Studies On Carbonation Behavior Of Concrete Mixed With Fly Ash And Granulated Blast Furnace Slag And Its Influencing Factors

The rebound method is widely used for the acceptance inspection of concrete quality at present. Owing to high volume mineral admixtures used, concrete has a deeper carbonation, and after correction according to the carbonation depth, the resulted strength values are always lower than the structure concrete. Disputes between the purchaser and the supplier frequently occur, which makes people believe that the concrete with mineral admixtures are not durable (for lower carbonation resistance). This in turn has limited greatly the extensive use of high volume mineral admixtures in concrete, which is not in consistence with the requirement of sustainable strategy. For the measurement of carbonation behavior, a standard method is normally adopted, which is significantly different from practical condition. For the mechanism of carbonation, it is normally considered that due to the dilution of alkalinity in concrete caused by the replacement of cement by mineral admixtures and the secondary hydration which will consume Ca(OH)₂ formed from cement hydration, the carbonation of concrete would be accelerated. These perceptions could be misunderstanding.The study measured the carbonation behavior under the ambient condition. The carbonation depths of concrete and mortar specimens with various water/binder ratios, dosage of fly ash / ground granulated blast furnace slag were tested. The influence of curing age, water/binder ratio and aggregate on carbonation are analyzed; The pore size and porosity of cement paste samples with the water/binder ratio same as the corresponding concrete were measured by volume method and mercury intrusion porosimetry method to analyze the relationship between the pore structure and carbonation behavior of concrete.It has been shown that within 28 days the change in carbonation speed of concrete with fly ash / ground granulated blast furnace slag could be reflect by the changes in porosity. The porosity of concrete with fly ash was increased with the increase of fly ash and the concrete carbonation resistance was decreased. The porosity of concrete with ground granulated blast furnace slag was seen increased within 14 days with the increase of slag content and the carbonation resistance was reduced accordingly, but up to 28 days, the porosity was reduced with the increase of slag content and the carbonation resistance was therefore enhanced. With the decrease of water/binder ratio, the porosity was decreased and the carbonation resistance enhanced. Longer standard curing duration before carbonation is beneficial to the enhancement of carbonation resistance. It was also shown that mortar and concrete specimens with the same parameters of proportion have the same behavior for carbonation.Specimens at age of 22.5 months were tested and it was shown that both porosity and pore size could affect the carbonation velocity of concrete. At the age, concrete with water/binder of 0.4 and 20% fly ash had a higher porosity but reduced average pore size in comparison with the plain concrete and the carbonation velocities of the two concretes were consistent. In this case, the effect of pore size reduction was more significant. High volume ground granulated blast furnace slag was found beneficial to decreasing carbonation velocity of concrete in the later ages. Concrete with 30% ground granulated blast furnace slag shows the best performance, the average pore size of the concrete was lower than that of the plain concrete. Even when the dosage was up to 60%,the concrete carbonation velocity was still lower other than that of the plain one.Analyzing the core samples of much older concrete aged 7 to 10 years shows that the carbonation zone had played an important role to hinder further carbonation of concrete. As the increase of service duration, the density and carbonation resistance of concrete with fly ash were significantly increased. As long as the mineral admixtures is used properly in concrete, the problem of carbonation would not be as serious as thought.