Hydration characteristics of portland cement after heat curing: II, evolution of crystalline aluminate-bearing hydrates

The development of crystalline aluminate-bearing hydrates in portland cemen t mortars during water storage at room temperature for periods of up to 1 y ear after an initial heat cure for 12 h has been observed by quantitative X -ray diffraction analysis and backscattered electron imaging. Ettringite wa s present in the mortars immediately after a short-term cure at 20 degrees and 60 degreesC, calcium carboaluminate (C(4)A (C) over barH(11)) at 60 deg reesC, monosulfate at 85 degreesC, and hydrogarnet at 85 degreesC and above . Ettringite started to form after an induction period ranging from several days to several months after the initial heat cure at 85/100 degreesC, and developed substantially during the period of expansion of the mortar assoc iated with delayed ettringite formation (DEF), Ettringite growth was also o bserved in the nonexpansive cement mortars. Development of the ettringite b ands occurred exclusively in the expansive mortars, Although monosulfate ob served in the mortars that had been heat cured at 85 degreesC sometimes inc reased in amount on initial storage at room temperature, it appeared to var y little in amount for up to 1 year. The amount of hydrogarnet in the heat- cured cement product did not change significantly during storage at room te mperature for more than 1 year. DEF expansion of the heat-cured mortars was attributed to ettringite band formation, which started to form at the surf ace of the cement product and gradually developed inwards.