Cradle-to-Gate LCA of Fly Ash Mortar for Low-Carbon Construction: GWP Reduction, Uncertainty Analysis, and Policy Relevance

This study has four objectives, namely 1) to quantify the cradle-to-gate Global Warming Potential (GWP) of mortar produced with 0%, 30%, and 50% fly ash substitution; 2) to identify the dominant emission contributors across material production, transport, and mixing; 3) to evaluate uncertainty and sensitivity in the Life Cycle Assessment (LCA) model; and 4) to interpret the mitigation potential in relation to Indonesia’s sustainable-construction and climate-policy context. This study was designed as a scenario-based cradle-to-gate LCA to quantify and compare the GWP of mortar mixtures incorporating fly ash. The functional unit was defined as 1 m3 of mortar targeted to an approximately 25 MPa compressive-strength class; however, the 30% and 50% fly ash mixtures were treated strictly as modelled LCA scenarios rather than experimentally validated mortar formulations. The system boundary was cradle-to-gate, covering raw material production, material transport to the batching plant, and mortar mixing. Fly ash was modelled using a cut-off allocation approach. A Monte Carlo simulation with 10,000 iterations was conducted for the 30% fly ash scenario. The baseline mortar without fly ash substitution generated the highest GWP, at 266.6 kg CO2-eq/m3. Replacing 30% of cement with fly ash reduced the GWP to 190.8 kg CO2-eq/m3, corresponding to a reduction of 75.8 kg CO2-eq/m3 or 28.4% relative to the baseline. A higher substitution level of 50% fly ash further reduced the GWP to 140.2 kg CO2-eq/m3, equivalent to a reduction of 126.4 kg CO2-eq/m3 or 47.4%. Overall, the scenario results indicate that fly ash mortar can be a promising low-carbon material option, but the findings should not be interpreted as proof of mechanical equivalence, and practical implementation requires compressive-strength verification, fly ash quality assessment, durability testing, site-specific transport data, plant-specific cement emission factors, and measured mixing-energy data.