Flexural Strength and Thermal Performance of Low-Carbon Cement Mortars Incorporating Microalgae Biomass

Abstract

This study investigates the potential of Arthrospira platensis (A. platensis) microalgae biomass as a bio-based, carbon-negative substitute for the partial replacement of Portland cement in low-carbon mortars. Mortar specimens were prepared by substituting 2.5%, 3.75%, and 5% of cement with A. platensis and evaluated in terms of mechanical, microstructural, thermal, and durability performance. At 7 days, the replacement with microalgae notably enhanced flexural strength by up to 100% compared with the control mix. However, at 28 days, flexural strength decreased with increasing microalgae content, indicating that the early-age benefit does not persist. These organic components promoted the formation of viscoelastic bonding networks within the cement matrix, improving energy absorption and resistance to crack propagation. However, compressive strength slightly decreased with increasing substitution levels due to partial hydration inhibition and elevated porosity. Microstructural analyses (SEM–EDS, XRD, FTIR, and TGA/DTA) confirmed that A. platensis reduces portlandite content through dilution and accelerated carbonation, confirming the absence of pozzolanic secondary C–S–H formation. Biomass‑related carbonate formation was observed instead. Thermophysical measurements revealed significant reductions in thermal conductivity (by 7.3%), indicating improved thermal insulation. While low substitution levels (≤ 2.5 wt%) offered an optimal balance between mechanical enhancement, thermal efficiency, and microstructural stability, higher contents (> 3.75 wt%) accelerated carbonation and increased porosity, potentially affecting long-term durability. Overall, the findings demonstrate that A. platensis, used as a partial replacement for Portland cement, acts as an eco-efficient, multifunctional replacement enhancing flexural performance and thermal resistance while contributing to the development of sustainable, low-carbon construction materials. © The Author(s), under exclusive licence to Shiraz University 2026.