Experimental Study On Water Stability Of High Ductility Magnesium Phosphate Cement-Based Composites

Magnesium phosphate cement?MPC?has fast setting and hardening,high early strength,good bonding performance,and small dry shrinkage,so it has a wide range of applications.High ductility fibre cement-based composites have the same excellent tensile strength as metal materials,and their ultimate tensile strain is very large.Adding fibre to cement products to form high-toughness materials is gradually becoming the mainstream in the field of repair and reinforcement materials.In view of the characteristics of MPC and the existing research basis,high ductility fibre-reinforced magnesium phosphate cement-based composites can give full play to the superior properties of MPC and solve the problem of large brittleness.However,when MPC is cured in water or in wet environment,the strength will be greatly reduced,and the durability will be poor,which is not conducive to large-scale use.Therefore,research on high ductility magnesium phosphate cement-based composites under water immersion conditions has a great impact on its practical application.Based on the previous research group,this paper first performed compressive strength and four-point bending tests by changing the age,changing the water-solid ratio,Mg O-KH₂PO₄ ratio?M/P?,fly ash?FA?replacement amount,nano-alumina?NA?content,and waterglass?WG?content.And calculate the strength retention rate and bending performance to study the impact on bending performance and water stability;secondly,select a representative mix ratio and analyze its mechanism at the micro level by scanning electron microscopy?SEM?.The main conclusions obtained are as follows:?1?Test the working performance of high ductility magnesium phosphate cement mortar through the slump expansion test.The test confirmed that when the water-solid ratio is 0.13,the slurry viscosity is good,the fibres are evenly dispersed,and the working performance is good;when the M/P is 4,the slurry's flow performance is the best;as the amount of FA substitution increases,the slump spread decreases,but the fibre dispersion is relatively good;the NA content has little effect on the working performance,and the working performance is relatively good at small dosages;the WG content has little effect on the working performance.?2?Change the age,cement base ratio and change the admixture and amount to study the compressive strength and water stability:after 28 days of natural curing,the water stability of the immersed specimens is higher than that of the corresponding age of 7 days after natural curing;as the water-solid ratio increases,the compressive strength decreases,and the strength retention rate reaches about 95%;with the increase of M/P,the compressive strength increases,andthe strength retention rate is the highest when M/P=5;with the increase of FA substitution,the compressive strength after adding NA is greater than that of the specimen without NA,and the retention rate of compressive strength is greater than 100%except NA=10%.Adding an appropriate amount of NA can improve water stability;when the amount of WG is greater than 2%,the compressive strength is lower than that of the undoped test piece.?3?Through the four-point bending test and bending performance evaluation,the bending performance of the test piece is calculated:regardless of whether it is immersed in water or not,the ductility after 7 days of natural curing is slightly greater than that of 28 days of natural curing;the mid-span deflections are similar except for the water-solid ratio of 0.11;the mid-span deflection increases first and then decreases with the increase of M/P.When M/P=4 and 5,toughness is relatively better,and the bending load is also relatively large;when FA=50%,the mid-span deflection is the largest;the addition of NA has little effect on the bending performance;except for WG=5%,the mid-span deflection is the largest after immersion in water.?4?Microscopic observation of the test piece by SEM found that:the incorporation of 40%FA fills some of the holes,making the fibre and matrix more tightly bonded,and the matrix structure is more dense,so the bending performance is better and the water stability is also improved;after 8%NA is mixed,the crystals are arranged closely,struvite is wrapped by the generated Al?OH?₃,the structural integrity is strong,and the water stability is good;after blending 5%WG,due to the wrapping effect of WG,the fibre is tightly bonded to the matrix and the bending performance is good,but a large amount of hydrated magnesium silicate gel is generated,doped with unreacted Mg O and KH₂PO₄ coagulation and precipitation The arrangement is chaotic,the gap between the crystals is increased,the mechanical properties are slightly weakened,and the water stability is reduced.