Research On Influence Of MgO Expansive Agent Dosage On Microscopic Structure Of Shrinkage Compensating Cement Paste

Shrinkage compensating cement is an important means to control the cracking of concrete structures,and MgO expansive agent is one of the admixtures for configuring shrinkage compensating cement.Too little MgO expansive agent cannot fully compensate shrinkage,but too much MgO may cause concrete stability problems and affect concrete performance.At present,the research on the dosage of MgO expansive agent is concentrated within 12%,and there is no stipulation on the use range of the dosage.Brucite crystals growth affects the pore structure of cement paste,which affects the strength and durability of cement-based materials.There are few researches on the pore structure changing of shrinkage compensating cement paste after adding MgO expansive agent.Microscopic pore structure is the main factor that affects the macroscopic properties of cement-based materials.Therefore,it is necessary to have a deeper understanding of the pore structure of the material,so as to explore the influence of MgO expansive agent on the macroscopic properties of the shrinkage compensating cement paste.Research on the effect of MgO expansive agent on the properties of shrinkage compensating cement-based materials is mainly focused on low dosage.There are few studies on the high dosage of MgO expansive agent.In practical engineering applications,it is often necessary to increase the dosage of MgO expansive agent to compensate shrinkage.In this research,an experimental study was carried out on the shrinkage compensating cement paste mixed with MgO expansive agent.To study the effect of the dosage（0%,6%,9%,12%,18%,36%）MgO expansive agent on the mechanical properties,deformation,pore structure and microtopography of the shrinkage compensating cement paste.The main research conclusions are as the following:（1）As the content of MgO expansive agent increases,the flexural and compressive strength of the shrinkage compensating cement paste continue to decrease.The flexural and compressive strength of the benchmark group basically no longer increase after 60 days of age,and the compressive strength of the 6%to 18%test group continues to increase after 60 days of age.The low dosage test group has no obvious effect on the mechanical properties of the shrinkage compensating cement paste,and the high dosage test group with a dosage of 18%or more has a significant decrease in mechanical strength.（2）The deformation of the shrinkage compensating cement paste continues to increase with the extension of the curing age,and it tends to be stable in the latter part of the curing age.The deformation of the shrinkage compensating cement paste increases rapidly within 90 days,and then slows down after 90 days.The full-curve model for free expansion rate and autogenous volume deformation have a good correlation.The correlation coefficients are all above 0.98.The high dosage test group has a better fit than the low dosage test group.（3）For the shrinkage compensating cement paste,the adding of MgO expansive agent will increase the pores of the paste with the same age in the range of 0.01 to 0.1μm.With the increase of the MgO expansive agent dosage,the integral curve and differential curve of the pore size distribution of the shrinkage compensating cement paste have a tendency to move to the right.The 36%dosage test group curve obviously moved to the right,and the most probable pore diameter gradually increased.With the increase of the MgO expansive agent dosage,the pore structure of the shrinkage compensating cement paste has a certain regularity in the early age,but it is not obvious.In the case of long age,the unit mercury intake,mean pore diameter,most probable pore diameter and median pore diameter all gradually increase,and the increasing trend is obvious.The unit mercury intake,the most probable pore diameter,and the median pore diameter of the 36%dosage test group are significantly higher than the low dosage test group,and the mean pore diameter of the lower dosage test group has a slight increase.Combining compressive strength and free expansion rate test analysis indicates the following.For the hardened paste of the low content test group,the hardened paste structure is denser and the strength is higher.Mg（OH）₂ has a weak ability to expand the pores,and the pore structure of the hardened paste gradually becomes dense with the extension of the curing age.Therefore,the compressive strength of the compensated shrinkage cement paste still tends to increase slightly after 60 days of age.For the high dosage test group,the amount of expansion source Mg（OH）₂ produced is larger,and the amount of expansion is larger.Due to the decrease of cement amount in the same cementitious system,the strength of the hardened paste is lower,the growth rate is slower,and the structure of the hardened paste is looser.Eventually,a large amount of Mg（OH）₂ produced a larger expansion force on the pores,and it gradually increased with age.Therefore,the increase rate of the unit mercury intake of the hardened paste is increasing,which eventually leads to a significant decrease in the compressive strength.（4）Through scanning electron microscopy experiments,the low dosage test group produced fewer needle-shaped magnesium hydroxide crystals,most of which were fluffy and denser.In the high dosage test group,obvious needle-shaped magnesium hydroxide crystals can be observed,and a large amount of magnesium hydroxide can be seen everywhere in the hardened paste.The needle-shaped magnesium hydroxide crystals,which are distributed in the C-S-H gel and the pores of the hardened paste,and can be clearly seen in the cracks,are within1μm in size.