Studies On Mechanical Performance And Durability Of Alkali-salt Compound Activated Bottom Ash-based Geopolymer

Bottom ash is an industrial waste of coal-fired boilers or other combustion equipment in thermal power plant after combustion. According to the different forms of boilers, they can be mainly divided into pulverized coal combustion bottom ash,auto-sulfur retention fluidized bed combustion bottom ash and fluidized bed combustion bottom ash without sulfur retentio. The ingredients of bottom ash varied with boiler types. The bottom ash emission from coal combustion is not only occupancy of land resources, but also leads to a variety of environmental pollutions.Therefore, it is particularly important to carry out the efficient recycling of bottom ash.Aiming at the subsistent problems with low usage rate of bottom ash, single variety and large dosage of activator during the preparation of geopolymer, the fluidized bed combustion bottom ash without sulfur retention was used as the only raw material mixed with alkali-salt activator for the preparation of bottom ash-based geopolymer in the first time. The mechanical properties, mineral phases and microstructure were characterized in the paper. The results demonstrated that when the dosage of alkali-activator Na2SiO3 was 7.7wt %, salt-activator Na2SO4 was 0.5 wt % in the alkali-salt compound activator, the bottom ash-based geopolymer showed the best compressive strength of 46.94 MPa in curing age of 28 d at room temperature, which increased by 15.8% comparing to the sample obtained by single alkali(Na2SiO3)-activation(AA). The XRD, SEM and IR results revealed that the alkali-salt compound activation(ACA) accelerated the mullite, lime hydration reaction in bottom ash, and generated the fibrous ettringite(AFt) and a large number of amorphousgeopolymer gel, so as to lead to the displacement of asymmetric stretching vibration and bending vibration absorptions of Si-O bonds.The brucite fiber reinforced alkali-salt compound activated(BFRACA) bottom ash-based geopolymer was synthesized in the paper. The results indicated that as the content of brucite fiber was 0.8wt%, the fiber reinforced geopolymer exhibited the highest compressive strength of 50.69 MPa and flexural strength of 9.14 MPa after 28 d age, in which the flexural strength increased by 26.6% compared to the zero fiber content. XRD, MIP and SEM results demonstrated that the addition of brucite fiber did not change the mineral phases of geopolymer, and the fibers were tightly wrapped up in amorphous gel generated in the hydration reaction so that the fibers paly on a role of bridge between pulling out and connecting. In comparison with bottom ash-based geopolymer, the brucite fiber reinforced bottom ash-based geopolymer have some advantages, such as the porosity decreased, the pore volume percent of less than 20 nm increased by 17.23%, average pore diameter decreased, the bulk materials more compacted and the mechanical properties improved.The durabilities of the brucite fiber reinforced bottom ash-based geopolymer as well as the bottom ash-based geopolymer were investigated. The results showed that the brucite fiber reinforced bottom ash-based geopolymer has much better durabilities to resistant to high temperatures, sea water and frost. With the increase of calcination temperature, the compressive strength showed the first increase and then decrease.When calcined temperature rises to 800?, the compressive strength of fiber reinforced sample is still up to 51.19 MPa. After the soaking and drying cycles for 28 times, the compressive strength of fiber reinforced sample is 41.83 MPa, flexural strength is 4.64 MPa. The fiber reinforced sample can withstand freeze-thaw cycles up to 40 times so that the frost resistance remains to be improved.