Low-temperature characteristics of asphalt mixtures with sewage sludge ash under varying pH moisture conditions

The generation of various waste materials across global industries has raised significant environmental concerns and economic challenges. One of these waste materials is the sludge produced by municipal wastewater treatment plants. Given the critical role of material recycling in the sustainable development of road infrastructure, this study evaluates the effect of using sewage sludge ash (SSA) as a replacement for fine mineral aggregates in hot-mix asphalt (HMA) on the low-temperature performance of the mixtures under varying pH moisture conditions. SSA was used to replace fine granite (passing sieve No. 50) at four different dosages: 25%, 50%, 75%, and 100%. Fabricated mastic and mixture specimens were tested using pull-off and semi-circular bending (SCB) tests under moisture conditions with different acidity levels (pH = 5, 6, 7, 8, 9). The test results revealed that different moisture conditions, particularly acidic ones, reduced the adhesive/cohesive pull-off strength (APS/CPS) in the control samples, leading to an approximately 40% decrease in the mixture's fracture energy (FE) and fracture toughness (FT). However, due to its unique chemical and physical properties, SSA successfully enhanced the mixture's adhesion and cohesion under various pH moisture conditions. On average, an approximately 60% increase in FE and FT confirmed this improvement. A stepwise increase in the SSA weight% within the mineral aggregate structure (25%, 50%, 75%, and 100%) led to a gradual enhancement in FE, FT, APS, and CPS. This demonstrates that incorporating SSA can adjust the destructive effects of moisture conditions. Furthermore, statistical analysis indicated that using SSA at a 75% weight replacement level or higher resulted in a statistically significant impact on FE, FT, APS, and CPS. Accordingly, samples containing 100% SSA under varying pH moisture conditions exhibited optimal fracture and adhesive/cohesive properties compared to the control samples.