Preparation And Hardening Mechanism Of Low Calcium Cementitious Material Made By Bayer Red Mud

The use of Bayer process red mud to prepare new low-calcium cementitious materials is the most promising topic for large-scale resource utilization of red mud.In this paper,a new type of low-calcium cementitious material was prepared by Bayer process red mud as the main raw material?>50%utilization rate?.The hardening process and hardening mechanism of the resulting low-calcium cementitious material was researched under different curing conditions,such as standard hydration curing,the carbonation curing,and the carbonation curing combined with subsequent standard hydration curing.The carbonation reaction kinetic model of the prepared low-calcium cementitious material was established based on the heat of carbonation reaction.The mechanism of carbonation curing to enhance the hardening properties of this cementitious material was also revealed.The specific findings are as follows.1.A new low-calcium cementitious material was prepared by calcining Bayer process red mud and limestone at 1200°C.The resulting cementitious materials were mainly composed of C₂S,C₄AF and C₁₂A₇.The formulation and mineral formation process of this cementitious materials was also proposed.The alkali in Bayer process red mud can stabilize the high-temperature metastable C₂S mineral crystal form??-C₂S?,and most of the alkali is mainly dissolved in the C₁₂A₇ mineral.The morphology and distribution of each mineral in low calcium cementitious materials are as follows:C₂S is mainly elliptical,C₄AF is needle-like particles,and C₁₂A₇ is filled in the middle of C₂S and C₄AF.The low-calcium cementitious material is capable of hardening after hydration.And the hydration and hardening process is as follows.The hydration of C₄AF and C₁₂A₇ minerals,to form C₃AH₆ and AH₃ gel in the form of foil,are the mainly reaction in the early stage of hydration.The hydration products with high porosity are not dense.In the late hydration stage,C₂S can react with the C₃AH₆ and AH₃ gel formed by early hydration to form silicon-containing C₃ASH₄,which increases the density.However,excellent compressive strength of this low-calcium cementitious material cannot be obtained after hydration curing process.And no calcium hydroxide is formed in the hydration product.2.Carbonation curing process can significantly enhance the early hardening performance of this cementitious material.At room temperature,the thermodynamic sequence of carbonation reaction of each mineral is C₁₂A₇>C₄AF>C₂S.The temperature rises caused by the carbonation of C₁₂A₇ mineral are the highest,followed by C₄AF minerals,and the C₂S mineral is the smallest.The carbonation degree of the three minerals is ranked as follows,C₁₂A₇>C₄AF>C₂S,which is consistent with the carbonation reaction activity of each mineral.The isothermal calorimeter to test the carbonation reaction heat of the test material was designed,and the carbonation reaction kinetic model of C₂S was established.The carbonation process of C₂S was divided into three stages,and different stages were controlled by different reaction processes.The effects of water to cementitious ratio,carbon dioxide pressure and other conditions on the carbonation reaction of dicalcium silicate were researched.The carbonation reaction is mainly focused in the initial 2 hours and subsequent carbonation rate will be greatly reduced,reaching a higher reaction degree within 24 h.By controlling the appropriate carbonation conditions,short-term carbonation curing can achieve the purpose of significantly improving the performance of the cementitious material.3.The main carbonation products of C₂S are CaCO₃ crystals and highly polymerized SiO₂ gel,and the carbonation products are mainly produced in the first 2 hours.After carbonation curing process,unreacted C₂S particles are coated with SiO₂ gel.CaCO₃crystals are filled between the pores of the particles to form a dense hardened structure.The carbonation products of C₄AF and C₁₂A₇ minerals are CaCO₃,AH₃ and FH₃ gels.Although the carbonation products of C₄AF and C₁₂A₇ minerals can be formed quickly,the carbonation products are poorly bonded resulting in much lower compressive strength than that of C₂S minerals.In the ternary composite low-calcium cementitious material system,C₂S can form a network skeleton of the hardened paste after carbonation.The appropriate content of C₁₂A₇ and C₄AF can further densify the microstructure of the hardened paste formed by the single C₂S system,which has higher compressive strength properties than the single C₂S system.The main carbonation products of low-calcium cementitious material are calcium carbonate,silica gel,aluminum hydroxide and iron hydroxide gel,forming a dense hardened microstructure,which contribute to the excellent compressive strength of this cementitious materials after carbonation curing process.4.Carbonation curing process can not only enhance the early compressive strength of this low-calcium cementitious material quickly,but also significantly increase the strength in the later stage of hydration curing,indicationg that the carbonation curing process can significantly enhance the hardening performance of this low-calcium cementitious materials.The un-carbonated minerals in the hardened paste after carbonation curing can continue to be hydrated.The reaction products are mainly CaCO₃,AH₃,FH₃ gels and there is a small amount of C₃AH₆ and C₃A·CaCO₃·H₁₁ existed,which is quite different from that of hardened paste after pure hydration curing process.After the carbonation curing process,the low-calcium cementitious material forms a dense hardened structure,and the microstructure of the hardened paste can be further densified after the further hydration curing.Compared with the hardened paste after pure hydration curing test block,the porosity and total pore volume of the hardened paste after carbonation curing process are significantly reduced,and the porosity and total pore volume can be further reduced when the carbonation hardened pastes exposed to a futher hydration curing process.The reaction product formed after carbonation curing process has higher density and elastic modulus,and the elastic modulus of the reaction product can be further improved after a futher hydration curing process.The mechanism of carbonation curing to enhance the hardening property of the low-calcium cementitious material is as follows.?1?The type of reaction product is changed,and more products such as CaCO3 crystals and SiO2 gels with highly polymerized degree are formed,forming a network skleton,and the cementing ability is enhanced.?2?The microstructure and distribution of the reaction product are changed.The foil-like hydration product disappears after carbonation curing,and a denser carbonation product such as CaCO₃crystals and SiO₂ gels is formed.Moreover,the carbonation product is tightly wrapped in the surface of the un-reacted particles and the carbonation products are closely bonded,and the microscopic density is obviously improved.?3?The porosity and total pore volume of the hardened paste are obviously reduced.?4?The cementation ability and the elastic modulus of the reaction product is remarkably improved.The results obtained in this work will provide some theoretical and technical guidance for the preparation of new low-calcium cementitious materials using Bayer process red mud and its application in prefabricated components.