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 st orage at room temperature. Large expansions have been observed after 2 1/2 years for prisms made with a certain portland cement. Backscatter ed electron imaging (bse) in a scanning electron microscope (SEM), com bined with x-ray microanalysis and quantitative x-ray diffraction anal ysis, was employed to examine the microstructure, the distribution and the quantity of ettringite formed. Bands of ettringite could be readi ly observed in a heat cured prism near one of its long surfaces, but n ot in the central region of the prism. X-ray microanalysis demonstrate d that S and Al species, initially dispersed in the outer and the inne r hydration products around cement grains during heat curing, diffused from these hydration products during the period of 2 1/2 to 3 1/2 yea rs water storage at room temperature to form the ettringite bands. Tre nds between the occurrence of ettringite bands and paste expansion sug gest that a causal relationship may exist between these factors. Copyr ight (C) 1996 Elsevier Science Ltd