Regulation Of Silica Fume And Na-HMP On The Cementitious System Of MgO-Gypsum Slag

Marine engineering cement is a kind of high performance special cement used in marine engineering construction.Due to the characteristics of high humidity and salt corrosion in marine engineering cement,higher requirements are put forward for cementitious materials.Compared with Portland cement,gypsum slag cement has natural advantages in marine engineering applications due to its good durability and anti-erosion ability,but its characteristics of low early strength and easy carbonization can not meet the technical requirements of marine engineering cement.In this paper,magnesium oxide is used to modify gypsum slag cement,and this paper aims at studying the regulation of silica fume（SF）and sodium het-hetaphosphate（Na-HMP）on the cementification system of magnesium oxide gypsum slag and the reaction mechanism of the system,in order to solve the problems of low early strength,easy carbonization and unstable mechanical properties.Meanwhile,the application prospect of CaO-MgO-SiO₂-CaSO₄-H₂O new cementification system in marine engineering has been explored.The main contents in this paper are as follows:（1）The change characteristics of macroscopic properties and reaction mechanism of cementified materials with different MgO content have been studied.MgO content has an important effect on hydration degree,hydration product characteristics and microstructure of hardened paste.The results show that too little MgO content will lead to insufficient hydration products,which is not conducive to the development of compressive strength.Excessive MgO content will increase the pH value of pore solution,which will accelerate the early hydration rate and promote the generation of AFt and Ht,so as to improve the compactness and strength of cement stone.However,dilatable hydration products will increase the number of microcracks in the material and affect the stability of the microstructure inside the material.In general,7.5 wt.%MgO content（M7.5）is the optimal content,and the maximum compressive strength（56 days）is 75.5 MPa.The main phases of MgO modified cementification materials are C-S-H gels,AFt and Ht.（2）The effects of silica fume and Na-HMP on the cementitious system of MgO-gypsum slag with different dosage have been studied.The results show that the introduction of silica fume and Na-HMP is beneficial to the stable development of long-term strength of cement.The addition of Na-HMP can promote the hydration of MgO and the dissolution of slag at the early stage of reaction,but Na-HMP chelates with Ca²⁺and Mg²⁺in the liquid phase,which hinders the early strength development of cement.In short,the optimal content of Na-HMP is1.0 wt.%of the total solid.And the introduction of silica fume promotes the formation of gel phase in the cementitious system of MgO-gypsum slag at the early stage of reaction,which is beneficial to improve the compactness and early strength of the slurry.（3）The reaction mechanism of silica fume and Na-HMP on the cementitious system with different dosage of magnesia oxide-gypsum slag has been revealed.The reaction process of CaO-MgO-SiO₂-CaSO₄-H₂O system is mainly divided into three stages.And the reaction process mainly depends on the dissolution rate of slag and silica fume,as well as the hydration rate of MgO,while the ion concentration and pH value in pore solution have a direct relationship to the formation of gel phase in the cementification system.It has been found that C-S-H gel can be preferentially generated at the initial stage of hydration,and the proportion of M-S-H gel will increase continuously with the increase of SiO₂content in the later stage of hydration.Besides,the initial Ca/Siand Mg/Siof the gel system can affect the molecular structure of the gel phase,and the introduction of Na-HMP is closely related to the degree of polymerization of the gel phase.