. After that, each of the high DEF risk cements was replaced with 30% fly ash (FA30) or 65% slag (BS65). Concrete specimens were mixed at a room temperature of 27℃, then exposed to a high temperature history with maximum temperature of 85℃, assuming the interior temperature of mass concrete in a warm climate region. An accelerated DEF test was performed in reference to the Duggan test. As a result, DEF had occurred in concrete specimens at several SO3 levels without fly ash or slag, showing extremely large expansion, with significant cracks including gaps around aggregate particles and random cracks that indicated formation of ettringite. In contrast, no abnormal expansion had not occurred in FA30 and BS65 concretes, suggesting that fly ash would be effective in suppressing DEF. An EDS analysis revealed that low Ca/(Si+Al) hydrates with fly ash or slag were advantageous in fixation of alkalis.
