Microscopic analysis of alkali–aggregate reaction products in a 50-year-old concrete

Fifty-year-old concrete from a large dam was examined in the scope of an investigation program concerning the properties of granite as aggregate material for concrete. Site inspection, which was developed in order to detect possible signs of deterioration of the concrete, revealed the existence of efflorescence and exudations. ng electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses were attempted to identify the composition of these materials and their morphology. From the analyses, it was concluded that some of the exudations were composed by alkali–silica gel. In these samples, an interesting behavior was observed in different moments after a 3-month interval. It was noticed that the initially noncrystalline alkali–silica gel transformed into sodium-rich needles and tablets after a few months kept in a desiccator in the laboratory. Therefore, it was concluded that the materials identified corresponded to different stages of evolution of an alkali–aggregate reaction product.