CO₂ curing of concrete blocks containing recycled aggregate

Abstract

For the purpose of environmental protection and reduction in resource consumption, construction & demolition (C&D) wastes have long been recycled and treated as a secondary aggregate for production of new construction materials. However, due to the intrinsically poor quality caused by the attached old cement mortar, recycled aggregates have been reported to weaken not only the mechanical properties, but also the durability of the resulting concrete products. Huge research effort is continuously required to develop innovative techniques to enhance the properties of recycled aggregate and related concrete products. Meanwhile, the calcium silicates phases in cement have been reported to react with carbon dioxide (CO₂) and water, leading to a rapid strength development for cement-based materials. At the same time, a certain amount of CO₂ can be sequestrated into the cement-based materials. Thus, in this research, experimental investigations are performed in order to study the beneficial use of CO₂ curing for producing concrete blocks prepared with recycled aggregates. The work conducted in this research program was consisted of three main phases. The work in Phase I was to explore the feasibility of using the CO₂ curing technique for the production of concrete blocks containing recycled aggregates. Recycled fine aggregates collected from a local recycling plant was used to replace the natural fine aggregate in concrete blocks. The CO₂ curing process was performed with a duration of 6 h, 12 h and 24 h, under the pressure of +0.1 bar. The experimental results proved that CO₂ curing process rendered the concrete blocks prepared with recycled aggregates with higher compressive strength and lower drying shrinkage than the corresponding moist cured blocks. However, curing time and amount of recycled aggregate present in the blocks had insignificant effects on the strength gain and CO₂ curing degree. The study in Phase II, mainly focused on evaluating several important factors that might influence the CO₂ curing process of concrete blocks made with recycled aggregates. There were two parts including materials characteristic and curing regime. In part 1, the effects of materials characteristics of the concrete block on CO₂ curing process were investigated, including moisture content, bulk density, aggregate to cement ratio, recycled aggregate content and types of binders. The results showed that, the initial moisture content and the aggregate to cement ratio in the concrete blocks significantly affected the CO2 curing degree and compressive strength of the concrete blocks. The bulk density and the recycled aggregate content also influenced the CO₂ curing process, but their effects on compressive strength were more complex. The work in this part further confirmed that the inclusion of recycled aggregates can improve the CO₂ curing efficiency. In part 2, the influence of several factors associated with the curing conditions on the curing degree and compressive strength of the concrete blocks were investigated, including curing time, temperature, relative humidity, pressure and post water curing after the pressurized CO₂ curing (PCC) method was employed. An experiment with flow-through CO₂ curing (FCC) method was also carried out. The results of PCC experiments showed that high percentage of the total considerable curing degree and compressive strength were attained during the first 2 hours of CO₂ curing period, and extending the curing time yielded slower gain of curing degree and compressive strength; the variation of temperature from 20 ℃ to 80 ℃ and relative humidity from 50% to 80% had limited effects on PCC; the effects of CO₂ gas pressure on the curing degree and compressive strength were the most pronounced between 0.1 bar and 0.5 bar within the first 2 hours of PCC; the post water curing allowed the concrete blocks to attain further strength gain. The results also indicated that, a lower curing degree and slower strength development at the early age were observed for the FCC-cured concrete blocks; however, after 24 hours of curing duration, they were comparable to those obtained from the PCC method. In Phase III, the intrinsic mechanism for rapid strength development of the CO₂-cured cement-based materials was studied, and the chemical and microstructural evolution of cement paste subjected to coupled CO₂- water curing was investigated. These was achieved through identifying the phase variations, change in porosity and morphological evolution of the structure of CSH gel. The results indicated that more than 2 hrs of CO₂ curing might lead to a lower long term strength development after the subsequent hydration period. The results of the solid state ²⁹Si MAS NMR showed that the CO₂ curing process can remove calcium ions from the interlayer of CSH, and promote the merging of dimers into polymers; and thus the formed CSH had a longer silicate chain length and a higher degree of polymerization. This is regarded as the main reason for the rapid strength development of the cement paste at the early age when subjected to the CO₂ curing process.