Strength, Transport Properties, and Life Cycle Impacts of Mortar Containing German Natural Pozzolan

Portland cement production is energy- and carbon-intensive. Substituting part of the clinker with natural pozzolans is a promising route to lower-impact mortars. This work evaluates mortar where Portland cement is partially replaced by a German natural pozzolan (12–56% by mass). Compressive and flexural strengths were measured at 7, 28, and 90 d. Water-accessible porosity (28 d) and 24 h water absorption were also determined. Strength development and water transport were interpreted using (i) a three-parameter strength–age model and (ii) a capillary–diffusive model. The results showed delayed reactivity typical of pozzolanic materials. At 90 d, 12% replacement slightly exceeded the control by 3.38% and 1.4% in compressive and flexural strengths respectively. Higher replacement levels caused a drop in strength at 90 d (18.3% at 36% and 42.5% at 56% in compression; 25.3% and 31.0% in flexure). Porosity and absorption increased with replacement, consistent with the mechanical trends. The compressive and flexural strengths were strongly correlated. Life cycle analysis showed a significant reduction in embodied carbon, reaching approximately 52% at 56% replacement. Overall, moderate replacement (12–21%) provides the best balance between performance and carbon reduction.