Design And Optimization Of Alkali-activated Red Mud Based Cementitious Materials And The Investigations On Its Properties

In present, as a type of solid waste, large amounts of red mud are annually generated by alumina industry in our country. Red mud occupied a lot of ground, and also causes environment pollution to soil, groundwater resources and air. Currently, the main applications of red mud are building materials, utilization in recycling valuable metals and absorbent materials for environmental protection fields. However, due to the high alkalinity,low metal content of red mud, the processing cost is very high; it is difficult to achieve the effect of circular economy and there is no effective way to deal with large-scale industrialization, thus the abundant red mud discharged from alumina industry was mostly left unemployed. In this thesis, the influence of red mud types, supplementary cementitious material and content and modulus of water glass on the composition and structure of red mud based alkali activated cementitious materials was investigated, and its basic mixture was determined; based on this, the performance and reaction products of red mud based alkali activated cementitious materials cured under different curing systems was also investigated.Finally, the thermal insulation mortar based on red mud based alkali-activated was prepared.The specific research results included:(1) The influence of red mud types, supplementary cementitious material(Fly ash and slag)and content and water glass modulus of water glass on the composition and structure of red mud based alkali activated cementitious materials was investigated. Alkali activated sintered red mud cementitious material presented good mechanical properties(3 days compressive strength was up to 36.3MPa without calcination and supplementary cementitious materials);however, it was difficult for alkali activated Bayer red mud cementitious material to set and harden. Under the room temperature curing, the early strength of alkali activated fly ash was low and the strength development was slow, indicating that fly ash was not suitable for preparing alkali activated sintered red mud cementitious materials. On the other side, a higher mechanical strength(especially the early strength) was related to a higher content of slag in red mud based alkali activated cementitious materials. The early strength of the red mud based alkali activated cementitious materials increased with the increase of the modulus and content of water glass; however, the influence of water glass on the strength decreased withthe curing age. Based on the above results, the basic mixture of red mud based alkali activated cementitious materials was determined: 70 % original sintered red mud, 30 % slag, and 5 %water glass with a modulus of 2.4. By using this mixture,when cured at room temperature, red mud alkali activated cementitious materials present a 3 days compressive strength higher than40 MPa, and 28 days compressive strength higher than 60 MPa. The setting time of red mud based cementitious material activated by water glass was about 30 minutes, and the final setting time was about 40 minutes. There were two main exothermic peaks on the heat release curve: the first peak at about 7 minutes was the dissolutions material exothermic peak of raw materials; the second peak at about 50 minutes was alkali activated reaction exothermic peak;the alkali activated reaction rate was fast and the main exothermic reaction was completed within 5 hours.(2)The effects of curing temperatures on the properties of red mud based alkali activated cementitious materials were studied. The experimental results indicate that increased curing temperature and extended curing time of the higher temperature lead to a denser structure of reaction products and improved early strength. When the curing temperature was in the range of 20℃-60℃, the early strength of red mud based alkali activated cementitious materials increased with the increase of curing temperature, increasing the curing temperature can promote the formation of reaction products, thus leading to a denser structure and significantly increased strength. The beneficial effect of curing temperature effect was more obvious for the early strength of red mud based alkali activated cementitious materials, when the curing age was extended, the later strength was similar at different curing temperatures.The curing system didn’t change the reaction products of red mud based alkali activated cementitious materials, the reaction products were amorphous substances, including hydrated calcium silicate(C-S-H) and hydrated sodium silicate(N-A-S-H), in some regions, Si was replaced by Al to form hydrated calcium alumino-silicate gel products.(3) The effects of aggregates, water glass, emulsion powders and water/cement ratio on the performance of the red mud based alkali activated thermal insulation mortar were investigated.The dry density and thermal conductivity of thermal insulation mortar were mainly influenced by the aggregates; the strength development of thermal insulation mortar was influenced by water glass; Re-dispersible emulsion powders didn’t effectively increase the strength ofthermal insulation mortar, but improved the deformation capacity and durability of mortar; the water/cement ratio changed the concentration of water glass, then influenced the activation effect and mechanical properties of red mud based alkali activated thermal insulation mortar.Based on the above results, the optimum mixture of red mud based alkali activated insulation mortar are as follows: 100 g red mud based alkali activated cementitious materials, water to cementitious materials ratio of 1.8, 100 g expanded perlite, 4 g emulsion powder, 60 g water glass. The prepared red mud based alkali activated thermal insulation mortar conform to the technical requirements of national standard II thermal insulation mortar.