Study On The Mechanism Of The Effect Of Maintenance Temperature On The Performance Of Fly Ash Magnesium Phosphate Repair Mortar

Magnesium phosphate cement（MPC）repair mortar has the characteristics of early strength,fast setting and fast hardening,dry shrinkage,good frost resistance,chemical corrosion resistance,wear resistance,high temperature resistance and good interfacial bonding strength with old concrete,but the high hydration temperature and high cost restrict its application,which can be alleviated to some extent by incorporating mineral admixtures.MPC repair mortars are mainly used in emergency repair works for in-service components.The geographical and climatic conditions of the cracked components determine the temperature and humidity of the environment,which have a significant influence on the hydration process,hydration products and pore structure,mechanical properties and water resistance of MPC repair mortars.Therefore,to promote the engineering application of MPC repair mortars,it is necessary to investigate in depth the mechanism of the influence of curing temperature and mineral admixtures on their physical and mechanical properties.In this paper,based on the preparation of fly ash MPC and its repair mortar,the mechanism of the influence of curing temperature and fly ash content on the setting and hardening process and physical and mechanical properties of MPC repair mortar is studied to reveal the mechanism of setting and hardening of MPC repair mortar at 0℃and-20℃,and to provide basic data for realizing its application in emergency repair works of in-service structures in severe cold areas.From the above research,the main conclusions are as follows:（1）Research on the structure and properties of fly ash modified MPC:The fluidity and mechanical properties of fly ash modified MPC decrease gradually,and the setting time decreases first and then increases.The strength retention of MPC increases first and then decreases,reaching the maximum value when the fly ash content is 40 wt%.The hydration temperature of MPC decreases first,then increases and finally tends to the constant temperature water tank temperature.MPC concentrated exothermic in the early stage,and the hydration heat of 100h decreased with the increase of fly ash content,and was lower than the theoretical hydration heat.The main mineral composition of fly ash modified MPC at different ages is Mg（NH₄）PO₄·6H₂O and unhydrated Mg O crystals,and no obvious characteristic peaks of mineral composition in fly ash are found.The characteristic peak strength of the hydration product struvite increases with age and decreases with the increase of fly ash content.（2）The influence mechanism of curing temperature on the physical and mechanical properties of MPC repair mortar:With the increase of fly ash content,the setting time of MPC repair mortar at-20℃and 0℃curing temperature decreases first and then increases,and gradually decreases at 20℃and 40℃.The fluidity,mechanical properties and water resistance of MPC repair mortar modified by fly ash increased first and then decreased with the increase of fly ash content.The interfacial bonding strength decreases with the increase of fly ash content at-20℃and 0℃,and increases first and then decreases at 20℃and 40℃.The MPC repair mortar modified by fly ash only has linear shrinkage rate during the drying process at-20℃and 0℃.During the drying process at 20℃and 40℃,when the fly ash content is less than15wt%,the linear expansion occurs.When the fly ash content is higher than 20wt%,the linear shrinkage occurs.The setting time of MPCM without fly ash and MPCM-10 with 10 wt%fly ash decreases with the increase of curing temperature,while the fluidity increases gradually.The compressive strength of each age increases with the increase of curing temperature,while the flexural strength decreases.The water resistance,interfacial bonding strength and dry shrinkage rate of MPCM and MPCM-10 decreased with the increase of curing temperature.