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

Abstract

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.