Mechanism Of Sulfate Attack On Cement-based Materials Under Severe Environment

Concrete structures,especially the piers and slope protections which partially immersed in brine and seawater in the South China Sea and salt lakes of western China region deteriorated early in practical applications and caused huge economic loss,which seriously restrict the development of Marine economy and the implementation of western development strategy,since the severe environment.The severe environment contains combined high salt,high temperature,low relative humidity in western China and combined high salt,high temperature,high relative humidity in South China Sea.The seawater and brine contain large amount of SO₄^2-ions,while the degradation of cement-based materials exposed to sulfate attack under severe environment is still“a confused world”.Therefore,the degradation mechanism of cement-based materials partially exposed to sulfate attack under severe environment and the influencing mechanism of severe environment on the damage are the main content of current research.These researches have important theoretical value and practical significance for the realization of the concrete structures life expectancy design.The main research contents and results are as follows:?1?The deterioration mechanism and influencing mechanism of cement-based materials partially exposed to sulfate attack under severe environmentThe damage progress of cement mortars with different w/c ratios and mineral admixtures partially immersed in sulfate solution under different temperatures and relative humidity are investigated in this paper.The results indicate the immersed portions of mortars partially exposed to Na₂SO₄ and MgSO₄ attack only suffer chemical sulfate attack,while the drying portions both suffer chemical and physical attack.The driving mechanism of specimen partially immersed in Na₂SO₄ solution is the physical sulfate attack in the drying portion,by comparison,the dominant mechanism of specimen under MgSO₄ solution is the chemical sulfate attack in the immersed portion.The decreasing of relative humidity can accelerate the water evaporation on the dry surface of mortars,lead the SO₄^2-ions accumulate in the dry-wet interface,form a supersaturated solution and accelerate the erosion of mortars.Cement mortars partially exposed to Na₂SO₄ solution under 35?erode faster than under 20?and 50?.?2?The physical attack mechanism of mortars under temperature-relative humidity cyclesThe damage progress of cement mortars partially exposed to sulfate solution under temperature cycles,relative humidity cycles and temperature-relative humidity cycles are investigated.The results indicate comparing to relative humidity cycles,the changing of temperature affect the degradation of cement mortars partially immersed in Na₂SO₄ solution more significantly.Which indicate that the driving physical attack mechanism for cement mortars is the crystallization pressure generated by the precipitation of Na₂SO₄ crystals.The deterioration of cement mortars partially exposed to Na₂SO₄ solution under temperature-relative humidity cycles is not only faster than under constant temperature and relative humidity,but also faster than under temperature cycles,relative humidity cycles.?3?The effect of surface treating on the damage of mortars partially exposed to sulfate attackEpoxy resin,polyurethane,silane and water glass are used in this paper to investigate the effect of surface treating on the damage of mortars under partially sulfate attack.The results indicate that Epoxy resin and polyurethane can form dense protective film and silane can form water-repellent film on the surface of mortar,which can restrict the water evaporation on the dry surface of mortars and improve the resistance of cement mortars to partially sulfate attack.Although water glass could react with the hydration products of cement and compact the surface the mortars,but water glass cannot change the hydrophilic property of mortar.So it has a poor effect on improving the resistance of cement mortars to partially sulfate attack.?4?the diffusion mechanism of SO₄^2-ions in cement mortars under severe environmentThe concentration of SO₄^2-ions in different heights of cement mortars partially exposed to Na₂SO₄ solution at different erosion ages are studied.The results indicate that in the early age of one-dimensional transportation for SO₄^2-ions in cement mortars,SO₄^2-ions mainly transport from the immersed part to the drying part by capillary adsorption.While in the latter age of erosion,SO₄^2-ions mainly diffuse to the interior of cement mortars caused by concentration gradient.?5?The degradation of cement mortars fully immersed in sulfate solution under severe environmentThe damage progress of cement mortars fully immersed in sulfate salt solution are researched and compared with the cement mortars partially immersed in sulfate solution in this paper to indicate the deterioration mechanism.The results indicate that comparing with the partially immersed cement mortars which both under physical and chemical sulfate attack,cement mortars fully immersed in sulfate solution only suffer chemical attack.Moreover,the damage is farmore slower than mortars partially exposed to sulfate attck.