Effect Of Mineral Admixtures On Frost-salt Resistance Of Metakaolin Based Geopolymer Concrete

The freeze-thaw damage of concrete is the main limiting factors to its durability in the cold regions,especially in area where the concrete is suffering from both corrosion of saline solution and freeze-thaw influence.As a result,it will accelerate the failure of concrete.Geopolymer is a new kind of Si-Al inorganic cementitious material with better performance compared with portland cement and has broad application prospects when used as rapid repair materials.So far,studies about geopolymer have proved that its frost resistance is unclear and the damage mechanism needs complement.Therefore,improving the frost resistance of geopolymer in salt solutions is valuable and significant to its further applications.In this paper,using the metakaolin,steel slag,slag,fly ash and silica fume as the study objects,the concrete was prepared with the activating of water glass and sodium hydroxide.With the evaluation of unit scaling amount,the effect of mineral admixtures on frost resistance of the geopolymer was studied.The volumetric porosity,saturation degree,phase composition,microstructure,thermal analysis as well as damaging form were tested to research the mechanism.The damage evolution equations were suggested by analyzing the damage during the freeze-thaw cycles.The results show that:1)Chloride environment accelerate the destruction process,the freezing injury factor reached at maximum about 1.05 with the remarkable concentration of 4%⁶% and the freezing expansion coefficient are significantly 14.9% when the concentration is 4%⁶.5%.2)The limit number of freeze-thaw cycles were improved by mineral admixtures.The natural volume aspiration ratio of geopolymer is in the range of 13%¹6% and the saturation degree is 0.92⁰.93,the mineral admixtures bring them down to 9% ¹3% and nearby 9% respectively.Also it delayed the saturation and the frost-salt resistance improved.High-calcium geopolymer performed best with the proportion of Si/Al=3.3,Na/Al=0.6,for low-calcium geopolymer it was Si/Al=3.5,Na/Al=0.7⁰.8.3)By XRD and DSC-TG analysis,steel slag and slag leaded in new type of hydration products such as C-S-H gel,C-A-S-H gel and other complex aluminumsilicate compound but fly ash and silica fume didn‘t.SEM pictures showed that there were many pores and microcrack on the surface of geopolymer,mineral admixtures made the hydration products more complicated.The improvment of frost resistance is primarily caused by the increasing varity of hydration products and improving of microstructure.4)The mineral admixtures delayed the DP growth and improved resistance to cracking.The ΔωM reduced and Pr5 was greater after 5 freeze-thaw cycles,especially for the high-calcium geopolymer,the ΔωM reduced 50% and the Pr5 was over 90%.SEM pictures showed that the G22 and K22 better maintained the microstructure before freezing but the P23 almost scattered.5)According to the freeze-thaw damage evolution equations of relative strength,base group subjected to single sectional damage model,experimental groups subjected to double sectional damage model.Mineral admixtures reduced the IDV and the early freeze-thaw damage rate of geopolymer could effectively reduce.The DA of F22,G22 and K22 after N₁₂ were less than P23,it meant that steel slag,slag and fly ash could restrain the growth of damage later.Silica fume could improve the early frost resistance before N₁₂.The frost resistance of geopolymer was effectively improved by steel slag or slag with a dosage of 15% individually,fly ash or silica fume influenced less but it would be remarkable when the fly ash used with steel slag,slag together.Improving the frost resistance of geopolymer is significant to its further applications in the cold regions.