Hydration And Pore Structure Of Magnesium Phosphate Cement In Natural Environment Of Severe Cold Area

In China,the cold areas mainly include high latitude areas and high altitude areas such as the Qinghai Tibet Plateau.These areas are cold in winter,and ordinary Portland cement-based materials are difficult to be hydrated normally.Magnesium phosphate cement,as a new kind of cementitious material,has the advantages of fast hardening,high early strength,compact structure,good durability,good bonding with old concrete interface,etc.,especially suitable for emergency repair of important projects in winter in cold region.At present,the hydration and microstructure of magnesium phosphate cement under-20 ℃ are still less studied.Therefore,based on the characteristics of natural climate and environment in Harbin,the construction,mechanical properties,pore structure and micro composition of magnesium phosphate cement in the cold winter are studied.The effects of magnesium phosphorus ratio,borax content and maintenance temperature and time on its properties are analyzed,The relationship between its macro performance and microstructure is summarized.The setting time of magnesium phosphate cement slurry decreases with the increase of magnesium phosphorus ratio and the decrease of borax content.When the ratio of magnesium to phosphorus is 3～5,the samples can set normally,and the setting time is within 20-40 min.When the magnesium phosphorus ratio is 2,the free water freezing may occur.The fluidity of the slurry decreases with the increase of magnesium phosphorus ratio and the decrease of borax,and the fluidity varies from75 to 160 mm.Considering the safety of magnesium phosphate cement structure in winter construction,the ratio range of magnesium to phosphorus is 3～5 and borax content is 2% in the follow-up study.The early strength of the two hours of positive temperature curing can reach50 MPa,and the strength can develop to more than 60 MPa at 24 h,and the strength is higher when magnesium phosphorus ratio is 3 or 4.When the temperature is negative,the higher the magnesium phosphorus ratio is,the higher the strength is,and the strength is 26.8MPa at 24 h when the magnesium phosphorus ratio is 5.The strength of the samples cured at negative temperature for 22 h and positive temperature for 2hours decreases with the increase of magnesium phosphorus ratio compared with the negative temperature 24 h.The higher the ratio of magnesium to phosphorus,the lower the curing temperature,the higher the porosity of magnesium phosphate cement,the lowest can reach 11%,and the maximum is more than 20%.The content of mesoporous under10 nm has a great influence on the compressive strength of magnesium phosphate cement.The smaller the ratio of magnesium to phosphorus,the more hydration products can be produced by complete hydration energy;The higher the ratio of magnesium to phosphorus,the faster the hydration reaction speed,the comprehensive effect of both determine the compressive strength of the two,which is different under different curing temperature.According to XRD,the smaller the ratio of magnesium to phosphorus,the more unresponsive phosphate,the greater the ratio of magnesium to phosphorus,the more unresponsive magnesium oxide.SEM images show that the higher the curing temperature,the denser the structure is;Another kind of hydrated product,DAP,was found in the samples with rapid exothermic.TG DSC curve showed that the struvite lost 5 crystal water between 100-120 ℃,and continued to lose water to produce magnesium pyrophosphate at 650 ℃;The higher the ratio of magnesium to phosphorus at the positive temperature,the less the hydration products,the smaller the mass loss.According to the above research,magnesium phosphate cement has the highest compressive strength,the most hydration and the best comprehensive performance under the condition of constant negative temperature when M/P is 5 in the cold winter natural environment.When m/p is 4,the performance of M/P will be improved rapidly once it is transferred to positive temperature,and the best performance is in the area with large temperature difference between day and night.Magnesium phosphate cement has a wide application prospect in high latitude and high altitude cold region.