Effect of type of aggregate on delayed ettringite formation

Portland cement mortars were made from natural quartz and crushed limestone sands, and combinations of thetwo. Three types of concrete were made from granite and quartz sand, coarse and fine flint, and coarse and finelimestone. Mortar and concrete prisms were cast and cured at 95–100°C and subsequently stored at roomtemperature under water. The quartz sand mortars exhibited large expansions within the first year but thelimestone mortars remained stable even after 6 years. The ultimate expansion of those mortars with combinationsof limestone and quartz aggregate had intermediate values. The granite aggregate concretes expanded readily butthe limestone and the flint concretes started to expand only after very long induction periods. X ray diffraction,scanning electron microscopy with back-scattered electron imaging and X ray microanalysis were used to examinethe composition and microstructure of the hydration products. Differences in the features of ettringite bands thatdeveloped in the various heat-cured mortars and concretes were observed. It was shown that the rough surface ofthe limestone sand and aggregate particles was able to interrupt the development of an ettringite band in thepaste-aggregate transition zone. Microstructural observations give support to the growth of ettringite bands asthe mechanism responsible for the observed expansion.