A Geopolymer Synthesized From Calcined Ore-dressing Tailing Of Bauxite And The Mechanism Of Performance Evolution

Geopolymer is referred to as a green building material and a new kind of eco-friendly cementitious material due to the low energy requirements compared to traditional OPC and associated low CO2 emissions from using common aluminosilicate wastes. The ore-dressing tailing of bauxite(hereinafter referred to as tailing), an aluminosilicate-rich byproduct of ore-dressing-Bayer process or ore-dressing-sintering process, is a new kind of solid wastes of Chinese alumina industry in addition to red mud. In order to use this aluminosilicate-rich tailing in preparation of this low-carbon emission geopolymer, a geopolymer with excellent performance synthesized for tailing is firstly investigated, and then the effect of various factors such as temperature on phase change, microstructure evolution and strength development is studied as well. The expected innovations and results are as follows:A geopolymer which can harden at normal temperature is synthesized from calcined tailing heated at 800 oC for 1 hour, slag and sodium silicate solution. It can provide excellent performance such as high strength(greater then 50.0 MPa for 28 days) and resistance to aggressive solution such as sulfate.Strength development, hydration products and microstructure strongly depend on the curing condition. The early strength can be enhanced by high-temperature curing in hot water or steam at 60 oC due to the acceleration effect of high temperature on geopolymerization. However, the deterioration is observed in middle-long-term mortars because of the formation of crystalline phase in gels and the resultant interface between gels and crystallization area. The strength of mortars cured at normal temperature for 6 years gradually increases because the unremitting geopolymerization leads pores to be fully filled with aluminosilicate gels and accordingly results in the more and more compact microstructure.The studied geopolymer shows its potential to be used in severe conditions such as low temperature or elevated temperature. The low temperature(1 oC and-13~17 oC) leads the lower strength compared to the standard curing because of the retarded effect of low temperature on reaction, but a gradual strength gain is found and 90 days strength thus can reach up to more than 60.0 MPa. The results of unreacted sodium silicate solution in TG-DSC analysis and unreacted tailing in ESEM analysis confirm the retarded effect of low temperature. Nevertheless, mortars cured in low temperature show the more compact microstructure due to the better quality of reaction which leads gels to fill pores well. After exposure to elevated temperature lower than 800 oC the residue strength of mortars is relative high. Despite dehydration and dehydroxylation occurrence, the aluminosilicate products of geopolymerization are still presented in amorphous gels, which leads mortars to survive thermal damage in elevated temperature up to 800 oC. After heating at 1000 oC, decomposition of gels, formation of crystalline phases and reconstructive phase transition of quartz from α-quartz to α-tridymite jointly lead macropores with dimension of micrometer to increase and thus cause a remarkable reduction of strength. The certain calcium species can accelerate the setting and hardening of geopolymer, but different calcium species has different acceleration effect because of their distinct characteristics. The order of acceleration is soluble salts > semisoluble calcium species > calcium oxide. Soluble salts such as Ca Cl2·6H2O intensively releases Ca2+ and immediately forms Ca(OH)2, thus produces many heterogeneous nucleation templates to accelerate reaction. In addition, Ca2+ simultaneously reacts with silicates in solution to form C-S-H gels with the low Ca/Si. So it can be concluded that heterogeneous nucleation templates and the formation of C-S-H gels collectively accelerate the setting and hardening. The primary accelertion mechanism of semisoluble calcium species such as Ca(OH)2 is gels formation because of the lower solubility and the slower release of Ca2+. The insuloble salt such Ca CO3 is just filler and has unconspicuous chemical effect. The special calcium species such as Ca O shoud initially react with water to form Ca(OH)2, and the resultant solid percipitate plays a role of matrix for heterogeneous unclration. In addtion, this reaction induces a sudden groth of local alkality in solution due to the comsumption of water and thus triggers gopolymerization of aluminum monmers and silicate monmers.