Effects And Hydration Mechanism Of Activator On Cementitious System Of Fly Ash-Titanium Gypsum-Calcium Carbide Residue

Industrial solid wastes such as titanium gypsum,fly ash,and calcium carbide residue is produced in massive amounts and piled up in the open air due to the difficulty of disposal.This not only wastes resources but also pollutes the environment.Therefore,in this paper,fly ash,titanium gypsum and calcium carbide residue were selected as raw materials for the preparation of alkali-activated cementitious materials,and the effects of Ca（OH）₂,Na OH,KOH,Na₂CO₃ and Na₂Si O₃ activators on the setting time,stability and mechanical properties of the system were systematically studied and selected to suit the system,and the leaching of harmful elements was determined to explore the curing effect.Using orthogonal experimental method,the effect of single activator and compound activator on the mechanical properties of the cementitious system was investigated.The hydration mechanism of different activators was analyzed by XRD and SEM microscopic characterization methods.The following conclusions were obtained.（1）The stability of each cementitious system meets the standard,and the addition of each activator can shorten the setting time of the cementitious system to varying degrees and improve the compressive strength of the system,among which the 28d compressive strength is the best when Na OH and Na₂Si O₃ are used as activators.（2）The leaching concentration of Mn ions in titanium gypsum is high,and the alkali-activated fly ash-titanium gypsum-calcium carbide residue cementitious material can effectively reduce the leaching concentration of Mn ions and the harm to the environment.（3）From the results of orthogonal experiments,it can be seen that the early strength of the Na OH system is higher and the late strength of the Na₂Si O₃ system is higher in the single activator cementitious system.Summer room temperature conditioning is more beneficial to the strength than standard conditioning conditions.A higher content of activator is beneficial to the early strength of the system,and a lower content is beneficial to the later strength development;titanium gypsum and calcium carbide residue participated in the hydration reaction to a lower degree in the early stage and had little effect on the strength;with the curing time prolonged,the hydration reaction is gradually involved,and the influence on the strength is enhanced.The strength of the system increased with the increase of content;The compressive strength of the system decreases with the increase of the water-binder ratio.（4）Under the standard curing conditions,the activation effect of the compound activators on the cementitious system is better than that of a single activator,and the 28d compressive strength of the cementitious system is higher,up to 17.51 MPa.The total content of activator is the main effecting factor of the compressive strength at all ages,and the optimal content is 6%;the small ratio of Na OH to Na₂Si O₃ is beneficial to the early strength,and the large ratio is beneficial to the later strength development;the compressive strength reaches its maximum at the water-binder ratio of 0.64.（5）The cementitious system’s main hydration products are C-（A）-S-H gel and ettringite.The activator generates OH^-through hydrolysis to improve the alkalinity of the system,which promotes the depolymerization of fly ash particles and the release of active silica-alumina substances to accelerate the generation of hydration products,while the presence of titanium gypsum is beneficial to the generation of ettringite,which fills in the gel pores to improve the strength of the system.（6）The activation methods of different activators are slightly different,when activated by Ca（OH）₂,Na₂CO₃ and Na₂Si O₃,the hydration products of the system rise continuously with the extension of the curing time;the high alkalinity of the Na OH-activated system inhibits the formation of ettringite,and a large amount of free SO₄^2-is adsorbed by the gel or replaces the silicate ions in the gel,prompting the transformation of ettringite to the AFm,while Fe participates in the reaction to replace Al in ettringite to form Fe-AFm,and the gel is the main hydration product in the system;in the KOH-activated system,SO₄^2-was reduced due to the early formation of transition product syngenite,which was not conducive to the formation of ettringite;with the extension of the curing time,the syngenite disappeared and released SO₄^2-,and the gels and ettringite gradually increased.