Research On The Phase Diagram Of MgO-MgSO₄-H₂O System And The Hydration And Hardening Rule Of Basic Magnesium Sulfate Cement

Basic magnesium sulfate cement（Basic magnesium oxysulfate cement,BMSC）is a new kind of green magnesia-based cementitious material,which is composed of magnesium oxide,magnesium sulfate,water and a small amount of chemical additives,sets and hardens in the air.The main hydration product is the basic salt hydrate of 5Mg（OH）₂·MgSO₄·7H₂O（5·1·7 phase）.BMSC has many excellent properties,such as light weight,low alkali,early strength,high strength,fire protection and reinforcement and so on.In this paper,the hydration process of MgO-MgSO₄-H₂O system was studied by means of isothermal dissolution method（Isothermal dissolution method,IDM-solid phase method）and electrochemical impedance spectroscopy method（Electrochemical Impedance Spectroscopy,EIS-liquid phase method）.The relationship between the molar ratio of raw materials and hydration products of MgO-MgSO₄-H₂O system and the phase transition rules of the hydration products were revealed,which enriched the basic theory of BMSC materials and chemistry.Through the test and characterization of basic physical properties and microstructure,the effects of chemical additives,concrete additives,mineral admixtures,CO₂ curing and carbonization on the hydration and hardening rules and microstructure properties of BMSC were studied.The mechanism of chemical additives and mineral admixtures on BMSC modification is revealed,which provided theoretical guidance and data support for the application research of BMSC and its products.The research contents and results are as follows:（1）The hydration product composition and its phase transformation rules of MgO-MgSO₄-H₂O system at 25℃were studied by IDM-solid phase method.The results show that the formation of hydration products in MgO-MgSO₄-H₂O system is closely related to the concentration of Mg²⁺in the liquid phase.When the concentration of Mg²⁺is low,the hydration products of MgO-MgSO₄-H₂O system without chemical additives are mainly 3Mg（OH）₂·MgSO₄·8H₂O（3·1·8 phase）and Mg（OH）₂,and the 3·1·8 phase is formed by the transformation of the unstable Mg（OH）₂·MgSO₄·5H₂O（1·1·5phase）into 5Mg（OH）₂·MgSO₄·3H₂O（5·1·3 phase）.The content of 3·1·8 phase increased,and the content of Mg（OH）₂ decreased with the increase of MgSO₄ concentration;the minimum concentration of MgSO₄ required for the formation of 3·1·8 phase increased with the increase of MgO mass percentage.When the concentration of Mg²⁺in the liquid phase reaches a certain level,5·1·7 phase can also be formed in the MgO-MgSO₄-H₂O system without chemical additives.However,at room temperature,the concentration of Mg²⁺in the liquid phase of MgO-MgSO₄-H₂O system is difficult to reach the concentration of Mg²⁺required to form 5·1·7 phase.The main hydration products of MgO-MgSO₄-H₂O system with chemical additives are 5·1·7 phase and Mg（OH）₂.The chemical additives can effectively promote the neutralization reaction of MgO particles through buffering and complexation,inhibit the hydration reaction of its to generate Mg（OH）₂,increase the concentration of Mg²⁺in the liquid phase,and reduce the minimum concentration of MgSO₄ required for the formation of the basic salt hydrate 5·1·7 phase.Therefore,the minimum concentration of MgSO₄ required for the formation of 5·1·7 phase decreases gradually with the increase of MgO mass percentage.The 5·1·7 phase is a metastable phase（or stable phase）formed by the transformation of the unstable 1·1·5 phase into 3·1·8phase.（2）The hydration process of MgO-MgSO₄-H₂O system with and without chemical additives was traced by EIS-liquid phase method.The electrical equivalent circuit（EEC）models of MgO-MgSO₄-H₂O before and after hardening were established by equivalent circuit fitting.The circuit codes are R_s（Q₁R_d）,R_s(C_d1(R_d(Q₂(R_ct1(Q₃R_ct2))))),R_s(C_d1(R_d(Q₂R_ct1))).Among them,the EEC of slurry with and without chemical additives is R_s（Q₁R_d）.The EEC of hardened slurry without chemical additives is R_s(C_d1(R_d(Q₂R_ct1)),while that of hardened slurry with chemical additives is R_s(C_d1(R_d(Q₂(R_ct1(Q₃R_ct2)))))in the early stage,and becomes R_s(C_d1(R_d(Q₂R_ct1)))in the later stage.The results of EIS showed that chemical additives could delay the hydration reaction of MgO-MgSO₄-H₂O system,promote the neutralization reaction of MgO particles,increase the concentration of Mg²⁺in the liquid phase,and make the R_s and R_ddecrease with the increase of reaction time before hardening of slurry.After hardening,the microstructure of slurry is gradually dense,and R_s,R_d and R_ct1 increase with the curing ages.