Study On The High-temperature Performance Of Magnesium Phosphate Cement-based Materials With Fly Ash/lithium Slag

MPC is a new type of unique cementitious material with fast curing at room temperature.Its curing mechanism is the acid-base neutralization reaction between refired Mg O and acid phosphate,a chemically bonded phosphate ceramic material with the characteristics of both cement and ceramic—industrial waste,etc.In addition,MPC has high-temperature resistance,it has been shown that using industrial waste materials as external additives in MPC fireproof coatings can improve their performance.Therefore,this paper used industrial solid waste fly ash and lithium slag as dopants to partially replace the refired Mg O and KH₂PO₄according to the dose of 0,5%,10%,20%,and 30%.The compound dose（fly ash+lithium slag）was 20%and30%to study the effect of the change of fly ash/lithium slag dosing on the physical phase composition and thermal and mechanical properties of MPC at room temperature and high temperature.The effect of the fly ash/lithium slag doping ratio on the high-temperature properties of MPC was investigated by characterization methods such as heat of hydration,X-ray diffraction,scanning electron microscopy,and simultaneous thermal analysis;at the same time,through the solidification of waste materials and the secondary use of blending materials,in line with the national solid waste utilization strategy development direction.The following conclusions were drawn from the above study and analysis of the characterization results.（1）Firstly,the effect of different M/P ratios and water-cement ratios on the compressive strength,setting time,and flow rate of MPC was investigated.It was found that as the M/P ratio increased,the compressive strength of MPC increased and then decreased while the setting time decreased;the early strength of MPC decreased significantly,and the flow rate increased with the increasing water-cement ratio.Based on the study’s results,subsequent tests were carried out on MPC specimens with a base ratio of 4:1 M/P and a water-cement ratio of 0.22.（2）The incorporation of fly ash can effectively reduce the rapid hydration rate of MPC at the early stage,among which 20%fly ash has the most significant effect and significantly improves the excessive exotherm of MPC at the early stage,with 21.32%less cumulative heat release and 19.42%lower exotherm of hydration compared with the blank sample.Regarding mechanical properties,the 28d compressive strength of the specimens was 62.66MPa at 20%fly ash dosing,which was the highest,an increase of 45.25%compared to the blank sample.At the same calcination temperature,the compressive strength of the fly ash-doped specimens increased to a certain extent compared to the blank examples,and the strength loss rate of the samples was significantly reduced.And different temperature conditions,the compressive strength of all specimens decreased gradually with increasing temperature.（3）The incorporation of lithium slag significantly reduced the pre-hydration rate of MPC,with the most significant effect of inhibiting the pre-hydration heat release of MPC at 30%of lithium slag,which reduced the heat release of MPC by 13.53%compared with the blank sample and increased the efficiency of hydration heat release rate.In addition,the mechanical properties of all lithium slag specimens were improved compared to the blank MPC specimens,with the 30%doping of lithium slag increasing the MPC strength by about 42.27%compared to the blank specimens and the loss of compressive strength at high temperature was lower than that of all lithium slag specimens.At the same time,the crystalline transformation of lithium pyroxene at 800°C by physical analysis resulted in a slight increase in the compressive strength of the lithium slag specimens at this temperature.（4）The mechanical properties of the 30%blended specimens were better than those of the 20%blended specimens,with a maximum compressive strength of 65.60MPa at 28 d,which was the highest among all the blended samples.Regarding thermal properties,the 30%compounded specimens showed a 10.43%reduction in heat release compared to the single compounded models,which improved the thermal properties better than the single compounded specimens.In addition,the filling and volcanic ash effects of the composite fly ash/lithium slag as an external additive resulted in a denser microstructure of the 30%compound specimens under high temperatures.