Experimental Study On Preparation Of Functional Pervious Concrete Via Multi-stage Utilization Of Coal Gangue Together With Multi-source Solid Wastes

China’s coal-based energy utilization structure makes coal gangue(CG)become one of the industrial solid wastes with the largest emissions.At present,CG has a high production intensity,a large cumulative pile stock,a low comprehensive utilization rate,and a large-scale utilization bottleneck that is difficult to be compatible with high added value,which has caused great pressure on human society and ecological environment,and has seriously restricted the sustainable development of coal industry.Promoting the large-scale consumption of CG resources is of great significance to improve the ecological environment,improve resource utilization efficiency,accelerate economic and social transformation and upgrading,promote the green,circular and low-carbon development of the coal industry and help the implementation of the "double carbon" strategy.However,there are some problems in the disposal and utilization of CG at present,such as high input cost,low added value of products,poor performance and short industrial chain,which limit the development of CG resource.The single value mining for coal gangue is often limited,but strengthening the organic collaboration between CG and multi-source solid waste is more conducive to realizing resource recovery and value enhancement.Based on the innovative concept of synergistic complementarity of solid waste,this paper proposed an innovative technical route for the preparation of functional pervious concrete via multi-stage utilization of CG together with multi-source solid wastes.First,CG based high belite-sulfoaluminate cementitious material(GBSAC)was prepared by synergistic multi-source solid wastes with CG and conducted pilot test.The synergistic properties of GBACM and GBSAC in thermal activated CG,mineral powder and calcium carbide slag terpolymer system were studied.Then,the prepared GBSAC and GBACM were used as cementing agent,and the functional pervious concrete was developed with CG as aggregate and multi-component solid waste.Finally,based on the life cycle assessment method and economic analysis,the environmental impact and economic benefits of CG based pervious concrete are evaluated,and a comprehensive integrated utilization and evaluation system of coal gangue is formed from the preparation of cementing materials to the development of functional pervious concrete to environmental impact assessment and economic analysis.The main work carried out in this paper is as follows:(1)GBSAC prepared from coal gangue,calcium carbide slag,aluminum slag,desulphurized gypsum and steel slag,and the influence mechanism of different calcination temperature and raw material ratio on mineral formation and hydration characteristics of GBSAC clinker was revealed.The mechanical properties and hydration products of GBSAC clinker-FG-SP ternary cementitious material system were analyzed.Based on the pilot test,the relevant process parameters and pollutant discharge were obtained.It was found that increasing the calcination temperature could promote the conversion of C2AS minerals and CaSO4 in the clinker to the target minerals β-C2S and C4A3?.The best results of compressive strength of GBSAC clinker were obtained when the calcination temperature was controlled at 1300℃ and the Al-Si and Al-Si ratios were 2.09 and 1.14,respectively.The addition of FG is beneficial to the development of compressive strength of GBSAC clinker.The synergistic effect of stone powder and FG further deepens the hydration degree of the system and makes the hardening matrix more dense.The optimum content of each component for the preparation of GBSAC cementitious material was 80%of GBSAC clinker,15%of FG and 5%of stone powder.The GBSAC clinker mineral prepared based on the pilot test was well formed,and the compressive and flexure strength of GBSAC reached the SAC cement standard of grade 52.5 in the national standard.The oxygen content,SO2,NOx,CO concentration and particulate matter content of the flue gas during the pilot test were all within the prescribed limit range.(2)The gangue-based composite auxiliary cementitious material(GBACM)was prepared by heat-activated coal gangue(ACG)in collaboration with slag powder(GBFS)and calcium carbide slag(CCS),and the synergistic performance of GBACM and GBSCA cementitious material of heat-activated CG unit system,ACG-CCS binary system and ACG-CCS-GBFS ternary system was investigated.The results show that the 10%doping of ACG is beneficial to the improvement of compressive strength of GBSAC and makes the structural compactness higher.The addition of CCS and GBFS can promote the compressive strength of the system,and an appropriate amount of CCS can promote the formation of AFt phase,but when the dosage of CCS is too large,the excessive alkali environment generated will inhibit the hydration ofC4A3? minerals to a certain extent;on the other hand,the admixture of CCS increases the alkalinity of the system and accelerates the formation of more C-(A)-S-H gels due to the volcanic ash effect of the active SiO2 and Al2O3 components in the ACG and GBFS.which promotes the densification of the matrix.The ternary system of ACG-CCS-GBFS has better synergistic effect with GBSAC.and the strength formation mechanism of the system is due to the high alkali environment provided by CCS on the one hand,which promotes the volcanic ash activity of ACG and GBFS to form more C-(A)-S-H gels filling more system pores,and on the other hand,the[SO42-]in GBSAC under alkali environment react with[H3AlO4]-and Ca2+to produce AFt phase.All AFt phases,Al(OH)3 and C-(A)-S-H composite gel systems are interwoven and filled to form a dense network structure that jointly supports the development of the system strength.