Study On Preparation And Properties Of Water-permeable Bricks Derived From High-content Fly Ash By Autoclaving

Fly ash（FA）is fine ash material captured from the boiler flue gas after coal combustion.With the improvement of industrialization and urbanization,the industrial solid waste production of China had also shown a rapid growth trend.With the explosive growth of China’s thermal power installed capacity,the amount of FA production had also increased dramatically.Its treatment and comprehensive utilization need to be solved urgently.FA can effectively replace part of the cement and is widely used in building materials due to its activity.However,the production of building materials generally owns a low value added.This paper studied the water-permeable bricks with content of FA more than 50 wt%by autoclaving,and explored the mechanism of strength formation and water permeability of water-permeable bricks.The durability such as the freeze resistance and carbonization resistance of water-permeable bricks were tested.Then,a pilot production line of water-permeable bricks was designed based on the above experimental results.The main contents in this study are presented as follows:1.Study on optimization of the process of water-permeable bricks prepared with high content of FA by autoclavingFor achieving the technical index of the FA content larger than 50%,the granular FA was produced by the mixing of part fine FA and granulated blast furnace slag（GBFS）with alkali activation.The strength of granular FA can reach 58.8 MPa.Granular FA need crushing and screening treatment,it can be put into the water-permeable brick as composite aggregate with fine FA,pebble,lime and sodium silicate.The effects of the strength and amount of granular FA,the amount of pebble,the amount of lime and the sodium silicate dose on the performance of water-permeable bricks were investigated.The optimal ratio of water-permeable brick was fine FA:granulated FA:pebble:lime=30:25:42:3,with the sodium silicate dose of 8 wt%as binder,and water/fine FA mass ratio was 0.15 under forming pressure of 10 MPa,autoclaving temperature of 180℃,steam pressure of 0.8 MPa and autoclaving time of 8 h.The compressive strength and the permeability coefficient of the prepared water-permeable bricks could reach 35.5 MPa and 1.044×10^-2 cm/s,respectively.2.Study on the mechanism of strength formation and water permeability mechanism of water-permeable bricks prepared with high content of FA by autoclavingThe mechanism of strength formation of the water-permeable bricks during autoclaving need to be explored by XRD and SEM.The correlations of porosity with compressive strength and permeability coefficient were researched.The results showed that the hydration products such as C-S-H,hydrated garnet,tombmullite and xonotlite were the main products on the strength of water-permeable bricks.With the increase of autoclaving time,honeycomb structure and plate-like product were formed,leading the formation of dense mesh structures finally.Porosity had a negative correlation with compressive strength and a positive correlation with water permeability coefficient.The water/fine FA mass ratio was the main factor affecting the porosity of the samples.The mercury intrusion porosimetry（MIP）and micro computer tomography（Micro-CT）explored the pore structure and distribution of water-permeable bricks by autoclaving.The higher the proportion of connected pores in the sample was,the higher the water permeability coefficient was.3.Study on durability of water-permeable bricks prepared with high content of FA by autoclavingThe durability of water-permeable bricks by autoclaving under optimal conditions was studied.Water-permeable bricks had good performance in freezing-thawing cycle.The compressive strength loss rate was 18%,which was less than 25%after 100 freezing-thawing cycles.And the mass loss rate was 0.23%,which was less than 5%after 100 freezing-thawing cycles.The water absorption and saturation coefficients increased,and the capillary absorption coefficients remained after 100 freezing-thawing cycles.With the progress of the freezing-thawing cycle,the microstructure of water-permeable bricks gradually deteriorated.The surface was no longer smooth with porosity increased,the amount of cracks increased,and the structure was to be loose.It increased the durability of carbonization resistance by resisting the loss of the water content in water-permeable bricks.When the water-permeable bricks were carbonized,Ca CO₃ were generated in the system.These materials fill the pores of the water-permeable bricks,reduced the porosity of the water-permeable bricks,increased the compactness of the water-permeable bricks and increased the strength.4.Pilot production line designed and construction engineering economic analysis for water-permeable bricks prepared with high content of FA by autoclavingRelying on the autoclave equipped with the FA aerated concrete block production line of Lanxi Tianda Environmental Building Materials Co.,Ltd.,a set of water-permeable bricks pilot plant was constructed.It mainly included the design of the pilot system,the selection of major equipment,the layout of equipment,the design of control methods,and estimating investment and operating costs.The total investment for the production line of an annual output of 4.8 million water-permeable bricks is about 5.01 million yuan.The production cost of each brick is about 1.82 yuan.The net profit of annual sales is 845,000 yuan.All investments can be taken back in 6 years,it can absorb 6000 tons of FA every year.It has the great socioeconomic and environmental benefits.This study took water-permeable bricks prepared by FA by autoclaving as the research object,and discussed the phase transition during autoclaving.The durability of water-permeable bricks prepared by FA by autoclaving were also studied.The pilot production line of water-permeable bricks was designed based on the experimental results.It provides experimental reference for the large-scale consumption of FA and high-value comprehensive utilization methods.