Performance of lime-SCM composites for masonry retrofitting: Role of silica fume and metakaolin

The development of innovative and sustainable cementitious materials is currently a topic of increasing interest, especially for building retrofitting. Such products need to comply with strict demands, both in terms of performance and reduced environmental impact. Ordinary binders have a noticeable potential in terms of mechanical properties, but have some limitations related to other requirements, such as hygrothermal compatibility with traditional substrates and low carbon footprint. Lime-based mortars have been used since ancient times and still demonstrate excellent durability and thermophysical performance, thus being a suitable option for historic building retrofitting. Additionally, pozzolanic additives can considerably improve the mechanical properties of lime mixtures, partially filling the gap with cement-based products. In this study, the effects of silica fume and metakaolin used as supplementary cementitious materials (SCM) in lime-based composites were investigated. Thermophysical, mechanical, and durability-related properties were assessed for increasing additive dosages, supported by microstructural analysis and complemented by environmental impact evaluation. The main outputs highlighted the superior performance and lower carbon footprint of silica fume, with an optimal dosage between 30% and 40%. The latter achieved a compressive strength at 28 days exceeding 32 MPa and emitted 80% lower carbon dioxide per MPa unit of compressive resistance compared to the mixture without SCM. This study paves the way for further steps towards the development of lime–SCM composites with enhanced strength, durability, and energy performance, offering promising solutions for sustainable retrofitting and strengthening of masonry structures.