Research On Accelerated Method For Sulfate Attack And Protection Technology Of Cement-based Materials Under Dry-wet Cycles

There are four great salt zones with thousands of salt lakes in the northwest region,which exist serious sulfate attack problem under dry-wet cycles because of the large temperature difference between day and night,windy and dry weather and high concentration sulfate environment.A large number of concrete construction in these areas usually failured before their design age.At present the sulfate attack experiments under dry-wet cycles in laboratories have many disadvantages including the high drying temperature,the big difference with the actual environment and without drying at room temperature and furnace cooling process.Besides,less attention has been paid to how to effectively improve the sulfate attack resistance of cement-based materials in a super high concentration environment under dry-wet cycles.Therefore,setting up a testing method which could not only avoid temperature stress for cement-based materials but also appropriately simulate dry-wet cycles in a actual environment,revealing the damage form and deteriorating law of sulfate attack under dry-wetting cycles,exploring the effective measures to improve the sulfate attack resistance of cement-based materials under this harsh condition,all of these have great scientific significance and application value for improving the durability of concrete construction and extending their service life.In this paper,the existing dry-wet cycles of sulfate attack in laboratory have been summarized and compared,the impact of immersion concentration,immersion temperature and drying temperature on acceleration of dry-wet cycles have been discussed.Meanwhile,the influence of materials factors and prevention technologies on sulfate attack of cement-based materials under dry-wet cycles have also been studied by macro and micro performance test including expansion,mass change,XRD,TG,etc.Main conclusions are as follows:(1)considering the performance result of expansion,visual appearance and XRD along with the sulfate concentration and attack mechanism in actual environment,5% concentration of Na2SO4 is the most suitable concentration for sulfate attack of cement-based when doing dry-wet cycles research in the laboratory.(2)Elevating the immersion temperature of cement-based materials decreases the acceleration of dry-wet cycles,while the 20?temperature of Na2SO4 solution has the largest acceleration for dry-wet cycles.(3)The acceleration for dry-wet cycles at 45? drying temperature higher than the automatic dry-wet cycles with natural air drying.Establishing a new dry-wet cycles method: the cement-based materials was soaked in 5% Na2SO4 solution at 20?for 96 h,then took out the cement-based materials and natural drying for 12 h,after that,put them into oven at 45?for 48 h,then turned off the oven and the cement-based materials cooled inside the furnace to room temperature for 12 h,a dry-wet cycles lasts for 7 days.(4)When adding protective materials in cement-based materials,hydrophobic admixture(GW(2))has a strong protective effect,but the protective effect of the compact admixtures show big differences from each other.When coating with protective materials on the surface of cement-based materials,both the hydrophobic(GW(1))and the compact(NF)protective materials have very significant protective effect,comparatively speaking the protective effect of the hydrophobic admixtures better than the compact admixtures.