Study On The Effects Of Chemical Admixture On Carbonation Of Hardened Cement Paste

Reinforced concrete is the most important and most common use form for the structure concrete. But safety accidents occur frequently due to durability problems, which cause huge losses to the state and society. The durability of concrete mainly includes reinforcement corrosion, freeze-thaw damage and fatigue damage, etc., among them reinforcement corrosion caused by carbonation are widespread and most likely to occur. Furthermore environmental changes like the increasing CO2 concentration and climate warming, make a great impact on carbonation of the concrete. Therefore, the understanding of concrete carbonation mechanism and rules of its nature, so as to truly achieve the durability design of reinforced concrete, has great significance in the academic. The three kinds of chemical admixtures such as air-entraining admixture expansion agent and superplasticizer could affect the pore structure of concrete, which is the important factors influencing the carbonation. Therefore to study the effect of chemical admixtures on carbonation has important practical significance.Hardened cement paste is the only component involved in carbonation, so the investigation of carbonation of it can provide more theoretical basis for carbonation of concrete. By using chemical analysis method, the effect of air-entraining agent, superplasticizer and expansive agent on cement carbonation was investigated and its mechanism was also been discussed in this study as well.The effect of air-entraining admixture on carbonation of hardened cement paste associated with water cement ratio. When water cement ratio was 0.4, air-entraining agent could entrain closed bubbles, which increased the permeability resistance of hardened cement paste, so both the two kinds of air-entraining agent squinty improved carbonation resistance of cement in different degree. While with higher water-cement ratio(> 0.4), the porosity increased with the dosage of air-entraining agent, which in turn accelerated carbonation, deteriorated the carbonation resistance of cement. Air-entraining agent had different effect on carbonation of cement in different relative humidity. When the humidity was higher, there was no obvious change on cement carbonation degree even extending the carbonation time; when the relative humidity was lower than 54%, the complete carbonation zone shortened. So air-entraining agent could improve hardened cement paste carbonation resistance in low relative humidity.The influence of the expansion agent on hardened cement paste carbonation was compromised by two factors, namely the amount of carbonized substance and pore structure. When the water-cement ratio was 0.4, the increase of carbonized substance was dominated which could improve the carbonation resistance. Whereas water cement ratio was increased to 0.5, expansion agent would cause deterioration of the pore structure, decreasing the carbonation resistance. The pore structure degradation was dominated on the effect of carbonation.When water cement ratio is less than or equal to 0.4, mixing with superplasticizer can increase the fluidity of cement slurry, but compared with the blank group there was no significant difference of carbonation degree. That means superplasticizer has no serious influence on the carbonation of hardened cement paste. When water cement ratio was 0.45, carbonation degree of hardened cement paste with 0.8% superplasticizer was increased. Higher dosage of superplasticizer will reduce actual water requirement of cement hydration. With the evaporation of excessive water, much more pore was formed in the hardened cement paste, which would provide channels for CO2 diffusion, thus increased the carbonation rate.The two kinds of air-entraining agent could increase the porosity of hardened cement paste, while the bubble parameters of hardened cement paste were different. These differences in characteristics of the internal porosity lead to different carbonation resistance of hardened cement paste. Without constrained, porosity of hardened cement paste would increase with expansive agent, and the pore can be connected and macro, which seems adverse to the carbonation resistance of hardened cement paste. The admixtures had different influence on mortar strength. Due to greatly increased the porosity, the mortar strength with air-entraining agent decreased obviously; when the water cement ratio was 0.4, mixed with superplasticizer resulted in the mortar bleeding which had adverse effects on strength; adding expansive agent mortar strength increased slightly.