Accelerated carbonation of alkaline construction sludge by paper sludge ash-based stabilizer and carbon dioxide

Construction sludge frequently has high alkalinity after its generation or during the intermediate treatment process. The aim of this study is to experimentally investigate the potential of combining accelerated carbonation and a paper sludge ash-based stabilizer (PSAS) to neutralize the alkalinity of construction sludge in a short period and to improve its strength for use as a recycled material. The experimental results indicate that the addition of a PSAS significantly granulated the alkaline sludge, and once granulated, the PSAS successfully accelerated the pH neutralization of the alkaline sludge. It was also found that the decrease in dry density ρ_d and the degree of saturation S_r of the PSAS-treated sludge was able to reduce the period required for the pH neutralization, t_N. The decrease in ρ_d is thought to allow fresh CO₂ gas to penetrate the specimen more easily. However, if S_r is below a certain limit, it does not strongly facilitate the reduction of t_N. This implies that pH neutralization cannot be accelerated when the amount of water in the sludge is below a certain level. Moreover, it was found that mean particle diameter D₅₀ also affected t_N. The strength development of the PSAS-treated sludge was evaluated using a series of cone index tests. It was found that the strength of the alkaline sludge without the PSAS was significantly decreased by accelerated carbonation, but was significantly increased even after accelerated carbonation when the PSAS was present. Due to the porosities of the remaining PS ash particles, most of the contribution of the water absorption and retention performance of the PSAS to the strength development of the PSAS-treated sludge was secured after accelerated carbonation. In addition, the granulated particles of the PSAS-treated sludge retained their granular shape to some extent. Therefore, it is presumed that the friction and interlocking of the particles did not decrease significantly. It was also found that, after carbonation, the q_c of the PSAS-treated sludge increased more rapidly than that of the alkaline sludge without the PSAS. A further detailed examination of the test results showed that under air-curing conditions, the q_c of the treated sludge with accelerated carbonation increased relatively gradually compared to that of the treated sludge without accelerated carbonation.     

Effects of primary curing conditions and subsequent crumbling on properties of compacted soils treated with paper sludge ash-based stabilizers

Soil stabilization using paper sludge ash-based stabilizers (PSASs) has been developed as a technique for using the sustainable materials generated from industrial processes in construction projects. PSASs can be produced by insolubilizing the heavy metals in original paper sludge (PS) ash particles, i.e., the waste generated by the incineration of the PS discharged from paper mills. The aim of this study was to investigate the effects of the primary curing conditions and subsequent crumbling on the physical, compaction, and strength characteristics of PSAS-treated soils, using two types of PSASs with different water absorption and retention performances. For comparison, the same investigation was conducted on soils treated with blast furnace cement type B (BFCB). The experimental results revealed that, after crumbling, the PSAS-treated samples produced sand and gravel-like granules, regardless of the sealed or air primary curing conditions. In addition, the lower the water content at crumbling, the smaller the particle size. This was also true for the BFCB-treated samples. Consequently, the compaction test results demonstrated that, for one of the two types of PSASs, the dry densities of the PSAS-treated samples with sealed primary curing were almost the same as those without primary curing. The same trend was observed for the BFCB-treated samples. However, for the other type of PSAS, the dry densities of the PSAS-treated samples with sealed primary curing were lower than those without primary curing. This could be due to the difference in the degrees of disturbance caused by crumbling, depending on the type of PSAS. The rapid formation of hydrates in one type of PSAS may have significantly disturbed the treated samples owing to crumbling, resulting in a decrease in dry density. Finally, after secondary curing in a soaked environment, cone index tests were conducted on the PSAS- and BFCB-treated samples. The results indicated that the cone indices of the PSAS-treated samples with primary curing were higher or lower than those of the samples without primary curing, depending on the primary curing environment, number of curing days, and type of PSAS. The different trends, depending on the conditions, were considered to be caused by the combined effects of the “strength reduction owing to crumbling,” and “strength increase owing to water content reduction at compaction.” These mechanisms suggest that, for PSAS-treated soils with early strength development, the strength reduction caused by crumbling must be considered. However, for PSAS-treated soils with slow strength development, adjusting the water content of the treated soils through primary curing before compaction is an effective approach. Moreover, it is suggested that the curing conditions used for the laboratory mixtures be designed and set to reflect the field conditions and to minimize any discrepancies between the field and laboratory observations for the PSAS treatment.     

