Study On Strength Formation Mechanism And Optimal Mix Proportion Of Red Mud-fly Ash Geopolymers

As a toxic industrial waste product,red mud causes severe environmental concern due to its strong alkalinity.In this paper,guided by the comprehensive utilization of red mud,geopolymers were prepared by using the red mud as raw material,supplemented by fly ash rich in active compounds under the action of alkali activator.The optimum ratio of raw materials,liquid-solid ratio,curing conditions and appropriate type and concentration of alkali activator were explored,and the mechanical and microstructural of geopolymers was further explored.The main conclusions are as follows:Effects of raw material ratio and liquid-solid ratio on the strength of geopolymers:When RM:FC=8:2,excessive red mud particles hinder the reaction of geopolymers,and the slow strength development leads to poor final strength,but the geopolymers has good toughness.When RM:FC=2:8,the active compounds in fly ash react rapidly in alkaline environment,and the strength of geopolymers develops rapidly but its brittleness is larger.When RM:FC=5:5,the strength and strain rate of the geopolymers are between the other two ratios,but it could effectively improve the utilization rate of red mud on the basis of ensuring the strength,and was selected as the optimal proportion of raw materials.Low liquid-solid ratio of geopolymers will lead to poor fluidity of paste,and appropriate increase of liquid-solid ratio is conducive to the strength development of geopolymers.However,when FA1 and FA2 are used as raw materials,large liquid-solid ratio will lead to excessive fluidity of paste.In order to ensure geopolymers can be demoulded and maintained,the liquid-solid ratio of 0.5 was selected.Effect of curing conditions on the strength of geopolymers:Most of RM-FA1-Na OH and RM-FA2-Na OH geopolymers cured at room temperature cannot be demoulded.Due to excessive alkaline matters,the strength of RM-FC-Na OH geopolymers decreases with the increase of Na OH solution concentration.When the curing temperature raised to60°C,the specimen can be demoulded after curing 24 hours,and the strength develops rapidly.In the dry curing group,the cracking was serious,and the RM-FC-Na OH type geopolymers showed shrinkage cracking,and the cracks were slender but not penetrating.RM-FA1-Na OH and RM-FA2-Na OH geopolymers exhibit expansion cracking with short and wide cracks.There was no cracking occurred in the control group after moisturizing curing.The curing of geopolymers should be carried out under certain humidity.Effect of different alkali activators on the strength of geopolymers:In the geopolymers with Na OH solution as alkali activator,the strength of RM-FC-Na OH type geopolymers increases with the increase of Na OH solution concentration,while the strength of RM-FA1-Na OH and RM-FA2-Na OH type geopolymers increases first and then decreases with the increase of Na OH solution concentration,and reaches the maximum when the Na OH solution concentration is 7.5 mol/L,and further increase of concentration will cause serious pan-alkali phenomenon.The strength of RM-FC-WG,RM-FA1-WG and RM-FA2-WG increased with the increase of mixed solution concentration.In addition,the overall strength of RM-FA-WG type geopolymers is higher than that of RM-FA-Na OH type geopolymers.Strength formation mechanism of geopolymers:The formation of geopolymers structure skeleton is divided into two steps:（1）Fly ash particles dissolve in alkaline solution to form geopolymers precursor.（2）The geopolymers precursor reacted with free metal cations in alkaline solution to obtain silicate aluminate gel.For RM-FC type geopolymers rich in Ca O,in addition to the above reactions,Ca O dissolves in alkaline solution to form Ca（OH）₂,and combines with Si O₂ to form hydrated calcium silicate gel C-S-H.In addition,red mud exists in the form of inert filler in geopolymers,and the degree of participation in geopolymers reaction is poor.