（3）Based on the hydrolysis qualitative experiment of Mg²⁺and the influence of different kinds of chemical additives（p H value）on the hydration and hardening rule and microstructure characteristics of BMSC,the formation process of the basic salt hydrate and the mechanism of formation of 5·1·7 phase induced by chemical additives were revealed.The results show that the formation of basic salt hydrate can be divided into four processes:（1）the dissociation of MgSO₄ in water;（2）the reactions of MgO in MgSO₄ solutions;（3）the hydrolysing-bridging reaction of mononuclear complexes;（4）the crystallization of the hydrate phases.The chemical additives can regulate the concentration of H⁺and OH^-ions in the liquid phase through buffering,promote the neutralization reaction of MgO particles and influence the hydrolysing-bridging reaction of Mg²⁺in the liquid phase,and inhibit the hydration reaction of MgO particles to produce Mg（OH）₂ through complexation,thus affecting the hydration and hardening process and microstructure properties of BMSC.The chemical additives with acid p H value can inhibit the hydrolysing-bridging reaction of Mg²⁺,which slows down the formation rate of 5·1·7phase,and prolong the setting time of BMSC and reduce its early compressive strength,but the later compressive strength is relatively high.The chemical additives with alkaline p H value can promote the hydrolysing-bridging reaction of Mg²⁺,accelerate the formation rate of 5·1·7 phase,shorten the setting time of BMSC and increase its early compressive strength.（4）The effects of different content of low calcium fly ash（L-FA）,high calcium fly ash（H-FA）,granulated blast furnace slag（G-BS）,silica fume（SF）and different fineness of quartz powder（SP）on the hydration and hardening rule and microstructure characteristics of BMSC were studied.The results show that as a component of BMSC,mineral admixtures can improve the mechanical properties and water resistance of BMSC by optimizing the composition and microstructure of its hydration products through morphological effect,microaggregate filling effect,dilution effect,nucleation effect and packing effect.However,due to the differences of physical properties and chemical composition of mineral admixtures,they have different effects on the hydration and hardening process and microstructure properties of BMSC.The effect of inert SP is not significantly different from that of L-FA.At the same consistency,the setting time of BMSC is shortened when mineral admixtures replace MgO.At the same water cement ratio,the setting time decreases with the increase of SP fineness.Under the standard air curing condition,BMSC with H-FA and G-BS shows excellent setting characteristics and high compressive strength,and BMSC with H-FA has good volume stability.The optimized effect of L-FA and H-FA on the microstructure of BMSC is better than that of G-BS,and the incorporation of SF has a good effect on the microstructure of BMSC.The volume deformation of BMSC shows the change rule of at first expands to a small degree and then shrinks with the increase of curing ages.L-FA can effectively increase the early expansion of BMSC and reduce its the later drying shrinkage.After immersion,the expansion stress and volume deformation caused by the transformation of residual MgO into Mg（OH）₂ are the reasons for the decrease of compressive strength of BMSC.At the same time,the pore filling effect of Mg（OH）₂ can effectively increase the compactness of BMSC microstructure,and make the compressive strength of BMSC at first decrease and then increase after immersion.（5）Taking water reducing rate and compressive strength as evaluation indexes,the suitability of water reducing agents and defoamer for ordinary concrete and BMSC was studied.The results show that melamine,sulfonated naphthalene formaldehyde,aminosulfonic-based and aliphatic sulphonate superplasticizer have the effect of water reduction on BMSC,but the polyester type high-performance polycarboxylate superplasticizer has no water reducing effect on BMSC.The superplasticizers can increase the porosity and early Mg（OH）₂ production of BMSC and reduce its mechanical properties.The defoamers can effectively improve the early compressive strength of BMSC and make up for the loss of compressive strength of superplasticizer-BMSC system.The effects of CO₂ curing and carbonization on compressive strength and hydration product composition of BMSC were studied.It was found that the incorporation of L-FA can increase the carbonization resistance of BMSC,while coral powder can decrease the carbonization resistance of BMSC.Under the action of carbonization,more Mg（OH）₂ in BMSC may undergo crystal transformation,which increased the content of amorphous phase in hydration products.In addition,a small amount of Mg（OH）₂ reacts with CO₂ to form Mg CO₃·3H₂O。...