Preparation Of Magnesium Phosphate Cement By Slag Doping And Its Properties

Magnesium phosphate cement is characterized by fast setting,early strength beating,low expansion rate,good adhesion to the interface with old concrete,etc.,and is widely used in housing construction projects,bridges and tunnels,roads and other fields.However,magnesium phosphate cement has large hydration heat release and fast hydration rate in the early stage of coagulation,which in turn affects its composition stability and material strength.Fly ash,boron sludge and other solid wastes have huge stocks in China and need to be developed and utilized.Therefore,the use of fly ash and boron has dual significance for the secondary utilization of solid waste resources and environmental protection.In this thesis,founding the mixing ratio that has the least effect on the setting time and compressive strength of magnesium phosphate cement was first investigated experimentally,and the optimal admixture amount of calcium lignoate and borax as composite retarder was optimized on the basis of the above experiments.Fly ash and boron slurry were then added to investigate the modification of the magnesium phosphate cement,and changes in performance such as initial set-up time and compressive strength.Finally,the hydration process of magnesium phosphate cement was clarified by combining XRD,SEM,and other analytical test methods.（1）The optimum content of magnesium phosphate cement was the water to cement ratio is 0.15;the phosphorus to magnesium ratio is 1/4,and the optimal content of borax and calcium lignate in magnesium phosphate cement was 5 wt.%and 1 wt.%,respectively,compared with single borax,the delayed setting time of magnesium phosphate cement was extended from 29.3 min to 38.5 min,but the compressive strength at 7d was reduced from 59.59 MPa to 56.21 MPa.（2）The performance of magnesium phosphate cement was further improved after modification of the fly ash and boron slurries,and the set-up time of magnesium phosphate cement modified with fly ash was increased from 38.5 min to 41.3 min,the compressive strength increased from 56.21 MPa to 58.3 MPa in 7 days,the setting time of boron mud-modified magnesium acid cement was extended from 38.5 min to 54.35min,and the compressive strength increased from 56.21 MPa to 58.7 MPa in 7d.（3）The mixing of fly ash and boron sludge has little effect on the hydration products of magnesium phosphate cement,and the hydration hardening process is:firstly,the soluble phosphate is dissolved in water to form an acidic solution,and then in the acidic solution,the Mg O particles are gradually dissolved to release Mg²⁺;Mg²⁺and PO₄^3-plasma react to form crystalline phosphate hydration products.The water inside the slurry gradually volatilizes and is consumed with the hydration reaction,and the hydration products form a three-dimensional network structure.Due to the increase in temperature during the hydration process of magnesium phosphate cement,part of the water vapor quickly escapes to form pores,which cannot escape before hardening and stays in the hardened structure to form closed micropores,resulting in a decrease in compactness.The fine particles of fly ash boron sludge can fill the voids formed during hydration,thereby improving the compressive strength.Magnesium phosphate cement modified with fly ash and boron slurry can meet the strength requirements of plain Portland cement,which not only reduces the cost of producing magnesium phosphate cement but also realize the secondary utilization of solid waste resources.