Study On The Degradation Mechanism Of Chloride And Sulfate Attack Of Cement Mortars Under Severe Environment

Concrete structures partially subjected to salt lakes are more prone to be damaged in the western region of China,due to the high concentration of sulfates and drastic changes in temperature and humidity.In this paper,in order to unveil the deterioration mechanism of cement-based materials exposed to sulfate attack,the mortars were fully and partially immersed in 5%Na2SO4 and 5%MgSO4 attack,with the weather condition cycling in 20?-80%RH and 35?-30%RH every other day.Meantime,various influencing factors of sulfate attack were studied,and the mechanism of chlorine-sulfate interaction in mortar and the protective effect of different coating materials on mortar against sulfate attack were explored.The main research work and conclusions are as follows:Firstly,Ordinary Portland cement(OPC)mortars and Calcium Sulfoaluminate cement(CSA)mortars partially and fully immersed in sulfate solution were adopted.The mass change,relative dynamic modulus of elasticity,compressive/flexural strength loss and distribution of SO42-of mortars were monitored to trace the damage process.In addition,Micro analysis methods such as the scanning electron microscopy(SEM),X-ray diffraction(XRD)and Mercury intrusion porosimetry(MIP)were adopted to unveil the damage mechanism.The results showed that OPC mortars in MgSO4 attack suffered worse chemical sulfate attack than that of Na2SO4 attack,when mortars fully immersed in sulfate solution;while mortars met worse deterioration in Na2SO4 attack in terms of partially immersion,due to the Na2SO4 physical sulfate attack.Meantime,in order to study the effects of water-cement ratio,mineral admixtures and chlorine salts on the deterioration of cement mortars partly exposed to sulfate under the cyclic weather,the degradation process and the change of porosity and pore diameter distribution of mortars were investigated.The results manifested that a low water-cement ratio can largely strengthen sulfate attack resistant of the cement mortars.Compared with the plain mortars,the fly ash or slag in blended mortars can reduce Na2SO4 chemical attack,but aggravate the physical Na2SO4 attack and chemical MgSO4 attack.Chlorine salt can inhibit the chemical and physical Na2SO4 attack,but it has no obvious effect on MgSO4 attack.In addition,in this paper,the mechanism of chlorine-sulfate interaction in cement mortar was explored by setting a series of chlorine-sulfate mixed solutions with different concentrations,and the protective effect of different coating materials on mortar against sulfate erosion was studied.The results showed that with the increment of chloride concentration,the content and the reaction coefficient of SO42-decreased rapidly.In the early stage of erosion,with the increase of Na2SO4 concentration,the chloride diffusion coefficient and concentration decreased successively.However,in the later erosion stage,high concentration of Na2SO4 can lead to the deterioration on mortars,thus increasing the diffusion coefficient and content of Cl-.When mortars subjected to Na2SO4 attack,the coating protection effects of difference coating materials are polyurea>glass flake epoxy resin>polyurethane>silicane>epoxy resin>sodium silicate.