Study On The Preparation Of Geopolymer From Pretreated Bay Red Mud And Mechanism Of The Strength Formation

The utilization of Bayer red mud (RM) is a worldwide problem due to its large quantity, fine particle size and strong alkaline. The Bayer red mud produced from the diaspore bauxite ores distributed in Henan, Shandong and Shanxi provinces consists of low content of Fe, and high contents of residual Na20 and Al2O3, which could be called as low-Fe Bayer red mud. It is feasible to be recycled as cementitious materials based on geopolymer technology. But its low Si/Al ratio and low activity of aluminosilicates are disadvantageous to the formation of geopolymer structure. This research studied the pretreatment of RM in order to improve the Si and Al leachability to activate the potential of RM to form geopolymer. The geopolymer formulations based on RM were optimized. The mechanism of the geopolymerization process was studied. The feasibility of using RM based geopolymer for the solidification/stabilization (S/S) of fly ash from municipal solid waste incineration (MSWI FA) was also studied. The main contents are as follows:1. Study on activation pretreatment processes for Bayer red mudDirect calcination and calcination with addition of alkali were conducted to obtain thermal activated RM and alkali-thermal activated RM respectively, to improve the dissolution of SiO2 and Al2O3 of RM. The transformations of mineral phase and chemical construction before and after the activation processes were studied. The activation effects were evaluated through the leaching experiment in alkaline solutions. The gibbsite, katoite, muscovite, cancrinite and calcite in the raw red mud decomposed sequentially after calcination. Nepheline, sodium aluminum silicate, gehlenite and some amorphous phases were formed in the thermal activated RM, whereas a-dicalcium silicate and peralkaline aluminosilicate were formed in alkali-thermal activated RM. The leaching ratios of SiO2 and Al2O3 reached the maximum values of 36.6% and 38.9%respectively, when the RM was directly calcined at 800? for 3 h. As for the RM calcined at 800? for 1 h with addition of 10-15 wt% Na2O, the leaching ratios of SiO2 and Al2O3 reached 38% and 60%, respectively.2. Sodium silicate activated geopolymer made from thermal activated RM and study on the geopolymerization mechanismA poorly crystalline C-(A)-S-H phase was formed after the thermal activated RM was activated by sodium silicate solution. But amorphous geopolymer gels were not formed in the binder due to its low Si/Al ratio. Thus, RM-GBFS geopolymer was synthesized by activating the mixture of thermal activated RM and granulated blast furnace slag (GBFS) with sodium silicate solution. The strength of the RM-GBFS geopolymer increased with the activity of the RM enhanced. The compressive strength of the geopolymer synthesized with a RM/GBFS ratio of 7:3, by adding 8 wt%(Na2O) of sodium silicate solution with a modulus of 1.7, reached 55 MPa at the curing age of 28 days.The geopolymerization mechanism study showed that by adding GBFS to improve the Si/Al ratio of the geopolymer formulation, amorphous geopolymer gels were formed in the binder. The active aluminosilicates in the solid materials dissolved in the alkaline solution to form Si(OH)4 and A1(OH)4-monomers, which would polymerize to form nano-sized geopolymeric micelles, and then the geopolymeric micelles aggregated to form homogeneous and high density matrices. Due to the synergistic effect between RM and GBFS, composite structure with C-(A)-S-H gels acted as framework and geopolymer gels acted as filler, which led to the improved mechanical properties of the RM-GBFS geopolymer.3. Synthesis of one-part geopolymer from alkali-thermal activated RM and study on the geopolymerization mechanismThe alkali-thermal activated RM could harden after blended only with water similar as ordinary Portland cement (OPC) and the obtained geopolymer was named as one-part geopolymer. The one-part geopolymer could obtain a compressive strength near 10 MPa at the curing age of 7 days. But the structure of the binder deteriorated after 7 days, with a loss of the strength. By adding 20?30 wt% of silica fume (SF) to improve the Si/Al ratio of the geopolymer formulation to the satisfactory range, the long-term strength of the RM-SF geopolymer increased significantly. The compressive strength of the one-part geopolymer synthesized with a RM/SF ratio of 3 reached more than 30 MPa at the curing age of 28 days, by adding 0.5 wt% of sodium lignosulphonate as dispersant to reduce the water/solid ratio to 0.45.The a-dicalcium silicate and peralkaline aluminosilicate in the alkali-thermal activated RM dissolved in water and formed flake-like hydration products, providing the early strength of the one-part geopolymer, and forming alkaline environment and providing free Na, Al and Si for geopolymerization process. But the flake-like hydration products dissolved again in the long-term curing due to the low Si/Al ratio of RM, leading to the increase of pH, damage of the structure and deterioration of the strength. The dissolution of SF could consume the OH-ions in the pore fluid, and form free Si to participate in the geopolymerization process, which thus improved the Si/Al ratio and led to the formation of nano-sized geopolymeric micelles. Typical amorphous geopolymer matrices were formed in the completely cured binder by the aggregation of geopolymeric micelles. In addition, MSssbauer spectra analysis showed that the Fe in RM existed in hematite and isomorphism of aluminosilicates in the form of Fe?. No significant transformation occurred after the geopolymerization process, which indicated that the Fe in RM had no significant adverse effect on the geopolymerization process.4. Research on the S/S of MSWI FA using RM based geopolymerBy using alkali-thermal activated RM as a S/S reagent for MSWI FA, synergistic effect was generated between the two materials. The MSWI FA acted not only as a S/S target, but also an aluminosilicate source for geopolymerization, which promoted the formation of geopolymeric micelles and geopolymer matrices, and improved the stability of the structure by improving the Si/Al ratio of the aluminosilicate gel. With the addition of S/S reagent in the range of 50?60 wt%, the S/S effects for heavy metals of the RM-based geopolymer were close to that of OPC, and the effects followed this descending order:Pb> Cu> Zn> Cr. Most of the Zn, Pb and Cu in the MSWI FA transformed from the exchangeable fraction and carbonates/specifically adsorbed fraction into the Fe-Mn oxides fraction and residual fraction that were hard or unavailable for leaching, thus the leaching risk decreased significantly.This research work provides new utilization approaches and theoretical bases for the recycling of Bayer red mud. It will also contribute to the comprehensive utilization of other solid aluminosilicate wastes through geopolymer technology.