Effect of type of aggregate on delayed ettringite formation

Portland cement mortars were made from natural quartz and crushed limestone sands, and combinations of the two. Three types of concrete were made from granite and quartz sand, coarse and fine flint, and coarse and fine limestone. Mortar and concrete prisms were cast and cured at 95-100°C and subsequently stored at room temperature under water. The quartz sand mortars exhibited large expansions within the first year but the limestone mortars remained stable even after 6 years. The ultimate expansion of those mortars with combinations of limestone and quartz aggregate had intermediate values. The granite aggregate concretes expanded readily but the 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 examine the composition and microstructure of the hydration products. Differences in the features of ettringite bands that developed in the various heat-cured mortars and concretes were observed. It was shown that the rough surface of the limestone sand and aggregate particles was able to interrupt the development of an ettringite band in the paste-aggregate transition zone. Microstructural observations give support to the growth of ettringite bands as the mechanism responsible for the observed expansion.