Study On Construction Of Carbonated Protective Layer On Cement Mortar Surface And Its Resistance To Chloride Ion Infiltration

The durability of concrete is mainly influenced by the carbonization and chloride ion corrosion.Spontaneous carbonization results in the decrease of p H in the pore solution and accelerates the chloride corrosion of the cement concrete.In response to this problem,this paper innovatively proposed a serious of surface treatment methods of constructing a dense carbonation protective layer on the surface of cement mortar,including in situ carbonation of Mg（HCO₃）₂ solution with different concentrations,accelerated carbonation by adding calcium source and in situ formation of hydrotalcite,the surface resistance of cement mortar to chloride ion penetration was improved.The composition structure of surface carbonated protective layer,the permeability resistance of mortar to chloride ions and the adsorption and migration mechanism of chloride ions were studied.The prime results are as follows:（1）The cement mortar was soaked in the Mg（HCO₃）₂ solutions with different concentrations,which promoted the formation of Ca CO₃ on the cement mortar surface.Compared with the mortar without surface treatment,the chloride ion migration coefficient was reduced by about 16.51%in 3 wt%Mg（HCO₃）₂ solution.The addition of 0.05 wt%Li₂CO₃reduced the chloride ion diffusion coefficient of mortar by 32.93%.The study of microstructure showed that Ca CO₃ was deposited on the surface of the mortar mainly in the form of flake calcite crystals,which filled and sealed the pores on the surface of the sample and built a protective layer,thus reducing the surface porosity and cumulative pore volume.Chloride ions penetrated through the pores into the interior of the mortar,the transport efficiency reduced,which caused the decrease of chloride ion diffusion coefficient accordingly.Under the experimental condition of adding Li₂CO₃,on the one hand,the content of CO₃^2-was increased and the calcite production was promoted.On the other hand,Li⁺induced the calcite to grow to flatten and formed the lamelliform calcite,which made the surface structure of mortar samples much densification and further reduced the diffusion coefficient of chloride ion.（2）The slag is rich in Ca²⁺,taking the alkali-activated slag as the external calcium ion additive,the surface treatment of cement mortar was carried out by spraying Mg（HCO₃）₂solutions with different concentrations and alkali excited slag suspension with different alkalinity（4 wt%,5 wt%,and 6 wt%）onto the surface.The results showed that when the alkalinity of alkali-activated slag was 4 wt%and the concentration of Mg（HCO₃）₂ solution was 3 wt%,the chloride ion diffusion coefficient of mortar sample was reduced to 9.14×10^-12 m²/s,which was reduced by 59.54%compared with that of the untreated mortar sample.The addition of alkali-activated slag accelerated the carbonation rate of the mortar surface,and the calcite content was greatly increased in short time.In addition,silica gel formed by the decalcification of calcium silicate aluminate hydrate（C-A-S-H）,filled the pores and interspace of calcite crystal.The increased denseness of the protective layer structure further reduces the transportation rate of chloride ions,thus,the chloride ion diffusion coefficient was further reduced.（3）MgO was added into the slag,Mg/Al molar ratios in the slag system were 3:1,4:1,and5:1,respectively.The composite activator made of potassium carbonate and 2.4 M potassium silicate was used to activate the slag,and the samples were sprayed onto the mortar surface after stirring for different times at low temperatures.With the Mg/Al molar ratio of 5:1 and 90 min of low temperature stirring,the chloride ion migration coefficient of mortar sample was reduced to7.71×10^-12 m²/s,and it was reduced by 65.86%compared with that of the untreated mortar samples.When MgO was added into the alkali-activated slag and stirred at low temperature,hydrotalcite was generated.Meanwhile,the presence of CO₃^2-in the composite activator could also promote the formation of calcite.The hydrotalcite and calcite were deposited on the sample surface to form a composite protective layer.In addition to the filling and sealing effect of calcite and silica gel in mortar pores,the anion exchange property of hydrotalcite enable it to adsorb the chloride ions.Therefore,the number of free chloride ions in the samples were decreased,which further reduced the chloride ion diffusion coefficient of mortar.