Delayed ettringite formation (DEF) is a concrete deterioration process that involves the growth over time of ettringite, Ca Al (SO ) (OH) ·26H O, crystals. This research investigated the effects of two 6 2 4 3 12 2 factors, thermal…
Delayed ettringite formation (DEF) is a concrete deterioration process that involves the growth over time of ettringite, Ca Al (SO ) (OH) ·26H O, crystals. This research investigated the effects of two 6 2 4 3 12 2 factors, thermal cycling and elevated potassium levels, on the mechanisms and kinetics of this process. The experimental approach involved casting three sets of concrete specimens under different conditions that were designed to either promote or inhibit DEF. These consisted of a control set cured at room temperature, a set that was subjected to thermal cycling and a third set which was made with an elevated potassium content of 1.72% K2O and also thermally cycled. The measurements included expansion, weight change and compressive strength and were taken periodically up to 350 days. In addition, fractured specimens were examined by Scanning Electron Microscopy (SEM) and Energy Dissipative X-ray (EDAX) which confirmed the presence of ettringite. Only the heat-treated specimens showed significant expansion. Analysis of The correlation of the expansion data with the weight change data revealed that deterioration process occurs in three distinct stages. In the first stage, the ettringite fills the pores with little or no expansion; in the second, the expansion mechanism appears to be creep due to expansive stresses in the filled pores and in the third, ettringite causes crack propagation leading to significant expansion and loss of compressive strength. Pre- existing microcracks are a necessary condition for DEF. The elevated potassum level did not affect the rate of ettringite formation, but resulted in a higher limiting value of the expansion.