Fundamental study on the acceleration of the neutralization of alkaline construction sludge using a CO2 incubator

Abstract The recycling ratio of construction sludge in Japan has gradually increased over the past 20 years. However, the increase in this ratio has become marginal. One of the main reasons for the stagnation is the alkalinity of the recycled sludge after treatment, as alkaline construction sludge can impact the environment owing to factors such as alkali leaching. Therefore, this study explored whether the neutralization of alkaline construction sludge can be accelerated with a controlled CO2 concentration during the production of recycled soils at recycling plants. A CO2 incubator was introduced in the laboratory to accelerate the neutralization of the alkaline sludge to investigate the effects of the initial water content and the layer thickness of the sludge on the neutralization process. The effects of the degree of CO2 concentration on the process were also examined, as were the contributions of other factors to the neutralization rate. First, the mixture conditions were explored to simulate alkaline construction sludge having a wide range of pH values (hydrogen-ion exponent) to investigate the effects of the initial pH on the neutralization rate. The results revealed that the neutralization rate of simulated alkaline sludge subjected to atmospheric curing was affected by the initial pH. Sludge with high alkalinity, such as a pH of 11 or 12, cannot fall below the upper limit of the pH regulated by the water pollution prevention act even after a dozen days. Consequently, sample surveys were conducted to capture the variation in the pH of actual construction sludge at several intermediate treatment facilities. The sample surveys showed that the pH of the sludge samples, which were in an either solid or semi-solid state, fell in the range of 6.9–13.0, with the samples having a pH of around 11 being dominant. Therefore, CO2 curing using a CO2 incubator was implemented in the laboratory on alkaline sludge that simulated the characteristics observed in the sample surveys. The laboratory test results revealed that, at a constant sludge height, a higher CO2 concentration increased the neutralization rate of the alkaline sludge. However, the contribution of the CO2 concentration to the acceleration of the neutralization decreased with a reduction in the initial water content. The test results also suggested that the contribution of the initial water content to the acceleration of the neutralization was more dominant than that of the sludge height for the present experimental conditions.