Study On Migration Characteristics And Control Mechanism Of CO₂ In The Process Of Carbonation Curing For Concrete

Carbonation curing of concrete is a new way of concrete curing. Its advantages include short production period, low energy consumption, high mechanical performance and excellent durability of concrete products. However, the technique of carbonation curing is not mature which leads to the low-level and uneven of the concrete. In particular, the migration law and influencing factors for CO₂ gas in the process of carbonation curing are not clear. Therefore, it is of great significance to master the diffusion of carbon dioxide in the process of mineralization and the control method of the concrete.In this paper, the carbonation depth is treated as the index of the degree for diffusion of carbon dioxide. In order to explore the influence of a single variable on the CO₂ migration, the structure of concrete and curing condition of carbonation are changed respectively. CO₂ migration coefficient and depth model are established on the basis of the carbonation depth results to investigate the diffusion rule and the control mechanism of CO₂ in carbonation curing of concrete. The transition for the internal structure of the cement matrix before and after carbonation are also investigated by the mercury intrusion porosimetry, scanning electron microscopy and X-ray diffractometer, and the main results are as follows:1 Microstructure and composition of carbonation cured concreteThe carbonation effect was enhanced with the increase of the w/c ratios. The carbonation effect of the paste with a 0.60 w/c ratio was the best after 4h carbonation. The carbonation depth was 16.43 mm when the compressive strength was higher than that of the un-carbonated paste, which was up to 90.21%,while the carbonation efficiency was 15.12% which was lower than that of the 0.50 w/c ratio. The occurrence of appropriate bleeding phenomenon inside of the paste leads the increase of amount and sizes for connected pores, which is useful for the migration of CO₂. The loss of cement clinker contributes to the too large w/c ratio weaken the capacity of CO₂ fix and the strength of paste. The influence of carbonation time is significant in the first 6 hours during the long-time carbonation curing, while the following curing showed low efficiency. The internal structure of paste is improved by the carbonation curing, when the w/c ratio was 0.45, the total porosity of the paste after carbonation decreased by 54.01% after carbonation. The volume for more hazardous pores whose sizes were larger than 0.2?m decreased by 84.25%. In the process of carbonation, the reactants of carbonation reaction is mainly C2 S, C3 S and Ca?OH?2, the amount of reacted C-S-H is very small.2 Study on the influence factors and control mechanism of CO₂ diffusion in concreteFor mortar, the mortar using sands with a high fitting degree of the ideal Fuller gradation curve get a lower carbonation depth, the carbonation depth of mortars using sands with a higher deviation degree is as high as 20.63 mm. When the used sand was single gradation, the CO₂ migration turned to be more difficult in the internal of the mortar. When the sand fineness is 2.16-4.78 mm, the carbonation depth is only 5.71 mm. With the sand binder ratio decreases, CO₂ migration ability become stronger; the addition of mineral admixture increased the carbonation depth and decreased the carbonation efficiency, which is directly related to the loss of the cement clinker.For concrete, when the coarse aggregates with a single particle size were used in the concrete, the decrease of gravel sizes making the CO₂ migration and carbonation efficiency higher, the best particle level is 5-10 mm. The increase of sand ratio and the decrease of the w/c resulted in the decrease of carbonation depth; the increase of the cement amount made the diffusion of CO₂ difficult, when the amount of used cement was 360 kg/m3, the carbonation depth was the maximum.For the carbonation process, the reasonable pretreatment method and time can effectively reduce the moisture content of concrete, and improve the efficiency of carbonation. The curing condition during the carbonation has a considerable effect on the carbonation curing: the increase of CO₂ pressure driving the CO₂ gas to penetrate into the inside of the specimen more easily, the carbonation depth of up to 17.25 mm when the curing pressure was 0.45 MPa. The carbonation effect was the best when the temperature was 65 oC and the RH was 70%. When the humidity and temperature large were higher or lower, the effect of the CO₂ migration was weaken.3 The establishment of the prediction model of the diffusion coefficient for CO₂ and carbonation depthAccording to the influence of water cement ratio, sand rate, cement usage, carbonation time, carbonation temperature, carbonation pressure on the carbonation depth, a multicoefficient CO₂ diffusion coefficient formula is established. The tests show that the CO₂ diffusion coefficient and water cement ratio show linear relationship, and have quadratic function relationship with the dosage of cement, carbonation pressure, while having a cubic function with the amount of cement and sand ratio. And the CO₂ diffusion coefficient was in a relation with square carbonation time. The carbonation depth prediction formula derived from the CO₂ diffusion coefficient formula is as follow:...