Effect Of Na-HMP And CaO On The Reaction Mechanism Of MgO-SiO₂-H₂O System

As a kind of eco-friendly,high-performance building materials,MgO-SiO2-H2O system with good cementitious performance,low pH value and low hydration heat,can be applied to the comprehensive utilization of magnesium mineral resources and industrial waste,and also other fields:such as the encapsulation of nuclear waste including metallic aluminum and magnox sludge.In order to investigate the reaction process of MgO-SiO2-H2O system,the quantitative analysis on phases,reaction kinetics analysis and molecular structure analysis have been studied.Sodium hexametaphosphate(Na-HMP)as the most effective additive can improve the working performance of MgO-SiO2-H2O system,and CaO is one inevitable composition of raw material source,therefore it is necessary to study the effect of Na-HMP and CaO on the evolvement mechanism of MgO-SiO2-H2O system and M-S-H gel.This paper aims at studying the competition mechanism and molecular structure of M-S-H gel and C-S-H gel.The specific research content and conclusions are showed as follows:1)The dissolution of SiO2 plays a key role in the reaction process of MgO-SiO2-H2O system.The results show that the dissolution rate of SiO2 is consistent with the formation rate of M-S-H gel in MgO-SiO2-H2O system,and is relatively slow compared to in the hydration rate of MgO.Based on the dissolution process of SiO2,the reaction kinetics model of SiO2 particle has been established in order to analyze the influencing factors and predict the reaction process.The effect of the intial Mg:Si molar ratio on the composition and molecular structure of M-S-H gel in MgO-SiO2-H2O system has been studied.The Mg:Si molar ratio of M-S-H gel is in the range of 0.75-1.37,and the average condensation degree first increases and then decreases with the increase of the Mg:Si molar ratio.2)The effect of Na-HMP on MgO-SiO2-H2O system has been studied in detail.Na-HMP can react with Mg2+ to form soluble[Mg2(PO3)6]2-resulted in the acceleration of MgO dissolution and the adsorption on the MgO to form insoluble[6MgOH+·(PO3)6]6-resulted in the inhibition of MgO dissolution.Na-HMP can inhibit the formation of Mg(OH)2 and increase the pH value in pore solution,which promotes the dissolution of SiO2,and accelerates the formation of M-S-H gel.The mixing amount of Na-HMP is controlled within the range of 1 wt.%?2 wt.%.If Na-HMP content is excessive,the MgO hydration will be inhibited and new phase(hylbrownite)will be formed.When the intial pH value in pore solution is higher than 13,MgO hydration and M-S-H gel formation will be restrained.The optimal alkali environment of M-S-H gel formation is from 8.5 to 10.5.3)In this paper,the effect of CaO on MgO-SiO2-H2O system has been investigated in detail.After adding a small amount of CaO,concentration of ions and pH value in pore solution changes significantly.The reaction process of system can be promoted when the mass of the CaO is less than 5 wt.%.While the formation of M-S-H gel is inhibited when the mass of the CaO is more than 10 wt.%,as C-S-H gel is largely formed.The effect of high Ca2+ concentration in the pore solution on the molecular structure of M-S-H gel is obvious.The "core-shell" structure of M-S-H gel has not been found in C-S-H gel.