Study On The Durability Of MgO Shrinkage-compensating Concrete With Different Strength Grades

The carbonization of concrete will cause the steel bar to be depassivated and corroded,which will lead to a series of adverse consequences such as the reduction of the cross-sectional area of the steel bar,the destruction of the bonding force between the concrete and the steel bar,the cracking of the concrete protective layer,and finally reduce the durability of the concrete structure and reduce the service life.Adding an appropriate amount of expansive agent has become an effective measure to improve the durability of concrete.Compared with traditional expansive agents,MgO expansive agent has the advantages of less hydration water demand,stable hydration products and adjustable expansion performance.However,the existing research on the effect of MgO on the durability of concrete is not systematic enough.In this paper,MgO concrete with three strength grades of C30,C45 and C60 was taken as the research object.The carbonation resistance,compressive strength,carbonation shrinkage compensation and chloride ion penetration resistance of MgO concrete under carbonation conditions were systematically studied.The carbonation mechanism of MgO shrinkage compensation system was studied by means of MIP,XRD and TGA.The main conclusions of this paper are as follows:（1）MgO is more conducive to the strength growth of concrete under carbonation conditions.For C30 concrete,within 28 days of carbonization,the carbonization enhancement effect of 6%dosage is the best,and the carbonization increase reaches 19.2%;after 28 days of carbonization,the carbonization enhancement effect of 12%dosage is the best,and the carbonization increase reaches 20.4%.For C45 concrete,the carbonation enhancement effect of 12%content is the best,but the carbonation increase is within 10%.Generally speaking,under the content of 0～12%MgO,both C30 and C45 strength grade concrete show that the greater the content of MgO,the better the carbonation enhancement effect,because the greater the content of MgO,the more types and quantities of carbonation products can be generated,the more obvious the effective filling effect on pores,so as to refine the pore structure and improve the strength of concrete;however,the higher the concrete strength,the weaker the carbonation enhancement effect.（2）The carbonation shrinkage of C30 and C45 concrete tends to be stable after 98 days of carbonation.The carbonation shrinkage of C45 concrete is higher than that of C30,because the lower the water-binder ratio,the smaller the porosity,and the higher the effective filling rate of Mg（OH）₂ to the pores.Therefore,at 12%MgO content,the carbonation shrinkage compensation of MgO to C45 concrete is higher than that of C30,but the compensation effect is lower,all within 15%.（3）For MgO paste,the larger the water-binder ratio,the faster the carbonation process and the worse the carbonation resistance.When the water-binder ratio is 0.5,the carbonization front reaches the core layer（20 mm from the surface layer）at 42d,while the water-binder ratio of0.3 and 0.4 reaches the core layer at 56d.Under the same water-binder ratio,the carbonation resistance of different MgO activity and content is inconsistent.Under the water-binder ratio of0.3,the anti-carbonization performance shows 6%content>12%content>benchmark group.Under 0.4 water-binder ratio,the carbonation resistance showed the benchmark group>12%content>6%content;in the comparison of different MgO activities,the carbonation resistance showed M type>R type.Under 0.5 water-binder ratio,the carbonation resistance of the reference group is better than that of the MgO group.Within 28 days of carbonization,the carbonation performance of 12%content is better than that of 6%content.After 28 days of carbonization,6%content is better than 12%content.（4）For concrete,the higher the water-binder ratio（the lower the strength）,the faster the carbonation process,the worse the carbonation resistance,and the greater the change in the p H value of each group.For C30 concrete,within 56 days of carbonization,the anti-carbonization performance shows M-type>benchmark group>R-type（6%>12%under each MgO activity）.After 56 days of carbonization,it shows 12%>6%>benchmark group（M-type is better than R-type under each dosage）.For C45 concrete,the carbonation resistance of the reference group is better than that of the MgO group within 42 days of carbonation,and the 6%content is better than the 12%content;after 42 days of carbonization,the carbonation resistance of the two MgO-doped test groups was better than that of the reference group.In the range of0～12%MgO content,the greater the MgO content,the stronger the carbonation resistance.For C60 concrete,the MgO-doped two groups are better than the reference group at each carbonization age,and the 6%content is better than the 12%content.（5）MgO is beneficial to improve the resistance to chloride ion penetration of concrete under carbonation conditions.In general,the three strength grades of concrete in each carbonation age（except C60 carbonation 28d）are more obvious to improve the resistance to chloride ion permeability of MgO group,because the effective filling effect of MgO related carbonation products on pores refines the pore structure and improves the resistance to chloride ion permeability of concrete,but the higher the strength grade,the smaller the resistance to chloride ion permeability.At 12%R-type MgO content,the maximum decrease in chloride ion permeability of C30 occurs at 28d of carbonization;C45 concrete can reflect the high resistance to chloride ion permeability requires carbonation for more than 56 days;the absolute decrease of C60 concrete reached the maximum at 14 days of carbonization,and the maximum relative decrease appeared at 84 days of carbonization.（6）Carbonization can refine the pore size,especially for large pores.After carbonization,the proportion of large pore volume decreases,and the carbonization products are deposited in the pores,filling the large pores into smaller pores,thus improving the strength,carbonation resistance and chloride ion penetration resistance of concrete.Carbonation,especially for 0.3-6R,0.4-6M,0.4-12R,0.5-B and 0.5-6R,has more obvious pore size refinement effect,so these groups have the best carbonation resistance.The carbonation degree of Ca（OH）₂ increased significantly with the increase of carbonation age,and the carbonation degree decreased gradually from the surface to the inside.The hydration degree of MgO in the core layer is significantly higher than that in the surface layer.The carbonation products of Ca（OH）₂ mainly include four kinds of Magnesite,Nesquehonite,Dypingite and Hydromagnesite,but the amount of carbonation products is less.There is a good correlation between the content of carbonized products obtained by HCl dissolution method and thermogravimetric analysis.