Study On Characteristics Of Decomposition And Reconstruction Of Ettringite Based On Rietveld Refinement

Delayed ettringite formation is a potential deterioration of autoclaved concrete,especially for pipe pile concrete.Based on the production technology of pipe pile concrete,it can be divided into two types:autoclave process and non-autoclave process.Even the curing temperature of autoclave-free process is above 70? which reaches the decomposition temperature of ettringite.In this paper,the DEF process of ettringite was studied from the decomposition and reconstruction of synthetic ettringite.In this paper,ettringite was synthesized from CaO and Al2(SO4)3·18H2O by liquid phase synthesis method using gypsum and NaCl as auxiliary raw materials.Ettringite was decomposed at 70?,100? and autoclave.The ettringite was reconstructed in fresh water at 20? and 40 ?,and the ettringite in sulphoaluminate cement was decomposed in different ways by using sulphoaluminate cement mortar.In the reconstruction process,the laser rangefinder is used to monitor the effect of DEF process on the volume of mortar specimen.The above samples were analyzed by X-ray diffraction(XRD)and scanning electron microscopy(SEM),and the synthetic ettringite samples were quantitatively analyzed by Rietveld refinement analysis.In this paper,it is found that ettringite and gypsum crystals can be found in the samples synthesized by liquid-phase synthesis in four different ionic environments for 4 hours.After three different decomposition methods,ettringite can be decomposed obviously in all samples,in which there is residual undecomposed ettringite in the samples with excess calcium oxide at 70?.The main crystal phase of decomposition products is gypsum at 70? and 100?,and anhydrite under autoclave decomposition conditions.There is no obvious diffraction peak of AFm in the X-ray diffraction curve.Ettringite reconstruction can occur quickly in water at 20? and 40?,and the reconstruction process can be accelerated by increasing the temperature of reconstruction.In addition,it can be seen that the excessive existence of chloride ion,gypsum and calcium oxide all accelerate the reconstruction of ettringite.For the sulphoaluminate cement mortar,steam curing at 70? can not decompose ettringite to a large extent,so the formation of ettringite is delayed after curing at 70?.After decomposition at 100 ?,the formation of ettringite was delayed after 100 days.After autoclave curing,it is difficult to delay ettringite formation.The phenomenon of delayed ettringite formation may lead to the expansion of the specimen.