Effects of stabilizers on CO2 fixation capacity in neutralization of alkali construction sludge

Construction sludge, generated from tunneling and piling, is typically in a liquid state. It can be improved via physical treatments, such as dehydration, and/or chemical treatments, using stabilizers, in order to to be recycled as construction material. To adjust the strength of sludge, chemical treatments are often preferred. However, chemical treatments frequently result in alkali leaching. Methods to reduce alkalinity by curing the alkaline sludge under CO₂ gas at a certain concentration have been proposed in Japan. In recent years, technologies that utilize CO₂ to improve the quality of cementitious material have received considerable attention in terms of carbon capture. Therefore, the effects of stabilizers on the CO₂ fixation capacity of alkaline sludge during pH neutralization were investigated in this study. Accelerated carbonation and carbonate content measurement tests were conducted to detect the CO₂ content fixed in alkaline sludge specimens treated with various stabilizers. The test results showed that the fixed maximum CO₂ content per gram of dry mass of sludge, (m_CO2)^max, increased with the calcium oxide (CaO) content of the stabilizer(s) per gram of dry sludge, C_CaO. However, the rate of increase in (m_CO2)^max with C_CaO was significantly affected by the type of stabilizer used. In the case of quicklime (QL), the ratio of (m_CO2)^max to C_CaO was approximately 0.5, whereas, in the cases of fly ash (FA) and steel slag (SS), the ratio was approximately 0.25. The ratios for biomass ash and paper sludge ash were between that for QL and that for FA and SS. Detailed analyses of the test results suggest that the CaO content per gram of stabilizer(s) in the sludge, C*_CaO, can provide an estimate of the fixed maximum amount of CO₂ per gram of stabilizer(s) in the sludge, (m*_CO2)^max. However, other factors, including the amount of water-soluble Ca, should be considered for a precise evaluation. Additionally, the experimental results showed that the decrease in pH owing to neutralization increases with the increasing C_CaO. However, the type of stabilizer did not significantly affect the relationship between the degree of CO₂ fixation and the degree of neutralization.     

Evaluation of CO2 captured in alkaline construction sludge associated with pH neutralization

Recently, the capture of carbon dioxide (CO₂) using alkaline waste and byproducts has garnered considerable interest. Construction sludge may be categorized as alkaline waste, as it often exhibits high alkalinity during its generation or intermediate treatment. Hence, researchers have attempted to accelerate pH neutralization and recycle alkaline construction sludge by curing it under a high CO₂ concentration. By exposing concentrated CO₂ gas to an alkaline sludge, cement hydrates such as calcium hydroxide and calcium–silicate–hydrate gels form calcium carbonate (CaCO₃). Subsequently, the generation of CaCO₃ is expected to reduce the pH of the sludge. However, the amount of CO₂ captured in sludge has not been investigated extensively, unlike those of other alkaline wastes. Therefore, the amount of CO₂ captured in alkaline sludge that is associated with pH neutralization is evaluated in this study. It is demonstrated that accelerated carbonation tests using a CO₂ incubator and carbonate content evaluation tests based on the gas pressure method successfully reveal the amount of CO₂ captured in the alkaline sludge that is associated with pH neutralization. Additionally, the test results show that the amount of m_CO2 (i.e., the amount of CO₂ captured per 1 g of dry mass of alkaline sludge) increases with ΔpH (ΔpH is the difference between the initial pH and the pH after the alkaline sludge is neutralized). A maximum of 0.021 g of CO₂ is captured per 1 g of dry mass of alkaline sludge when the addition ratio of quicklime A_QL = 3% and 0.040 g when A_QL = 6%. The CO₂ capture ratio m_CO2/m_CO2^max, which represents the ratio of CO₂ captured in the sludge to the maximum capturing capacity, increases with ΔpH. CO₂ capture ratios of up to 90.0% and 84.9% are recorded when A_QL = 3% and A_QL = 6%, respectively. It is discovered that a higher A_QL results in a higher m_CO2. Moreover, the test results indicate that a higher A_QL causes a more significant change in the CO₂ capture ratio, even when the pH decreases slightly.     

The influence of particle size on microbial hydrolysis of protein particles in activated sludge

The influence of particle size on hydrolysis rates was evaluated in batch respirometers using protein particles derived from hard-boiled egg whites and activated sludge. It was found that initial hydrolysis rates for large particles were low but increased over time, contradicting commonly used mathematical modeling approaches to describe hydrolysis. A modified mathematical model was proposed based on particle breakup that described the observed hydrolysis kinetics for small and large protein particles. In the particle breakup model, hydrolysis results both in the release of readily biodegradable substrate and, at the same time, breakup of larger aggregates resulting in an increase of the specific surface area available for hydrolysis. Using this model, initial specific hydrolysis rates were calculated for the small and large particles, and ranged from 0.038 to 0.24 d(-1) and 0.19 to 0.98 d(-1) for large and small particles, respectively. The specific hydrolysis rate at the point of maximum overall hydrolysis rate ranged from approximately 2 to 3 d(-1) for all particles.