Study On Salt Freezing Performance Of High Toughness Cementitious Composites Prepared By Desert Sand Under Single-Side Freeze-Thaw Conditions

High toughness cementitious composite overcomes the shortcomings of low tensile strength,brittle damage and other shortingcomings of concrete.ECC has strain hardening and multi-crack cracking properties,and is a new type of building material with high toughness and high durability.High toughness cementitious composites(DSECC)were prepared using low-cost poly-vinyl-alcohol(PVA)fiber as a toughening material and Tengger Desert Sand(TDS)and Mu Us Desert Sand(MDS)as fine aggregates,and their mechanical properties were systematically studied.It was found that desert sand type and water-to-binder ratio have a large influence on the mechanical properties of ECC and further work on its durability is necessary.The combination of salt solution and freeze-thaw is one of the main reasons for the lack of structural durability.In this paper,DSECCs prepared from Tengger and Mu Us desert sand(named TDSECC and MDSECC,respectively)were studied,and the effects of desert sand type,water-to-binder ratio(0.27,0.30,0.32,0.35)and freeze-thaw medium type(3%NaCl,3%NaCl+3%Na2SO4)on the salt freeze resistance of DSECC were analyzed by combining SEM,EDS and XRD tests.The main conclusions are as follows.(1)Under the chloride salt-freeze-thaw environment,TDSECCs resistance to freezing and chloride ion permeability is similar to that of RSECC and better than that of C30 concrete,while MDSECCs resistance to freezing and chloride ion permeability is better than that of C30 concrete and slightly worse than that of TDSECC and RSECC.(2)Under the chlorine salt-freeze-thaw environment,the compressive strength of TDSECC and MDSECC decreases with the number of freeze-thaw cycles,and the slope of the stress-strain curve,the peak stress and the area surrounded by the curve all show a decreasing trend with the number of freeze-thaw cycles.At the end of 32 freeze-thaw cycles,the compressive strength loss was C30>MDSECC>TDSECC>RSECC,and the TDSECC and RSECC compressive strength loss rates were similar.Under the chlorine-sulfate-freeze-thaw condition,TDSECC and MDSECC compressive strength changes are basically the same as under the chlorine salt-freeze-thaw condition,and the compressive strength loss rate is higher.(3)Under the chlorine-sult-freeze-thaw environment,the water-to-binder ratio of 0.27 and 0.30,TDSECC has better anti-freezing and anti-chlorine ion penetration performance,and the worst water-to-binder ratio is 0.35;TDSECC has the highest compressive strength at 0.27,similar at 0.30 and 0.32,second to 0.27 and better than 0.35.The effect of water-to-binder ratio on compressive strength under chlorine-sulfate-freeze-thaw conditions is essentially the same as under chlorine salt-freeze-thaw conditions.(4)SEM electron microscopy scan results show that as the number of freeze-thaw cycles increases,the TDSECC cement matrix is fully hydrated,and the desert sand-cement matrix interface transition zone(ITZDS)has better bonding performance,which is similar to RSECC and is superior to MDSECC.It proves that TDSECCs macroscopic frost resistance is superior to MDSECC.Combining with EDS and XRD,the types and elements of hydration products of TDSECC,MDSECC and RSECC are similar,which proves that the preparation of DSECC by desert sand instead of RS is effective and feasible from the micro level.(5)The DSECC freeze-thaw damage equation is proposed by fitting the relative dynamic elastic modulus,electrical flux and compressive strength damage coefficients to the number of freeze-thaw cycles by Origin,and the equation curve is in good agreement with the measured data.Based on the relative dynamic elastic modulus and compressive strength,the service life of the DSECC in the actual environment is predicted based on the freeze-thaw cycle life in the laboratory.It is recommended that the optimum DSECC ratio is 0.27/0.30 for water-to-binder,0.46 for sand-to-binder,50%for fly ash replacement and TDS for desert sand.