Preparation And Performances Of High Strength Autoclaved Bricks From Ceramic Waste Residue

A large amount of ceramic wastes derived from the production of building ceramic(especially polished tiles)occupy a large number of land and cause serious environmental pollution and resource waste,becoming a heavy burden of restricting the sustainable development of building ceramics industry.On the other hand,with the increasing demand of building wall materials and the implementation of the policy of "sticking the ban",it has been focused to fabricate novel wall materials using renewable resources.Therefore,it is very urgent and necessary to explore the techniques and methods for preparing high-performance wall materials using ceramic wastes,which is of great social and economic significance.In this paper,high-strength silicate autoclaved bricks were fabricated using ceramic wastes as the main raw materials.Based on a series of experiments and analytical characterization,the physical and chemical properties of the ceramic wastes were studied.The relationship between the components and the properties of the materials was investigated.The orthogonal experiment was carried out to establish the optimum ratio of raw materials.The effects of technological conditions and technical parameters(mixing,ageing,and forming process and autoclaving system)on the performance of autoclaved brick were studied.The results show that the total content of SiO2 and Al2O3 in ceramic wastes reaches up to88.4% while the total content of K2 O,Na2O and MgO is very low.The crystal phase is mainly quartz and mullite,and the content of glass is 52.3%.The powders have fine particles with an average particle size(D50)of 10.09 ?m.Based on the properties of the above-mentioned ceramic waste residue,SiO2 can be partially dissolved under high temperature alkaline conditions and react with CaO via hydrothermal reaction to form hydration products such as tobermorite and CSH gel,laying the foundation for the preparation of high strength silicate autoclaved materials.The effects of ceramic waste,lime,aggregate,seeding agent,activator and their contents on the performance of autoclaved bricks were discussed by single factor variable testing.Hydrated calcium silicate cement phase can be obtained by high-temperature hydrothermal reaction of silica and calcium offered by ceramic wastes and lime.Skeleton structures are fabricated by aggregates to share external force.Hydrated calcium silicates containing CSH gel and tobermorite cement together with the aggregates and unreacted material particles to constitute the overall strength of the material.The crystallization agent acts as a crystallization center to accelerate crystallization of the hydrated products,which is beneficial for the durability of the product.Activator can promote the effective release of SiO2,thus improving the formation rate of hydrated calcium silicate,and promoting the enhancement of strength.The optimizing composition of raw materials confirmed by orthogonal testing is that ceramic wastes: aggregates: lime: seed agents: activator = 50:33:15:2:0.5(wt.%).The optimum particle gradation of the aggregate is D(2.36 to 5 mm): D(1.7 to 2.36 mm): D(0.83 to 1.7 mm): D(0.38 to 0.83 mm): D(0.15 to 0.38 mm)=10:15:25:40:10(wt.%).On this basis,the preparation conditions and technical parameters were studied.The various ingredients such as porcelain slag powders according to stoichiometric ratios were mixed thoroughly,stirred and aged with water,and then pressed and molded.To ensure full lime digestion,pug should be aging 2 h,followed by the second mixing and molding at 25 MPa pressure.After treatment under constant pressure of 1.2 MPa for 6 h in saturated steam,autoclaved brick samples are obtained.XRD,DTA and SEM were used to study the phase composition,hydration product type and microstructure of the products under different constant temperature and constant pressure.The effect of autoclave system on the performance of autoclave was discussed.The results show that C2SH(A)and other high-alkali hydrated calcium silicate are formed via the reaction of SiO2 with Ca(OH)2 at the initial stage of the hydration reaction,and then become to CSH(B)?tobermorite and other low-alkali hydrated calcium silicate as prolonging temperatures.The hydration products after autoclaving for 4 h is CSH gel and a small amount of fine flakes of tobermorite.As the autoclave time is prolonged to 6 h,CSH gel amount decreases,flaky tobermorite crystal become larger,a variety of material particles are intertwined and the density increases.As the autoclave time is prolonged to 8 h,the crystal volume and internal pores increase and form a network structure with lower density.The hydration product is mainly CSH gel with a small amount of small tobermorite at pressure less than 1.1 MPa.The production of tobermorite increases and crystal becomes larger at pressure higher than 1.2 MPa.Meawhile,the microstructure is transformed from finely divided microcrystals into patchy interspersed densities.But at pressure higher than 1.3 MPa,tobermorite gradually transformes into fibrous xonotlite,resulting in reduced product density and strength.The autoclaved brick samples prepared under the experimental conditions were tested by GB/T2542-2012(“brick test method”)and GB/T11945-1999(“autoclaved lime brick”).The average compressive strength is 44.5 MPa,and the compressive strength and mass loss rate after cyclic freezing and thawing are 41.8 MPa,0.99%,respectively.The comprehensive performance is better than that of the national standard in the highest grade autoclaved brick requirements.The preparation of high-strength autoclaved bricks using ceramic waste can consume a large amount of ceramic waste and resolve the land and pollution problems effectively,thus achieving high value-added recycling and providing excellent performance of the new wall materials for modern buildings.