Compositional variability of natural hydraulic limes (NHL) in Europe. Part 3: Mechanical performance of mortars under different curing regimes

In this paper, the mechanical performance of mortars made with different types of natural hydraulic lime (NHL2, NHL3.5 and NHL5) produced by six European manufacturers, was analysed. A total of 17 NHLs were used to produce 204 mortar specimens, cured at 95 ± 5% RH during the first 7 days and then maintained at either 60 ± 5% or 90 ± 5% RH, for testing at 28 days and 6 months. Results reveal significant mechanical differences between the mortars that cannot be explained solely by their hydraulicity. and arise above all due to 1) the type and content of calcium silicates, which can vary between NHLs belonging to the same hydraulic class but made by different manufacturers; and 2) the presence of either under-burnt or over-burnt lumps, due to differences in the calcination temperature, which can influence NHL reactivity. The petrographic changes in the matrix were investigated to link curing conditions, NHL type, and mechanical performance, providing the first systematic assessment of this aspect. The findings of this study have significant practical implications for building practitioners, heritage professionals, and technical committees working with NHL mortars in both conservation and sustainable construction contexts. From a sustainability perspective, the capacity demonstrated here of all NHL classes to meet minimum strength requirements under two-stage regime consisting of 95% RH for the first 7 days followed by RH ≥ 60%, supports the broader use of NHL mortars in contexts where sustained high-humidity curing is not feasible, expanding their possible use in sustainable and low-carbon construction without compromising structural performance.