Study On The Basic Performance And Hydrated Mechanism Of Composite Limestone Powder-Tailings Mixed Sand Concrete

At present,the concrete industry pays more and more attention to the concept of sustainable development of’energy saving,green and environmental protection,’and replacing raw materials with industrial waste residue has become a trend of industry development.Replacing part of cement with slag,limestone powder,fly ash and silica fume in three or four ways,and replacing natural sand with tailings mixed sand to prepare concrete have better economic and environmental benefits.In this paper,the mechanical performances,bonding behavior between concrete and steel bar,hydration heat properties,early cracking performance and fine microstructure of composite limestone powder-tailings mixed sand concrete are studied by physical test and numerical calculation.The microscopic hydration mechanism of macroscopic performance change is revealed by theoretical analysis from the microscopic level.The main conclusions are as follows:1.The development law of mechanical properties of composite limestone powder-tailings mixed sand concrete is clarified,and the strength calculation model is obtained.The failure modes of concrete mechanics test can be divided into two types:the failure at the bond of aggregate slurry and the joint failure of aggregate slurry.The incorporation of auxiliary cementitious materials reduces the early mechanical properties of concrete.After tailings mixed sand replaced natural sand,the compressive strength of concrete aged 28 days increased by about 9%.When the limestone powder content is 5%,the early mechanical properties of high strength concrete do not decrease significantly.When the limestone powder content further increases,the mechanical properties of concrete will decrease.When the silica fume content is 10%,the improvement effect on mechanical properties is the best.The compressive strength of concrete is positively correlated with the proportion of manufactured sand in tailings mixed sand.When the proportion of manufactured sand is 75%,the compressive strength of concrete is the highest.The calculation model of compressive strength and splitting tensile strength of concrete can be fitted by relational expression Y=aln t+b.2.The evolution law of bond performance between composite limestone powder-tailing mixed sand concrete and reinforcement was revealed,and the bond-slip constitutive equation was established.The increase of water-binder ratio or the incorporation of auxiliary cementitious materials reduces the bonding performance between concrete and steel bars.After replacing natural sand with tailings mixed sand,the ultimate bond strength of concrete and steel increases,and the peak slip decreases.When the proportion of manufactured sand is 75%,the ultimate bond strength is the largest,and the relative slip between steel bar and concrete before yielding is the smallest.The ultimate bond strength decreases with the increase of anchorage length,and the peak slip shows a similar rule.The relationshipτ_u/f_ts=a+b*d/l_abetween ultimate bond strength and anchorage length is obtained by fitting.The range of parameters a and b in the bond-slip constitutive equation was determined by analyzing the test data.3.The evolution law of hydration heat performance of composite limestone powder cementitious system was obtained,and the influence of limestone powder and silica fume on the early hydration process was clarified.The gelling system with low water binder ratio releases most of the heat in two days.Larger limestone powder or silica fume content has obvious’peak cutting’effect on hydration temperature.The early hydration rate and heat release increased first and then decreased with the increase of limestone powder content.The addition of silica fume can promote the end time of induction period and the time of entering the hardening deceleration period in advance,but the promotion effect on hydration reaction is limited when the silica fume content is large.4.The change rule of early cracking performance of composite limestone powder-tailings mixed sand concrete is given,and the early cracking trend of concrete is analyzed.The early cracks of concrete developed rapidly within 6 h,and then developed slowly.When the content of limestone powder is 10%,the total cracking area and the maximum width of cracks reach the minimum.When the content of silica fume increases within 15%,the crack resistance increases.The crack resistance of tailings mixed sand concrete is better than that of ordinary concrete.The maximum crack width and total crack area of concrete are negatively correlated with the proportion of manufactured sand in mixed sand.Standardized elastic modulus and tensile strength are introduced to analyze the early cracking trend of concrete.5.The microstructure and phase composition of hydration products of composite limestone powder-tailings mixed sand concrete were proved,and the mechanism of mechanical properties,hydration heat properties of cementitious system and early cracking performance of concrete were revealed from the microscopic level.In the diffraction images at the early hydration stage,C₃S and C₂S obviously existed.With the increase of age,the peak value of C-S-H increased significantly.Appropriate amount of limestone powder was added to improve the connection between aggregate and slurry.With the increase of silica fume content,more C-S-H and C-A-H are generated.When the proportion of manufactured sand is low,the internal structure of concrete is uneven and not dense,which reduces the performance of concrete.This paper has 74 figures,67 tables,109 references.