Research On Properties Of Low-heat Portland Cement Under Simulated Severe Environment Of Plateau

Traffic engineering construction in some plateau areas of China often needs to face severe engineering environment,including strong wind,dry,large temperature difference environment during bridge construction and high geothermal environment during tunnel construction,which brings new challenges to the existing cement materials.Low-heat Portland cement is a kind of special cement with dicalcium silicate as the dominant mineral.It has the characteristics of less heat of hydration,good volume stability,stable strength growth at high temperature,which can effectively guarantee the quality of traffic engineering construction in plateau area.Based on laboratory environment simulation tests,this study carried out research on the properties of low-heat cement under the plateau traffic engineering environment,revealing the evolution law of mechanical properties,drying shrinkage properties and mechanical properties of low-heat cement under the bridge construction environment of strong wind,dry and large temperature difference,and high temperature environment.The influence mechanism of high wind,dry,large temperature difference and high temperature environment on the properties of low heat cement was expounded.The low-heat cement with low magnesium content has higher mechanical properties than the low-heat cement with high magnesium content under the condition of similar drying shrinkage characteristics,which is more suitable for traffic engineering in plateau area.In the environment of dry,dry strong wind,dry strong temperature difference,dry strong wind and dry strong temperature difference coupling,water loss leads to the blockage of hydration process and slow cement strength.In the above environment,the strength of cement mortar at the same age is in the following order: dry large temperature difference > dry > dry strong wind large temperature difference coupling > dry strong wind;In the above environment,the development of low-heat cement strength is hindered more obviously than that of ordinary Portland cement.By extending the pre-curing time,the influence of the above environment on the strength growth can be effectively alleviated.When the pre-curing time is extended to 14 days,the strength of low-heat cement in the above environment at 28 days is comparable to that of ordinary Portland cement.In addition,the dry shrinkage rate of low-heat cement at each age is 30%～40% lower than that of ordinary Portland cement.The wind factor will lead to the intensification of 3 d～14 d dry shrinkage of cement,and the temperature difference factor will not lead to the obvious change of dry shrinkage rate.According to the calculation of capillary stress and phase analysis,the lower drying shrinkage of low-heat cement is mainly due to the lower capillary stress before 28 d and the higher proportion of C-S-H gel in the hydration products.The simulation method of heat curing directly after forming is more consistent with the construction environment of high geothermal tunnel.Under this condition,the strength of low-heat cement can maintain a stable increase,while the strength of Portland cement at 56 d shrinks compared with 28 d when the temperature is higher than 60℃,and the strength of 28 d and 56 d decreases with the increase of temperature.When using standard curing for 1 day and then thermal curing,both low-heat cement and Portland cement can ensure the relatively stable development of strength;The bending strength of low-heat cement and Portland cement decreased with temperature for 28 d and 56 d,whether it was hot curing directly after forming or hot curing for 1 d after standard curing.The microscopic test results show that: The strength damage behavior of cement under high temperature conditions is due to the degradation of microstructure in the later stage of hydration,but this behavior is closely related to the heating in the early stage of hydration.The low hydration rate of low-heat cement at high temperature makes its hydration products uniform and dense,and the pore structure of the slurry does not deteriorate significantly with the increase of temperature and the change of heating mode,so its strength can still maintain a steady increase at high temperature.