Research On Frost Resistance Of Modified Coal Gangue Aggregate Concrete

The solid waste coal gangue produced in the mining process of the coal industry has become one of the largest industrial wastes in China.Shaanxi is a large province of coal gangue discharge,and the coal gangue discharge in northern Shaanxi reaches the largest in the province.The large accumulation of coal gangue poses a serious threat to the environment.How to effectively convert coal gangue into available resources and reduce environmental pollution is imperative.As a coarse aggregate for construction,coal gangue is an effective means of resource utilization.However,its application in civil engineering is limited due to its high crushing index and large water absorption,resulting in low application rate.In view of the above shortcomings,this paper first carries out surface modification treatment on coal gangue,and carries out cement-based materials（cement,cement+fly ash,cement+silica fume）or cement-based materials+epoxy resin coating treatment on coal gangue.CO₂curing is carried out under different curing temperatures（20°C,40°C）,CO₂concentrations（65%,95%）and curing times（2h,4h,6h）.The enhancement effect of coal gangue aggregate under different curing temperatures,time and CO₂concentration is studied.The carbon fixation rate was tested,and the frost resistance test of modified and unmodified coal gangue aggregate concrete was carried out.The influence of different aggregate modification methods on the frost resistance of coal gangue aggregate concrete was analyzed,and the corresponding damage layer thickness calculation model was established.The main results of this paper are as follows:（1）The performance of coal gangue aggregate can be significantly improved by CO₂curing.Prolonging the curing time,increasing the CO₂concentration and appropriately increasing the curing temperature are more significant for improving the performance of coal gangue aggregate.The crushing index,firmness and water absorption of coal gangue aggregate can be reduced by 58.82%,38.57%and 19.15%.The crushing index and firmness of aggregate are improved from grade II to grade I.Among them,the macroscopic properties of cement package after CO₂curing are the best,followed by the package with 5%silica fume.Increasing the curing temperature and CO₂concentration can also increase the carbon fixation rate of aggregates.The carbon fixation rate of coal gangue cured with carbon dioxide can reach up to 20%.（2）Aggregate modification can significantly improve the frost resistance of coal gangue aggregate concrete.The number of freeze-thaw cycles of unmodified coal gangue aggregate concrete is 150 times,and the number of freeze-thaw cycles of modified coal gangue aggregate concrete can reach 200 times.The strength loss is significantly reduced by 13.51%～51.72%,and the relative dynamic elastic modulus is significantly increased by 8.33%～22.92%.The frost resistance of coal gangue aggregate concrete wrapped by cement+5%silica fume and cured by CO₂is the best.（3）In the process of freeze-thaw cycle,the change rule of damage layer thickness of coal gangue aggregate concrete is consistent with the change rule of relative dynamic elastic modulus.The thickness of the damage layer of the unmodified coal gangue aggregate concrete increases sharply after 50 freeze-thaw cycles,and the thickness of the damage layer of the surface modified concrete increases significantly after 100freeze-thaw cycles.The ultrasonic velocity V_dtin the damaged layer of concrete is significantly smaller than that in the undamaged layer,and with the increase of the number of freeze-thaw cycles,the ultrasonic velocity V_dtin the damaged layer gradually decreases,with a decrease of 23.81%～36.16%.