(3)A high-performance pervious concrete(HPPC)with both high strength and high permeability was prepared by using a composite cementitious material composed of GBSAC and GBACM as the binder.The mechanisms of different water-binder ratio,aggregate-binder ratio,molding method and GBACM content on the performance of HPPC were revealed,and the durability of HPPC was evaluated by frost resistance and abrasion resistance tests.The study showed that the HPPC prepared based on GBSAC with GBACM as binder has both high strength and water permeability,and the best molding process is to control the vibration time at 6-8s and the molding pressure at 700N under the mixing method of one feeding.The compressive strength,connected porosity,water permeability coefficient and hardened density of HPPC obtained at a water-binder ratio of 0.22 and aggregate-binder ratio of 4.0 were 34.8 MPa.19.80%,7.95 mm/s and 1930.8 kg/m3,respectively,which satisfied the requirements of C30 grade index as stipulated in the industry standard CJJ/T135-2009.The incorporation of 10%GBACM can accelerate the hydration reaction process of the gelling system,so that the compressive strength,frost resistance and wear resistance of HPPC can be improved.In addition,the concentration of leached heavy metals in HPPC is significantly lower than the concentration limits specified in the standard,and the immobilization rate of the hardened matrix for all detected heavy metals is over 90%,revealing that HPPC has excellent environmental friendliness.(4)The coal gangue based pervious concrete(CGPC)was prepared by using self-ignited coal gangue aggregate(SCGA),raw coal gangue aggregate(RCGA)and dynamic coating modified coal gangue as aggregates(MCGA).The physical and mechanical properties,frost resistance,hydration products and heavy metal leaching characteristics of CGPC were investigated.The strength development mechanism of SCGA-based CGPC was analyzed from the structural improvement of the interface transition area between aggregate and matrix.The results showed that the compressive strength and hardening density of CGPC prepared by SCGA and RCGA decreased with the increase of the substitution level of coal gangue aggregate for limestone aggregate,but the permeability coefficient and connected porosity increased to a certain extent.At the same substitution level,CGPC prepared by SCGA has better freezing resistance and compressive strength than CGPC prepared by RCGA.The compressive strength and frost resistance of CGPC prepared by modified coal gangue aggregate have been greatly improved,and all properties far exceed the index requirements of CJJ/T135-2009 industry standard.The strength formation mechanism of SCGA-matrix is the sealing effect of the "shell"structure with low water-binder ratio formed by dynamic coating modification and the volcanic ash effect of GBACM.which significantly improves the structure of the interfacial transition zone and makes it more dense.Secondly,the high strength AFt crystals self-extend and grow in the widely distributed micropores on the SCGA surface to form an interfacial structure resembling "rivet connection",and the resulting "bridge effect" strengthens the interfacial bonding strength between aggregate and matrix.(5)A low-alkalinity cementing material(LACM)was prepared by using GBSAC as substrate and multi-source solid wastes.The effects of the content of each component on the alkalinity,compressive strength and hydration products of LACM were investigated.Then,a low-carbon gangue-based plant-compatible pervious concrete(GPPC)with fertilizer selfrelease characteristics was prepared via using the optimized LACM as binder in collaboration with SCGA.The basic physical property parameters and plant compatibility of GPPC were investigated,and the self-release mechanism of fertilizer was analyzed.The results show that the synergistic effect of phosphogypsum(PG)and quarry tailings(QT)is more conducive to the balanced development of alkalinity and strength of the system.According to response surface analysis,the optimal LACM allocation ratios were 58.3%GBSAC,16.7%PG,8.3%QT.12.5%SCGP and 4.2%GBFS.The GPPC prepared based on LACM and SCGA has compressive strength greater than 10MPa,pH value less than 9.0,water permeability coefficient greater than 10mm/s,connected porosity between 25%-38%,hardening density between 16001800kg/m3 and sustainable leaching characteristics of phosphorus elements.GPPC has good compatibility with tall fescue,ryegrass and zoysia grass.The self-release mechanism of fertilizer in GPPC is that CaHPO4·2H2O and Ca(H2PO4)2 as chemical fertilizers can be selfformed in the matrix for a long time due to the continuous reaction of PO43-in PG with alkaline hydration products and CaCO3 in the matrix.On the other hand,it is related to the low solubility of CaHPO4·2H2O and Ca(H2PO4)2 in pore structure adsorption water and their transport and precipitation in matrix capillary channels.(6)Based on the actual production data and pilot test data of enterprises,the life cycle assessment theory was used to evaluate the environmental impact and carbon emission reduction effect of gangue based pervious concrete,and compared with conventional pervious concrete.The evaluation results show that among all 18 types of environmental impact,gangue based pervious concrete generally has lower environmental impact than conventional pervious concrete,and the total environmental impact value of the two is 0.107 and 0.329.respectively,the former is only 32.5%of the latter.For every 1m3 of concrete produced,gangue based pervious concrete can reduce carbon emissions by 205.55 kg CO2eq compared with conventional pervious concrete.Therefore,the production process of gangue-based pervious concrete is more green and low-carbon.With an annual output of 350,000 m3 concrete mixing plant production line,the production cost of gangue-based pervious concrete and conventional pervious concrete is 205.6 ￥/m3 and 286.65 ￥/m3,respectively,the former is 28.3%lower than the latter.The annual consumption of coal gangue is 572,600 tons,while the consumption of 46,000 tons of calcium carbide slag,30,000 tons of aluminum slag,15,800 tons of desulfurization gypsum and 13,600 tons of steel slag were coordinated,reducing about 72,000 tons of CO2 emissions.The Gangue-based pervious concrete has good environmental and economic benefits.