Research On Durability Of Volcanic Rock Powder Concrete Under Sulfate Freeze-thaw Coupled Conditions

Xinjiang is located in the western part of the motherland,far from the mainland cities and underdeveloped industries,and its high-quality admixtures appear to be scarce,especially in the vast southern Xinjiang region.During the investigation in Hotan area,a natural volcanic rock resource was found,whose composition is dominated by Si O₂ and Al₂O₃,which has certain activity and can be used as mineral admixture.After a feasibility study,the team found that this admixture has a catalytic effect on the long-term strength development of concrete,and was successfully applied to the Jiyin Water Conservancy Hub Project in Hotan,Xinjiang.In order to better apply the volcanic rock powder in engineering,this paper,based on the research of the previous team and the geographical conditions in Xinjiang,investigates the durability of volcanic rock powder concrete under the coupled sulfate freeze-thaw conditions.This research not only makes a great contribution to the engineering construction in Hotan area and even in the whole territory,but also promotes the local economic development and is in line with the development law of"solid waste utilization"advocated by the society nowadays.The research results are as follows:（1）During the freezing process of sodium sulfate solution,below 5%concentration,its freezing point decreases with increasing concentration;above 5%the freezing point hardly changes.At low temperature,the expansion rate of sodium sulfate solution is larger than that of clear water,and the larger the concentration of the solution,the smaller the volume expansion rate;as the freezing temperature decreases,the freezing time and freezing time of the solution are gradually shortened;at the same temperature,the larger the concentration of the solution,the shorter the freezing point time and freezing time.（2）Volcanic powder concrete after 28d curing,its strength can reach more than 90%of the fly ash group,can reach 80%to 89%of the pure cement group;compound concrete after 60d strength close to fly ash concrete;under the sulfate freeze-thaw cycle test,the durability performance of each admixture concrete specimens in order,fly ash concrete is the best,compound concrete is the second,pure cement concrete and volcanic powder concrete approximate and the worst;due to 5%sodium sulfate solution at low temperatures,its low freezing point,small expansion rate and freezing point time and freezing time are less than 1%solution and clear water.Therefore,the durability of concrete specimens in 5%sodium sulphate solution is higher than 1%sodium sulphate solution with water.（3）The bubble spacing coefficient test was carried out on air-entraining specimens under 5%sodium sulphate solution to explore the variation pattern of closed pores inside the specimens under the effect of sulphate freeze-thaw coupling.The analysis shows that the air content,average bubble diameter and bubble spacing factor of concrete specimens before and after freeze-thawing have increased to different degrees,while the bubble specific surface area has decreased.（4）The XRD tests on the volcanic powder specimens show that the hydration products of the specimens include Ca（OH）₂,Ca（SO）₄,Afm,Aft,C-S-H,C-A-H and part of the unhydrated cement clinker;Ca（OH）₂ and Ca（SO）₄ account for the major part of them,and the semi-quantitative calculation of the characteristic peaks shows that the erosion products in the volcanic powder specimens increase with the sodium sulfate solution;the erosion products in clear water and 1%sodium sulphate solution mainly consisted of Aft and Afm,and those in 5%sodium sulphate solution mainly consisted of Ca（SO）₄;from SEM and EDS analysis,it can be seen that the volcanic powder specimens were mainly damaged by freeze-thaw in the early stage,and with the increase of sodium sulphate concentration,the internal hydration and compactness of the volcanic powder specimens The internal hydration and compactness of the volcanic powder specimens increased significantly with the increase of sodium sulfate concentration.