دانلود رایگان مقاله دوام بتن تولید شده با استفاده CO2 به عنوان ترکیب شتاب

Abstract

Carbon dioxide was investigated for use as a beneficial admixture to concrete as it was truck mixed. The reaction between the CO2 and the hydrating cement creates finely distributed calcium carbonate reaction products that thereby influence the subsequent hydration. Comparisons of the fresh, hardened and durability properties were made between a reference concrete batch, a batch that contained a conventional accelerating admixture, and three batches subjected to a carbon dioxide addition. The optimum dose of carbon dioxide was found to reduce the time to initial set by 40% and increase the one and three day compressive strengths by 14% and 10% respectively. In comparison to the CO2 batch, the conventional accelerator provided greater reductions in set time but lower early strength. Concrete durability test results indicated that the carbon dioxide process did not compromise the expected durability performance of the treated concrete. Carbon dioxide is a viable admixture to improve concrete performance.

5. Conclusions

A series of 4 m3 concrete mixtures were produced in concrete trucks using injection of carbon dioxide during their mixing. The injection of waste CO2 into the concrete mixtures accelerated the hydration and strength development without affecting the fresh properties. The time to initial set was accelerated by 95e118 min (an average 25% time reduction) and the final set was accelerated by 103e126 min (an average 23% time reduction). The mixture batched with the conventional non-chloride accelerator offered 173 and 162 min improvements to the times of initial and final set, respectively. Isothermal calorimetry further supported the conclusion that the CO2 injection accelerated early hydration reactions and indicated that the carbon dioxide reacted with the silicate phases whereas the non-chloride accelerator is normally considered to react with the aluminate phases. A compressive strength benefit was observed for the concrete that received the lowest dose of CO2 but interpretation was complicated by differences in air content (however, other trials have suggested that a strength benefit is readily achievable outcome). The batches with the two higher doses of CO2 did not show a strength benefit but the reference concrete contained a greater addition of a strength enhancing/retarding water reducer. The durability testing showed that the CO2-injection process had a neutral to positive effect on concrete durability. Suitable chloride penetration resistance, drying shrinkage, freeze-thaw, and de-icer salt scaling resistance performance of the CO2-treated concrete was assured through testing. The acceleration benefits are associated with the in-situ development of uniformly distributed nano-carbonate reaction products. The products act as nuclei during early hydration and/or the lower Ca/Si silicate hydrate gel that forms alongside the carbonate products is less dense. The economics of using an injection of carbon dioxide as a set accelerator are favourable as compared to use of a non-chloride accelerator. However, the acceleration effect of the CO2 injection does not appear to be as potent, lending thought towards using it in combination with a reduced dose of accelerator.