Examination of the concrete from an old Portuguese dam: Texture and composition of alkali–silica gel

Exudations and pop-outs were identified in the interior galleries of a large dam built in the 1960s. The samples collected were examined by a Scanning Electron Microscope. A dense material with a smooth surface and drying shrinkage cracks or a spongy texture were observed in the samples. The semi-quantitative composition was obtained by energy dispersive spectrometry (EDS) and it was concluded that this material corresponds to alkali–silica gel, composed of SiO2–Na2O–K2O–CaO. A viscous white product in contact with an aggregate particle in a cone sampled from a pop-out was observed through use of the scanning electron microscope and it has characteristics similar to the gel present in the exudations and cavities. Reference is made to the potential alkali reactivity of the aggregate present in the concrete. The texture and composition of the products probably resulting from an alkali–silica reaction are presented, set out in ternary diagrams, and discussed.