Preparation, Characterization And Performance Of Geopolymer Binder Based On Tailings Of Vanadium Extraction From Stone Coal

Geopolymer is a new kind of inorganic cementitious material that consists of SiO4and AlO4tetrahedra linked alternately by sharing the oxygen atoms. It is usually prepared from industrial waste residue as source materials, and the production process is relatively simple. Hence, it has good prospects. With the rapid development of vanadium extraction from stone coal, large amounts of tailings of vanadium extraction from stone coal (called "vanadium-extraction tailings" for short) have been produced, which not only occupy vast land but also cause secondary environmental pollution. The vanadium-extraction tailings consist mainly of Si and Al, thus they might be utilized as source materials for geopolymer synthesis. However, compared with conventional aluminosilicate source materials such as metakaolin (MK) and fly ash (FA), the tailings contain markedly higher Si and lower Al contents, and their reactivity are low. Hence, the tailings have seldom been used as base materials for geopolymer synthesis.In this study, wet-alkaline roasting, dry-alkaline roasting and dry mechanical ball milling were employed as the pretreatment for activation of the raw vanadium-extraction tailings (RVT). Reactivity of the RVT and the activated vanadium tailings (AVT) were evaluated via alkaline dissolution tests, X-ray diffraction (XRD), Fourier transform infared spectroscopy (FTIR) and scanning electron microscopy (SEM). The experimental results showed that, the wet-alkaline roasting pretreatment was the best way to promote the decomposition of the vanadium-extraction tailings'crystalline structure, and thus it can enhance reactivity of the vanadium-extraction tailings most effectively.Combined with the aluminum correcting materials (including sodium aluminate, MK, FA and aluminate cement), the selected AVT were mixed with alkaline silicate solution (which was created by the reaction between silica fume and sodium hydroxide solution) to prepare geopolymer products. Moreover, considering that the "alkaline silicate solutions" used as activators in most studies were corrosive and unfavorable for the practical application of geopolymer, the RVT were simultaneously used as base materials to create the geopolymer cement powder. With compressive strength as the investigation index, orthogonal experiments were conducted to optimize the geopolymer composition for each type of aluminum correcting materials (or calcium-bearing source materials). For the optimal geopolymer samples, autoclaving process was adopted to further enhance their compressive strength. The early-aged geopolymeric reaction and microstructure of the hydration products were characterized by SEM, Environmental Scanning Electron Microscope (ESEM), FTIR, attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra,27A1solid Magic Angle Spinning-Nuclear Magnetic Resonance (27Al MAS-NMR), XRD and TG-DSC analyses. The results indicated that, both microstructure and compressive strength of the geopolymer samples can be effectively improved by autoclaving process, and the FTIR and27Al MAS-NMR analyses confirmed that during geopolymeric reaction the AlO4tetrahedra was linked to the SiO4tetrahedra, forming three dimensional network structure. Moreover, the ESEM and ATR-FTIR in-situ analyses revealed that the alkaline roasting process can enhance the rate and extent of the early-aged geopolymeric reaction, thus the later hardened mortar samples' compressive strength and microstructure can be improved. The compressive strength of the geopolymer cement mortar samples which adopted ore residue (KZ) as calcium-bearing source materials were generally higher than those of the geopolymer cement mortar samples which adopted steel residue (GZ) as calcium-bearing source materials. The XRD, TG-DSC and FTIR analyses jointly confirmed that aluminosilicate gel and hydrated calcium silicate (C-S-H) gel coexisted in the hydrated products of the geopolymer cement powder, and the appropriate amounts of CSH can improve the microstructure and early compressive strength of the samples.Subsequently, the durability and thermal stability of the selected geopolymer samples and ordinary Portland cement (OPC) mortar sample were investigated. The durability included the following aspects:chemical errosion resistance, freeze-thaw resistance and impermeability resistance. The thermal stability was evaluated in terms of the change of compressive strength and microstructure after heat treatment. The results revealed that, durability and thermal stability of the partial optimal geopolymer samples were similar or superior to those of the OPC mortar samples.Finally, further studies were conducted to decrease the efflorescence behavior of geopolymer sample. The geopolymer samples with the greatest presence of efflorescence, were selected and subjected to calcination at600℃for3h and autoclaving process at150℃for5h, respectively. Considering that efflorescence was caused by the transportation of soluble salts from inner of the structure to the surface of the samples, the geopolymer samples were immersed in deionized water for certain period, and the extent of efflorescence was evaluated by analyzing the Na concentration in the leaching liquid. The results demonstrated that, by means of the calcination pretreatment and/or autoclaving process, the Na concentration in the leaching liquid can be significantly decreased, thus the efflorescence behavior of geopolymer sample can be effectively prevented.This study has confirmed that the vanadium-extraction tailings can serve as favorable aluminosilicate source materials for synthesizing geopolymer binder materials.