Study On The Damage Mechanism Of Mortar Exposed To The Cyclic Drying-wetting And Sulfate Attack

Under combined sulfate attack and cyclic drying-wetting process,complex physical and chemical deterioration occurs in mortars.During this process,internal microcracks initiate and propogates,accompanied by the consumption of cement hydrates and formation of erosion products.The sulfate attack is detrimental to mechanical properties/behaviors as well as microstructure of mortar,such as degradation of mechanical properties,suface spalling and material expansion.Until now,no standard drying-wetting scheme was found in the literature.According to literature review,it is found that sample size,temperature,relative humidity and duration time of a drying-wetting cycle significantly affect drying-wetting scheme.Owing to the damage induced by high temperature,oven drying,is abandoned in this study.On the premise of the sample size used in this study,a 1-day immersion followed by 2-day drying drying-wetting scheme is employed in this work.The macroscopic mechanical properties/behaviors of mortar under sulfate attack and cyclic drying-wetting have been studied in the previous works,which is not repeatedly worked on in this study.The main object of this study is to investigate the deteriorational mechanism of the mortar subject to dual actions of sulfate attack and cyclic drying-wetting by means of multiple techniques,such as Mecury intrusion porometry(MIP),Nuclear magnetic resonance(NMR),X-ray diffraction(XRD),Thermo Gravimetry Analyzer(TGA),and Scanning electron microscope(SEM).The main findings of this study are as follows:(1)By comparing the MIP and NMR pore size distribution curves,it can be found that,with the increase of dyring-wetting cycles,the volume of large capillary pores and/or micro-cracks greater than 100 nm increases,which is responsible for the deterioration of mortar.In the late cycles,capillary pores(< 100 nm)were found to be finer than that at early cycle,this is attributed to the filling of the formation of AFt.(2)Based on the pore size distribution curves form MIP,fractal dimensionof pore space and permeability coefficient of mortar are determined.The porefractal dimension can be used to characterize the porous characteristics of the mortar,which shows a variety of changes under the influence of the filling of ettringite,sulfate ion concentration and cycle period.The change trend of the water permeability coefficient can well reflect the degree of damage and impermeability of the mortar.(3)Qualitative and quantitative analysis of the erosion process is carried out in combination with XRD and TGA tests.The main products of sulfate attack under drying-wetting cycles are ettringite and gypsum crystals,which were also detected.Based on the TGA results,the amount of portlandite was quantitatively calculated,and the overall trend showed a decreasing trend with the cycles.(4)Scanning electron microscopy combined with digital image correlation technique to study the damage mechanism of interface transition zone:(a)The damage morphology images inside the mortar samples were analysed,and the formation of ettringite crystals around the transition zone,the expansion of micro-cracks and the dual effects of ettringite crystals on micro-cracks and filling were discussed.(b)Based on digital image correlation analysis(DIC),the crack propagation and expansion in the interface transition zone were explored.The full field strain of the mortar surface and the crack propagation results around the aggregate were obtained,which is consistent with the SEM results mentioned aboved.