Study On The Effect Of MgO On Chloride Ions' Solidification Behavior Of Alkali-activated Slag Mortar

Considering the lack of river sand resources,the use of sea sand as a raw material in concrete will increase the concentration of chloride ions in concrete and further accelerate the corrosion of reinforcement bars in concrete.Compared with the low chloride ions' solidification rate and large energy consumption of Ordinary Portland Cement,the alkali-activated slag cementitious materials have their great advantages in durability,energy saving and environmental protection.In contrast to ordinary portland cement mortar(OPCM),the influence of MgO on the solidification of chloride ions of alkali-activated slag mortar(MAAM)was studied by using sea sand as aggregate,and alkali-activated blast furnace slag with MgO as cementitious material.Activators(containing water glass and sodium hydroxide)were applied to study the effects of MgO with different activities(high and slight)and contents(0%,2.5%,2.5%and 7.5%)on the compressive strength,flexural strength,chloride ions'solidification rate and the reinforcement bars corrosion rate of MAAM under curing conditions(standard and autoclaved).The hydration product types,morphology of MAAM and OPCM were studied by XRD,IR,FSEM-EDS and NMR.According to micro and macro experimental results,the mechanism of chloride ions' solidification rate was studied.The research shows that:1.Using water glass or NaOH as the activator,the hydration product types of MAAM are the same.The main hydration products of the MAAM are C-S-H and hydrotalcite-like(Ht)under standard curing.Under autoclave curing,the hydration products are mainly C-S-H and Tobermorite,and Ht is available.2.Highly active MgO promotes the formation of Ht.The substitution of Al3+ for Mg2+ results in the formation of a permanent positive charge in the Ht and the chloride ions are solified by the charge balance of Ht.Highly active MgO increases the substitution of Al3+ for Si4+ in C-S-H,and the formed negative charges absorb cations(Ca2+ and Mg2+,etc.).The chloride ions are solified by the charge balance of C-S-H.3.Under standard curing,if water glass is used as the activator,the chloride ions'solidification rate increases and then decreases as the highly active MgO's content increases.When the content of MgO is 5%,the chloride ions' solidification rate reaches the maximum value.When the content of slightly active MgO is less than 5%,the chloride ions' solidification rate of MAAM is less affected.When the content of MgO exceeds 5%,the chloride ions' solidification rate obviously decreases.4.Under standard curing,if NaOH is used as the activator,the chloride ions'solidification rate increases as the highly active MgO's content increases.When the content of MgO is 7.5%,the chloride ions' solidification rate reaches the maximum value at the 56th day.The effect of the chloride ions' solidification rate of slightly active MgO is similar to that of water glass as the activator.5.Under autoclave curing,using water glass or NaOH as the activator,the influences of MgO's activity and content on the chloride ions' solidification rate of MAAM are basically the same,and the solidification rate of MAAM of water glass as the activator is relatively higher.Despite the different activity levels of MgO,chloride ions' solidification rate increases as the content of MgO increases.The solidification rate of highly active MgO is higher than that of slightly active MgO.6.Compared with OPCM,the chloride ions' solidification rate of MAAM is higher.The rebar corrosion rate is not only related to the chloride ions' solidification rate,but also the internal micro-cracks of MAAM and so on.