Utilization of Sargassum Natans in Sand Crete Block Production for Sustainable Walling Solutions

Seaweeds are a diverse and abundant group of marine algae that offer a promising sustainable alternative to conventional cementitious materials, which are increasingly associated with high environmental impact. This study investigated the feasibility of incorporating locally sourced Sargassumnatans powder into cement–sandcrete blocks as a bio-based additive for sustainable walling. Seaweed collected at Eleko Beach, Lagos State, Nigeria, was washed, sun-dried, oven-dried, pulverized, and sieved before incorporation at 0% (control), 5%, and 7.5% by mass of cement in a 1:5 cement-to-sand mix. Specimens were produced in 230 × 110 × 80 mm moulds and evaluated using material characterization, sieve analysis, specific gravity, compressive-strength testing at 7, 14, and 28 days, water absorption, thermal screening, efflorescence observation, and a stakeholder perception survey (n = 55). This study evaluates the physical, chemical, mechanical, and thermal properties of seaweed-based (Sargassum natans) sandcrete blocks to support more environmentally friendly and cost-effective construction practices. Locally sourced Sargassum natans was processed into powder and incorporated as a cement additive at 5% and 7.5% by weight. Laboratory tests—including chemical composition analysis and microstructure using (optical microscopy and SEM-EDX), sieve analysis, specific gravity, compressive strength, water absorption, and thermal conductivity—were carried out in line with relevant BS and AASHTO standards. Results showed that Sargassum natans contains substantial amounts of SiO₂ (41.6%) and CaO (29.9%), indicating pozzolanic potential. The mix with 5% additive achieved at 28-days compressive strength of 8.22 N/mm², closely matching the 7.5% mix (7.08 N/mm²) and satisfying the Nigerian Industrial Standard (NIS 87:2000) for load-bearing blocks.