Study On The Mechanism Of Sulfate Attack And The Testing Methods For Sulfate Resistance

The mechanism of sulfate attack on hardened cement paste and mortar bars has been investigated with different concentration of Na2SO4 and MgSO4 and by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The demolishing behaviors of the specimens also have been observed. It is found that in Na2SO4 solution, when the concentration is low, a large mount of Ettringite produces, when high, a large mount of gypsum produces. While in MgSO4 solution, except for producing Ettringite and gypsum respectively in low and high concentration, it is also found that the C-S-H gel has been decompounded. The larger and wider cracks appeared on the surface of the specimens in Na2SO4 solution, however, the specimens in MgSO4 solution become loose and the strength become lower.C-S-H was prepared by synthesizing in solution, hydrating from calcium silicate (C3S and C2S) and extracting from hardened cement paste, and immersed in Na2SO4 and MgSO4 solution respectively, in order to investigate the mechanism of sulfate attack on C-S-H. The results show that in lower concentration solution, C-S-H decompounded, while in higher concentration solution, except for decompounding, a large mount of gypsum produced. It is known from these results that even if there exists no aluminate, sulfate can make C-S-H decompounded.The existing methods for testing the sulfate resistance were analyzed and discussed. It can be known from the experiment results that these methods show some shortcoming, for example, the state of specimens attacked by MgSO4 solution can't be reflected well. The results also show that the expansion measuring method and the drying-wetting cycling method are not suitable to evaluate the resistance performance of cement in MgSO4 solution, while the long-time immersing method has the advantage to test the sulfate resistance in MgSO4 solution.