Evaluation of the mechanical properties, microstructure, and environmental impact of mortar incorporating metakaolin, micro and nano-silica

The optimized use of nanomaterials in concrete for developing resilient and eco-friendly infrastructure is vital for our built environment. Therefore, the current study aims to optimize the dosages and particle sizes of colloidal nano-silica in developing high-performance, sustainable cement mortar. For this purpose, compressive strength, water absorption, porosity, and microstructure tests were performed on mortars containing different dosages and particle sizes (NS80, NS250, NS250Al, NS500) of colloidal nano-silica. For comparison, tests were also performed on control mortars and mortars containing metakaolin and silica fume. The microstructure of cement pastes was examined through SEM-based BSE imaging with elemental mapping through EDS and FTIR analyses. Additionally, global warming potential (GWP) as well as embodied energy (EE) were also analyzed for all mortar mixtures. The results demonstrated that smaller nano-sized particles (NS500) at a lower dosage (3%) showed improved compressive strength and densified microstructure compared to all other mixes. However, larger nano-sized particles (NS80) at medium (5%) and high dosages (10%) had the highest impact on compressive strengths due to the formation of low and medium Ca-based C-S-H and C-A-S-H phases. Furthermore, the results of greater compressive strength of NS80 mortars under medium and high dosages were supported by their lowest water absorption and apparent porosity. When compared with the control and all other mixes, the mortar mixes containing medium and high doses of NS80 had lower embodied energy/MPa and CO2-eq intensity/MPa, indicating their significant contribution to producing green concrete with a lower environmental impact.