Preparation Of Autoclaved Brick By Waste Concrete And Waste Clay Brick

In recent years,with the acceleration of China’s urbanization and urban infrastructure construction,the output and stock of construction waste,mainly including waste concrete and waste clay bricks,have been increasing year by year,and become an important source of urban solid waste.Recycling of construction waste is of great significance to saving resources and protecting environment,which is directly related to the sustainable development of building materials industry and construction industry.From the perspective of resource utilization,the calcium component（CaCO₃）provided by coarse aggregates and the silica component（SiO₂）provided by fine aggregates are the main components in the waste concrete,which account for about 80%of the total composition of concrete.The abandoned clay bricks are mainly clay minerals with low calcium and SiO₂accounts for about 70%of the chemical composition.Therefore,in theory,waste concrete and waste clay bricks can be used as calcareous and siliceous raw materials for the preparation of autoclaved silicate materials.At present,the research on the preparation of autoclaved silicate products from construction waste mainly focuses on the wall materials such as autoclaved brick and autoclaved block.In related studies,construction waste is only used as siliceous feedstock or as aggregates.In fact,there are sufficient calcium resources in the form of calcite and C-S-H gel in construction waste.After calcination at high temperature,CaO can be obtained and participate in hydrothermal synthesis reaction to provide calcium raw materials for autoclastic silicate materials.In this paper,the waste concrete and waste clay bricks are used as the main calcareous and siliceous raw materials,and the crushed and screened waste concrete can be used as fine aggregates to prepare autoclaved bricks by hydrothermal synthesis,which provides a new technology for the development of green wall materials while realizing the efficient resource utilization of construction waste.The research content of this paper mainly includes two aspects:firstly,taking the mixture（containing 70%waste concrete and 30%waste clay bricks）as the object,with the goal of reducing calcination energy consumption and making full use of effective calcium resources in the mixture,the appropriate calcination process for the mixture was determined by studying the effects of heating rate and reaction atmosphere on the decomposition of CaCO₃in the waste concrete.On this basis,the calcined mixture and two kinds of fine aggregates（recycled fine aggregates of waste concrete and natural sands）were used as raw materials to prepare autoclaved bricks.The effects of water-solid ratio,cement-aggregate ratio,fine aggregates gradation and calcium-silicon ratio of mixture on the compressive strength of autoclaved bricks were studied,and the optimal raw material ratio was optimized by micro-analysis.The influences of autoclaved pressure,autoclaved holding time,molding pressure,load-holding time and static stop time on the compressive strength of autoclaved brick were studied,and the suitable preparation process was optimized based on microanalysis.Furthermore,the pore structure of the autoclaved brick in the preferred group was studied.The main research conclusions are as follows:（1）With the heating rate accelerating,the decomposition temperature of CaCO₃in the waste mixture increases.The effect of heating rate on CaO content and reactivity is related to calcination temperature.At 760℃,CaO content and reactivity decrease with the increase of heating rate,and the effect of heating rate is weakened with the increasing of calcination temperature.Water vapor reaction atmosphere can promote the decomposition of CaCO₃.The content and reactivity of CaO increased with the content of adding water.Under the experimental conditions of this paper,the optimal calcination process of the mixture is as follows:calcination temperature is 760℃,heating rate is 13℃/min,100g mixture added with0.3L distilled water.（2）Both the water-solid ratio and the cement-aggregate ratio have significant effects on the specimen forming and strength,and there is a matching relationship between them.The reclaimed fine aggregates group should adopt high water-solid ratio and high cement-aggregate ratio.The optimum gradation of reclaimed fine aggregates and natural sands is similar.It is beneficial to improve the strength of autoclaved specimens that the content of large aggregates is relatively small and the content of medium and small aggregates is moderate.Under different autoclaved systems,the effect of calcium-silicon ratio on the compressive strength of autoclaved specimens is different.Under the experimental conditions of this paper,the optimal raw material ratio scheme of recycled autoclaved brick is as follows:recycled fine aggregates group（water-solid ratio 0.15,cement-aggregate ratio 6/4,calcium-silicon ratio 0.7）,natural sands group（water-solid ratio 0.15,cement-aggregate ratio5/5,calcium-silicon ratio 0.9）.（3）Autoclaved pressure,molding pressure and static stop time have significant effects on the compressive strength of autoclaved specimens,while the influence of holding time has no obvious rule,and the influence of load holding time can be ignored.With the autoclaved pressure increasing,the strength of autoclaved specimens mixed with recycled fine aggregates increased significantly on the whole.The strength of autoclaved specimens mixed with natural sands increased significantly at first,then decreased,and even decreased in groups with calcium-silicon ratio of 0.7 and 0.9.Under the condition of higher calcium-silicon ratio,the strength of autoclaved specimens mixed with recycled fine aggregates showed an overall increasing trend with the extension of holding time.With the increase of forming pressure,the strength of the autoclaved specimens at the inflection point of 25MPa increases significantly first and then decreases slightly.With the prolongation of static stop time,the strength of autoclaved specimens increases first and then decreases.Under the experimental conditions of this paper,the optimal preparation process of autoclaved brick is determined as follows:autoclaved pressure is 1.0MPa,holding time is 8h,forming pressure is 25MPa,static stop time is 1h in the reclaimed fine aggregates group,and static stop time is 4h in the natural sands group.（4）The hydrothermal products of autoclaves are mainly CSH（B）and tobermorite.At low autoclastic pressure,the hydrothermal products are mainly CSH（B）phase.Under the optimal cement-aggregate ratio,when autoclaved pressure is over 1.0MPa and heat preservation is 8h,a small amount of acicular calciosilicate phase can be formed.（5）Under the best experimental conditions of this paper,the maximum compressive strength of the autoclaved specimens mixed with recycled fine aggregates and natural sands is44.93MPa and 51.1MPa,and the porosity is 37.15%and 30.5%.