Microstructural and microanalytical studies of sulfate attack. V. Comparison of different slag blends

Abstract

Pastes of several slag blends that had been stored in 0.25 mol I −1 solutions of sodium or magnesium sulfate were studied by scanning electron microscopy using backscattered electron (BSE) imaging and X-ray microanalysis of polished sections. In general, attack resulted in decalcification of C-S-H and the formation of ettringite. With Na 2So 4, little or no gypsum was formed, but with MgSO 4, brucite, poorly crystalline serpentine (M 3S 2H 2) and gypsum were also formed. Visual examinations and BSE images indicated that the relative susceptibility of the blends to attack by Na 2SO 4 increases with the content of Al 2O 3 available for the formation of ettringite; this includes that present in AFm phases but not that present in C-S-H or hydrotalcite. The rank order of susceptibility to attack by MgSO 4 was broadly similar to that by Na 2SO 4, but the slag blends performed more poorly relative to plain Portland cement or sulfate-resisting Portland cement. Resistance to attack by Na 2SO 4, is favoured by the use of high proportions of slag, slags low in Al 2O 3, or sulfate resisting Portland cements and by supplementing the gypsum in the Portland cement component by additional calcium sulfate.