Delayed ettringite formation in 4-year old cement pastes

Cement paste prisms (40 x 40 x 160 mm) made from portland cement have been cured at 100 degrees C for 3 hours, followed by extended water storage at room temperature. Large expansions have been observed after 2 1/2 years for prisms made with a certain portland cement. Backscattered electron imaging (bse) in a scanning electron microscope (SEM), combined with x-ray microanalysis and quantitative x-ray diffraction analysis, was employed to examine the microstructure, the distribution and the quantity of ettringite formed. Bands of ettringite could be readily observed in a heat cured prism near one of its long surfaces, but not in the central region of the prism. X-ray microanalysis demonstrated that S and Al species, initially dispersed in the outer and the inner hydration products around cement grains during heat curing, diffused from these hydration products during the period of 2 1/2 to 3 1/2 years water storage at room temperature to form the ettringite bands. Trends between the occurrence of ettringite bands and paste expansion suggest that a causal relationship may exist between these factors. Copyright (C) 1996 Elsevier Science Ltd