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Applied electric fields, the reason behind stray currents, accelerate the ingress of sulfate into cementitious materials. To identify mitigation approaches, this study investigates the effects of slag and fly ash on sodium sulfate attacks of cement pastes under a constant electric current. The mineralogical alterations induced by the attacks were analyzed using X-ray diffractometry, thermogravimetric analysis, scanning electron microscopy, and thermodynamic modeling. The dissolution of aluminates in the slag and fly ash induced a monosulfate-rich area (>∼20 mm from the cathode surface) to form next to the ettringite-rich area on the sample surface (<∼20 mm). This effect reduced the availability of SO 4 2− in the pore solution, thereby hindering the penetration of sulfate. Meanwhile, the consumption of portlandite by the pozzolanic reaction lowered the decalcification resistance of the materials. This produced a wide area that endured the decomposition of portlandite and carbonate-AFm. Overall, blending 30% fly ash did not improve the resistance of the material to either sulfate attacks or decalcification; blending 50% slag can effectively mitigate sulfate ingress, though decalcification may become a governing mechanism in the degradation process. [ABSTRACT FROM AUTHOR]

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, 1H, 27Al and 29Si 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. 29Si 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 Al3+ 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. [ABSTRACT FROM AUTHOR]

This work aims to investigate the sulfate resistance of geopolymer composites prepared by alkali activation of metakaolin (MK) and reinforced with polypropylene fiber (PP), polyvinyl alcohol fiber (PVA) and wollastonite (WS). Compressive and flexural strength were measured to determine the optimum mix proportion of the composites. The sulfate resistance properties were evaluated by strength loss, mass loss, microstructure and pore structure changes after sulfate exposure. The experimental results show that the two groups containing 1% of PP + 1% of PVA and 2% of PVA +15% of WS, respectively, exhibit the highest compressive strength and flexural strength compared to non-reinforced mixtures. After sulfate exposure, the decreased compressive strength and mass loss are correlated to the concentration of sulfate and exposure period. In the simulated extreme marine environment, the deterioration of composites induced by high concentration of sulfate after 28 days of exposure reaches to the maximum level or a threshold. The average pore size and porosity increase after the sulfate exposure. The role of fibers in mechanical properties and microstructure after sulfate exposure were also investigated. The uses of hybrid organic fiber and inorganic mineral microfiber reinforcement are effective to enhance the resistance to sulfate attack. Image 1 [ABSTRACT FROM AUTHOR]