Study On Soundness And Expansive Mechanism Of MgO Shrinkage-Compensating Cementitious Materials

Adding proper expansion agent has become an effective measure to improve the crack resistance and durability of concrete.Compared with traditional expansive agents,MgO expansive agent has the advantages of less hydration water requirement,stable hydration products and adjustable expansion performance,but its delayed expansion may bring soundness and expansion is difficult to predict quantitatively,which is not conducive to the promotion and application of MgO expansive agent.In this paper,the influence of MgO expansion agent on the volume soundness of cement paste,cement mortar and concrete was systematically studied,and the volume change rule of compensating shrinkage concrete with different active MgO expansion agent was also studied under standard curing condition,and the action mechanism of MgO expansion agent was also discussed.The main conclusions are as follows:（1）In the same system,when the activity of MgO expansion agent is the same,the higher the content of expansion agent,the higher the autoclave restrained expansion rate of pressure evaporation.When the content of MgO expansion agent is the same,the higher the activity of expansion agent is,the smaller the autoclave restrained expansion rate is.Under the influence of the absolute content of MgO expansion agent per unit volume and the uniformity of aggregate in the system,the stable content of MgO expansion agent increases successively according to the cement paste,cement mortar and concrete.The influence between expansion and strength is intensified under pressure and high temperature and high pressure.The strength of hardened slurry decreases while the expansion increases.After the simplification of the autoclave process,the change rule of the restrained expansion rate of different systems and different dosages is consistent with the rule before simplification.（2）The expansion time window（expansion growth stage to the expansion stability stage）of compensated shrinkage concrete with different active MgO expansion agent is different.In strength grade C30 and C50 concrete,the expansion time window of R-MgO expansion agent is between 0d and 28d,the expansion time window of M-MgO expansion agent is between 7d and 90d,and the expansion time window of S-MgO expansion agent is between 28d and 180d.When the activity is consistent,the larger the dosage of MgO expansion agent is,the longer the time to enter the expansion stable period is,and the larger the final expansion amount is.When the activity and dosage of MgO expansion agent are the same,the restrained expansion amount of C50 compensated shrinkage concrete is larger than that of C30 compensated shrinkage concrete.By fitting with the existing prediction model of the restrained expansion rate of MgO compensated shrinkage concrete in the long age period,the test proves that the prediction of the restrained expansion rate of small dosage（<6%）in the age period from 120d to 240d is more accurate.（3）Under the condition of autoclaving at high temperature and high pressure,the crystal mineral composition of hardened slurry in MgO-cement binary cementification system mainly consists of Ca（OH）₂,crystallineα-C₂SH,unhydrated clinker minerals（C₃S,C₂S）,Mg（OH）₂ and a small amount of MgO.The degree of hydration of MgO expansion agent is affected by MgO activity,dosage and curing conditions.The higher the activity,the higher the dosage and the higher the degree of hydration.After the autoclaved test,the total volume and porosity of the holes increased,and the number of harmful and harmful holes increased significantly,but the number of harmless and less harmful holes decreased.After high temperature and high pressure,when the activity of MgO expansion agent is consistent,the microstructure of cement paste changes from compact to loose with the increase of expansion agent content.（4）Mg（OH）₂ is generated by hydration of MgO in cement paste.Some Mg（OH）₂ grows in situ and attaches to the surface of MgO expansion agent particles,and some migrates and grows on the surface of cement hydration products or in micropores.Under high temperature and high pressure,the diffusion rate of Mg²⁺is accelerated,the range of dispersion is larger,and the formation location of Mg（OH）₂ crystal nucleus is more dispersed.The generated Mg（OH）₂crystals are mostly pepetal shaped,and the crystals overlap to form scaffolded Mg（OH）₂ crystal clusters,expand the pores,and produce uniform macroscopic volume expansion in the hardened slurry.Most of Mg（OH）₂ crystal clusters are enriched around hydration products Ca（OH）₂ and C-S-H gel,and a small number of Mg（OH）₂ crystal clusters are dispersed in the tiny pores between hydration products,and the pores are mainly 20～60nm dense.