Influence Of NaNO₂/NaNO₃ On The Material Properties,microstructure And Resistance To Chloride Attack Of OPC-MK-DM Mixed Paste

Studies have shown that the addition of dolomite powder can greatly enhance the formation of the hydrotalcite phase in the hardened cement paste,and the hydrotalcite can greatly improve the resistance to chloride corrosion,sulfate corrosion,and carbonate corrosion of the hardened cement paste.Activated aluminum is a necessary condition for the formation of hydrotalcite.In this study,the composite cement formed by replacing some ordinary Portland cement?OPC?with metakaolin?MK?rich in activated aluminum and dolomite?DM?rich in magnesium will probably have stronger durability than ordinary cement,and Na NO₂ and Na NO₃ are added to further enhance the rust-inhibiting effect.The effects of different?MK+DM?ratios,the incorporation of Na NO₂/Na NO₃ on the material properties,microstructure,and resistance to chloride attack of the mixed paste were explored.The main research contents are as follows:?1?Design the mixing ratio and study the material properties of OPC-MK-DM mixed paste:The fluidity,initial setting time and final setting time were tested according to the specifications,and the internal factors were analyzed in combination with the characteristics of the raw materials and the composition of the reaction products.The compressive strength of the mixed paste was tested at the ages of 1d,3d,7d,14d,28d,and 56d;and the pore size distribution of the mixed paste was measured at 28d by the mercury intrusion porosimetry?MIP?,and the relationship between the compressive strength and porosity were analyzed.It was found that when the content of?MK+DM?was 30%,the workability was the best,the compressive strength was the best,and the pore structure was more dense.?2?Study the microstructure of OPC-MK-DM mixed paste:X-ray diffraction?XRD?was conducted to characterize the phases of reaction products at 7d and 28d by comparing the 2?angles of the characteristic peaks.Thermogravimetric analysis?TGA?was conducted to analyze the composition of reaction products at 7d and 28d by comparing the characteristic temperature of the weight loss peak.Observe the morphology at the nanometer level and chemical element distribution of reaction products by scanning electron microscope?SEM?,the chemical element ratios of reaction products?such as Mg/Al,Ca/Si,Si/Al?were obtained to determine the composition of the reaction products according to the energy dispersive spectrometer?EDS?point-scan analysis.It was found that the addition of MK to provide a sufficient amount of activated aluminum did not promote the formation of more hydrotalcite.Instead,due to the strong pozzolanic reaction of MK,a large amount of Ca?OH?₂ was consumed,which inhibited the reaction of dolomite and the formation of hydrotalcite;and carbonate AFm phase was still formed,maintaining the stability of ettringite;the addition of Na NO₂ and Na NO₃ mainly changed the type of AFm phase without adverse effects.?3?Study the chloride ion adsorption capacity of OPC-MK-DM mixed paste:The specimens of each group were cured to 28d and ground into powder,dispersed in Na Cl solution of four different concentrations,adsorbed for 30 days to equilibrium.Calculate the amount of free chloride ion and combined chloride ion for each group by X-ray fluorescence?XRF?;and draw the combined chloride ion-free chloride ion curve.Analyze the chloride ion adsorption capacity of OPC-MK-DM mixed paste.XRD was conducted to characterize the phase change of the reaction products after adsorption,focusing on the formation of chloride-fixed hydrotalcite and Friedel salt phase,the peak intensity changes of hydrotalcite,carbonate AFm phase,nitrite AFm phase,and nitrate AFm phase,further clarify the ability to fix chloride ions.It was found that although no hydrotalcite was produced,when the content of?MK+DM?was 30%,the ability to adsorb and fix chloride ions of the paste was still strong;adding Na NO₂and Na NO₃ had no adverse effects.?4?Study the chloride ion transmission law of OPC-MK-DM mixed paste under dry and wet cycling conditions:Put the samples cured to 28d into the wet and dry cycle device,dry for 24h,wet for 24h,cycle for 2 months to balance,and then follow the interval of 2/2/2/2/3/3/4/4/5mm layered grinding,using XRF to measure content of the chloride ion at different depths from the immersion surface of the samples,and plot the chloride ion conten-distance surface depth curve to analyze the effects of different?MK+DM?ratios,the incorporation of Na NO₂/Na NO₃ on chloride transport in OPC-MK-DM mixed paste under dry and wet cycling conditions.It was found that the degree of pore penetration has a decisive effect on the ability to resist chloride ion erosion under dry and wet cycling conditions.