Effect of environmental pH values on phase composition and microstructure of Portland cement paste under sulfate attack

The degradation of material induced by sulfate attack is major durability problem facing cementitious materials. In this paper, to illustrate the nanoscale chemical degradation, Portland cement samples were exposed to 5 wt% NaSO 4 solution with different pH values (pH = 13, 8 and 5) as the function of corrosion ages (28 and 180 days). The effects of pH value on the hydrate phase, morphology and microstructure of cement hydration products under sulfate attack were characterized in combination with XRD, SEM-EDS, 1 H, 27 Al and 29 Si NMR. The results demonstrate that reducing pH value of sulfate solution facilitates the formation and precipitation of columnar and platelet gypsum crystals in cement paste, whereas inhibiting the ettringite formation. A slight increase in the proportion of deleterious micropores larger than 1.6 μm as the sample exposed to sulfate with low pH value. The ettringite induced by sulfate attacking is transformed to AFm at early period and reversal transformation happens at later period. Reduction of the pH value of sulfate solution accelerates the transformation reaction of the hexa-coordinated aluminate phases. 29 Si NMR test revealed that with pH reducing from 13 to 5, the hydration degree increase nearly 18%, the mean silicate chain length increase by 66.2% and Al[4]/Si ratio increase by nearly 30%. Also, the silicate polymerization enhancement is closely related with Al 3+ ions extraction at early sulfate-attack period and re-entering at late sulfate-attack period. This research might provide nanoscale insights to guide more durable cement-based material in detrimental environment.