Fundamental Study on the Effect of CO2 Curing on the Strength Development of Alkaline Construction Sludge

Abstract

Construction sludge is known as waste material generated from construction industry. Since the construction sludge is usually very soft and at a high water content, it is often necessary to treat it with cement or lime to be recycled as construction materials. However, after the treatment, the increased pH sometimes cannot satisfy environmental regulation. By injecting high-pressure CO2 gas into the specimens, neutralization of alkaline construction sludge was accelerated. This research aims to study the effectiveness of using cement, CO2 gas, and paper sludge (PS) ash for not only lowering the pH level but also strengthening the alkaline construction sludge to be re-used in long-term durability. Alkaline simulated sludge having similar pH levels and state with the actual sludge were produced in the laboratory. The observed results showed that pH was decreased from more than 11.5 to around 8.6 after 7 h curing by the injection of CO2, whereas the addition of PS ash shortened the CO2 curing time. The strength of simulated samples was decreased during CO2 curing but then rapidly increased under air curing conditions. Strength of the samples with PS ash was seen significantly improved compared to that of samples without PS ash. After CO2 curing, under air curing conditions, the pH of treated samples tended to rebound to a certain level less than the original value, but then gradually decrease. It can be seen that the maximum value of pH rebound depends on the pH value obtained at the end of the CO2 curing.