| As a new type of permeable paving material,permeable bricks have promoted the development of"sponge city" and improved urban ecological benefits,and most of the preparation of existing permeable bricks uses natural aggregates.The use of recycled crushed stone instead of natural crushed stone to prepare recycled permeable bricks can not only save natural resources,but also alleviate the environmental problems caused by construction solid waste.Fly ash and mineral powder are common solid waste materials,and their incorporation affects the performance of recycled permeable bricks,and its compressive strength,water permeability and durability are the key factors that determine whether recycled permeable bricks can be widely used.In complex environments,freeze-thaw and sulfate erosion are the main reasons for the decline in durability of recycled permeable bricks.The addition of mineral admixtures can effectively improve the internal pore structure of recycled permeable bricks,slow down the rate of SO42-erosion,and improve their durability.Based on this background,this paper deeply and systematically studies the effects of fly ash and mineral powder doping on the compressive strength,water permeability,sulfate erosion resistance and salt frost resistance of recycled permeable bricks,and analyzes their influence mechanism,which provides a basis for durability evaluation and life prediction of recycled permeable bricks in complex environments.The research content and conclusions of this paper are as follows:First,determine the preparation process of recycled permeable bricks:choose the "aggregate wrapping method" stirring,hydrostatic molding method,forming pressure 5MPa,loading rate 500N/s,holding time 45s and standard maintenance.The effects of water-to-adhesive ratio,target porosity and recycled aggregate particle size grade paired with regenerated permeable brick compressive strength and water permeability coefficient were analyzed by univariate analysis method,and it was concluded that the recycled permeable brick had the best performance under water-adhesive ratio of 0.32,target porosity of 15%and particle size gradation of 4.75～9.5mm.At this time,the compressive strength is 33.6MPa,which meets the requirements of Cc30 strength grade in the specification "Permeable Brick"(JC/T9452005);The water permeability coefficient is 3.12×10-2cm/s,which meets the requirements of Class A permeability grade in "Permeable Pavement Bricks and Permeable Pavement Panels"(GB/T25993-2010).Secondly,the effects of single-doped fly ash and mineral powder on compressive strength,porosity,water permeability coefficient,water retention and frost resistance of regenerated permeable bricks under different substitution rates(0%,10%,20%,30%,40%,50%)were studied.The results show that the incorporation of appropriate amount of fly ash and mineral powder can help to improve the comprehensive performance of permeable bricks,and the enhancement effect of mineral powder on the performance of recycled permeable bricks is stronger than that of fly ash under the same amount of mineral admixture.Among them,the performance of permeable bricks is best when the fly ash content is 10%,the compressive strength is 34.7MPa,the water permeability coefficient is 3.30×10-2cm/s,the water retention is 0.73g/cm2,the mass loss rate after freeze-thaw is 2.24%,and the strength loss rate is 8.96%.The performance of the permeable brick is the best when the mineral powder content is 20%,the compressive strength is 36.3MPa,the water permeability coefficient is 3.31 × 10-2cm/s,the water retention is 0.72g/cm2,the mass loss rate after freeze-thaw is 2.06%,and the strength loss rate is 8.13%.Then,on the basis of single doping,the effects of fly ash and mineral powder re-mixing rate(30%,40%,50%)and the ratio between the two(1:2,1:1,2:1)on the compressive strength,porosity,water permeability coefficient,water retention and frost resistance of recycled permeable bricks were explored.The results show that the comprehensive performance of permeable bricks gradually decreases while the re-doping rate increases.When the re-mixing replacement rate is 30%,the comprehensive performance of recycled permeable bricks is the best,and the recycled permeable bricks in each proportion can meet the requirements of the specification.When the re-mixing replacement rate is 40%,there is an optimal ratio,which is 2:1,at this time,the compressive strength of the permeable brick is 31.8MPa,the water permeability coefficient is 2.87×10-2cm/s,and the water retention is 0.61g/cm2,which meets the requirements of the specification.When the re-mixing replacement rate is 50%,the compressive strength of the optimal mixing ratio under this dosage no longer meets the requirements of the specification,and the re-mixing replacement rate is not recommended.Then,the effects of fly ash and mineral powder single doping and compound doping on the sulfate erosion resistance and salt frost resistance of recycled permeable bricks were studied.The results showed that:① The appropriate amount of fly ash and mineral powder helped to improve the sulfate erosion resistance of the recycled permeable brick,which was optimal when the single fly ash and mineral powder were 20%,and the compressive strength after erosion was 34.7MPa and 36.7MPa,respectively,which were 2.96%and 8.9%higher than the single-doped benchmark group.When the total substitution rate of compound mixing is 40%,the optimal ratio of fly ash and mineral powder is 2:1,and the compressive strength at this time is 32.1 MPa,which is higher than that of fly ash and ore powder under the same dosage,and is 20.22%and 2.56%higher respectively.②With the increase of the substitution rate of single-doped fly ash and mineral powder,the salt frost resistance of recycled permeable bricks first increased and then decreased,among which the strength loss rates were 10.32%and 9.37%,respectively,under the conditions of fly ash content of 10%and mineral powder content of 20%,respectively.The total substitution rate of compound mixing is 40%,and the salt frost resistance is better than that of single 40%mineral admixture when the ratio of fly ash and mineral powder is 2:1,and the strength loss rate at this time is 16.7%.Finally,the porosity,pore diameter,pore contribution rate and pore structure of the regenerated permeable brick in the initial state,after salt freezing and after sulfate erosion are analyzed with the help of Image-Pro-Plus software.The results show that:①Appropriate amount of fly ash and mineral powder can help to improve the pore structure of recycled permeable bricks.②Compared with the single-doped benchmark group,when the fly ash content is 10%,the porosity of recycled permeable bricks is increased by 5.80%,which is 14.6%,and the pore contribution rate of 3～7mm is increased by 22.92%,which is 59%;When the mineral powder content was 20%,the porosity of recycled permeable bricks increased by 14.49%to 15.8%,and the pore contribution rate above 7mm decreased by 44.12%to 19%.When compounded,the porosity was 15.6%and the pore contribution rate was 54%when the ratio of fly ash and mineral powder was 2:1.③Regardless of single or compound doping,the porosity after salt freezing and the pore contribution rate above 7mm increased by about 5%and 15%respectively compared with the initial state;After sulfate erosion,compared with the initial state,the porosity of the regenerated permeable brick decreased,about 3%,and the pore contribution rate below 3mm increased significantly,about 10%. |
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