| Magnesium phosphate cement(MPC),which has the advantages of quick setting,early strength,high bond strength and waste solidification,has been a major concern of domestic and international academics.However,the problems such as rapid solidification and poor water stability have not been solved effectively.In this paper,a new-type magnesium potassium phosphate cement(Magnesium silicon potassium phosphate cement,MSPPC for short),with no retarder and good water stability,was prepared by silica fume,dipbtassium hydrogen phosphate and magnesia.Firstly,the influences of basic components of MSPPC(P/M change from 1/2 to 1/5,W/C change from 0.16 to 0.20,the content of silica fume change from 0%to 20%)on the basic physical properties(compressive strength,flexural strength,setting time and fluidity)were investigated,the experimental age is 3 d,7 d,28 d,56 d.Secondly,the influence of the basic components(P/M,W/C and silica fume content)of MSPPC on water resistance were studied.Finally,the macro-properties,water stability and setting time of MSPPC were investigated through microscopic tests,such as the change of PH value of hydration process,pore structure analysis,XRD,SEM and TG-DSC.The mechanisms of water resistance,retarding and hydration reaction of MSPPC were also revealed.The main conclusions are drawn as follows.1.With the decrease of P/M,the compressive strength and flexural strength in each age of MSPPC increase at first and then drop.When the P/M is 1/3,the mechanical properties are the best,the 28 d compressive strength and flexural strength are 113.13 MPa and 18.07 MPa respectively.With the increase of W/C,the compressive strength and flexural strength of MSPPC decrease.With the increase of silica fume,the compressive strength and flexural strength of MSPPC increase at first and then drop,and the mechanical properties are the best when the content of silica fume is 15%.With the decrease of P/M,the setting time of MSPPC increases.And the setting time increases with the increase of W/C or the content of silica fume.With the increase of P/M or W/C,The fluidity of MSPPC decreases.With the increase of the content of silica fume,The fluidity of MSPPC increases at first and then drop,when the content of silica fume is 15%,the fluidity of MSPPC is the best.2.With the decrease of P/M or W/C,the strength retention rate of MSPPC under the condition of water curing for 56 d increases,and with the increase of the content of silica fume,the strength retention rate also increases.Under the condition of natural curing,with the decrease of P/M,the.porosity of MSPPC increases after an initial decrease,and when the P/M is 1/3,it reaches a minimum porosity.With the increase of W/C,the porosity of MSPPC increases.With the increase of the content of silica fume,the porosity of MSPPC increases after an initial decrease,and when the dosage of silica fume is 15%,the porosity reaches the minimum value.Under the condition of water curing,compared with the one under the condition of natural curing,the porosity of MSPPC increases,and the ratio of the volume of macropores increases obviously.3.The hydration product of MSPPC is composed of MgKPO4ยท6H2O(MKP),MgSiO3 and amorphous magnesium silicon phosphate.The water stability of MSPPC is related to the content of residual phosphate,the type of hydration product,the crystallinity of main product MKP and the porosity of hardened cement.4.The setting time of MSPPC can be controlled by adjusting phosphate ion concentration,the initial pH value of reaction and the content of Mg2+ in the reaction system.The concentration of phosphate ion plays a leading role in the hydration process of MSPPC.5.On the one hand,silica fume can consume OH-in reaction system and reduce the pH value of reaction system,so that the reaction environment is favorable for the formation of MKP.On the other hand,silica fume after the reaction can react with free Mg2+ to form MgSiO3,and regulate the concentration of free Mg2+ in the reaction system. |
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