Basic Mechanics Of Sea Sand Geopolymer-based Recycled Coarse Aggregate Concrete

As a kind of inorganic polymer concrete,the CO₂emission of geopolymer concrete is much lower than that of ordinary concrete.It has the advantages of abundant raw materials,low environmental pollution,early strength,fast hardening and high density.It is considered to be a substitute for ordinary concrete with great development potential in the21st century.At present,the aggregate used to prepare geopolymer concrete is mainly natural aggregate.However,the increasing scarcity of natural resources,construction consumption,reducing its exploitation has become inevitable;on the other hand,a large number of construction solid waste can not be degraded and affect global climate change.The rational use of marine resources and the recycling of construction solid waste have become a hot research field of modern building materials.Therefore,the development of new ecological building materials with low emission,low cost,full solid waste and high durability has become an urgent problem to be solved.Sea sand geopolymer-based recycled coarse aggregate concrete（SGRAC）was prepared by using fly ash and slag as cementitious raw materials,recycled coarse aggregate and sea sand as aggregates,sodium silicate and sodium hydroxide compound solution as alkali activator.On the basis of realizing the efficient utilization of bulk solid waste,construction solid waste and marine resources,it is ensured that SGRAC has stable basic mechanical properties,which lays a solid theoretical foundation for the popularization and application of SGRAC in the field of green low-carbon recycling building preparation and promotes the achievement of“double carbon goals”.The main research and findings of this paper are as follows:（1）By studying the effects of alkali concentration,slag content,fine aggregate type and curing age on the internal structure and basic mechanical properties of SGRAC,the optimum ratio of SGRAC was determined.On this basis,the basic mechanical properties of SGRAC in each group were measured,such as compressive properties,splitting tensile properties,elastic modulus and shear properties.The results show that the optimal mix ratio of SGRAC is that the alkali concentration is 10mol/L and the slag content is 60%,and the corresponding 28d compressive strength,tensile strength and shear strength are75MPa,3MPa and 5MPa respectively.With the increase of curing age,the compressive strength of SGRAC at 1d and 7d can reach 46%and 88%of 28d,respectively,which has the characteristics of early strength and high strength.（2）The strength growth characteristics of SGRAC were studied from the microstructure,macro cube compressive and shear test system.The microstructure and hydration reaction characteristics of SGRAC were analyzed by SEM.The results show that the activation degree and microstructure of SGRAC show a progressive development at different curing ages,which is mainly manifested in its internal color,residual amount of raw materials and hydration products.The content of slag and alkali has a significant effect on the formation of N-A-S-H gel,C-A-S-H gel and C-S-H gel reaction products.Too much fly ash and too little alkali content will lead to the reduction of gel reaction products,which will affect its basic properties.Through the comparison of river sand at different ages,the influence of sea sand and river sand on concrete strength is less than 5%,which has stable mechanical properties.Finally,the prediction formula of cubic compressive strength with age is given.（3）Referring to the Guozhenhai model,combined with the compressive performance characteristics of SGRAC,the parameters of the Guozhenhai model were modified,and a piecewise compressive constitutive model suitable for SGRAC correction was established.Referring to ordinary concrete,the relationship between cube compressive strength and cylinder compressive strength,splitting tensile strength and elastic modulus of SGRAC is fitted.The empirical formula is obtained by regression analysis,and the correlation coefficient is high.（4）The shear strength,ductility coefficient,shear stiffness and expansion angle of SGRAC specimens were systematically studied,and the regular response of SGRAC with the change of compressive stress ratio under the optimal ratio was obtained.Based on the damage evolution of recycled concrete,the SGRAC damage evolution model is determined,and the correction coefficient with the change of compressive stress ratio is given.The full curve results of the model are in good agreement with the experimental results.Furthermore,theξ-r failure criterion and theσ_oct-τ_octfailure criterion are used to compare and analyze the SGRAC under the optimal ratio.Based on the experimental data,a failure criterion based on octahedral spatial stress for SGRAC is proposed.