Rainwater interactions with ISSA-modified mortars: pH evolution, CO₂ buffering and implications for urban infrastructure and waste valorization

Urban rainwater represents a realistic but often simplified exposure pathway for cement-based construction materials used in small-scale urban infrastructure. In this study, we investigated the time-dependent interaction between natural rainwater and cementitious materials, focusing on pH evolution, CO₂-related processes, and elemental mobility in both normative mortars and systems incorporating incinerated sewage sludge ash (ISSA). Rainwater collected in Kraków (southern Poland) exhibits near-neutral pH values that decrease slightly with storage time, reflecting equilibration with atmospheric CO₂ and the absence of strong acidic inputs, consistent with buffering from alkaline, potentially carbonate-bearing, urban aerosols.

Leaching experiments conducted over 1, 3, and 6 months show systematically higher pH values in leachates compared to the original rainwater, reaching approximately 8.1–8.4 after one month and gradually decreasing toward near-neutral values (≈ 7.0–7.3) after six months. These pH variations demonstrate effective alkalinity buffering by the cementitious matrix, dominated at early stages by portlandite dissolution and alkali release. With increasing exposure time, leachate pH shifts toward that of the incoming rainwater. 

Mortars containing ISSA exhibit pH trends comparable to those of conventional systems, with slightly moderated alkalinity release, suggesting the influence of additional aluminosilicate, phosphate, and iron-bearing components on the overall buffering capacity of the composite matrix. The observed pH evolution and associated changes in elemental mobility are linked to early alkalinity buffering, intermediate carbonation, and long-term diffusion-controlled stabilization. Throughout the exposure period, near-neutral to mildly alkaline pH conditions suppress the solubility of trace elements and promote sorption and encapsulation mechanisms, with no evidence of delayed contaminant release.

The results indicated that under realistic urban rainwater conditions, both conventional and ISSA-containing cementitious materials maintain chemical stability and environmental compatibility. Therefore, it is essential to consider natural rainwater chemistry and time-dependent pH evolution when evaluating the long-term durability and environmental safety of small-scale infrastructure. In addition, ISSA, when incorporated into cementitious matrices in appropriate proportions, does not represent a secondary source of contamination and may be valorized as a construction additive rather than disposed of in landfills

Acknowledgment: The research for this publication has been supported by the budget of the Anthropocene Priority Research Area (Earth System Science Core Facility Flagship Project) under the Strategic Programme Excellence Initiative at Jagiellonian University