Toward sustainable self-compacting concrete: Rheological, mechanical, durability, and microstructural evaluation of biomaterial-based cement substituents

This study investigated the incorporation of egg shell powder (ESP) and fish scale powder (FSP) as partial cement replacements in self-compacting concrete (SCC) to advance sustainable construction practices. A comprehensive experimental program was conducted to evaluate the rheological, mechanical, durability, and microstructural properties of SCC mixtures with varying dosages of ESP (4–16 %) and FSP (1–4 %). The results revealed that while all the mixtures retained acceptable rheological behavior, ESP16 exhibited a 3.8 % reduction in slump flow and a 17.6 % increase in V-funnel time compared with those of the control mixture (CM), indicating a slight decrease in flowability. Mechanically, the compressive strength of ESP16 reached a peak of 35.30 MPa at 28 days, which was equivalent to that of CM, whereas that of FSP2 reached 30.71 MPa, corresponding to a 13.2 % reduction. FSP4 attained the highest split tensile strength of 4.68 MPa (a 5.2 % increase over CM), whereas ESP16 had the lowest at 1.39 MPa, representing a 68.8 % decrease. The flexural strength was better in ESP4 (5.63 MPa) than in CM (5.85 MPa), whereas the flexural strength of the FSP mixtures generally decreased. The durability results from the rapid chloride penetration test (RCPT) demonstrated significant improvements, with FSP4 achieving the lowest charge passed (971.50 Coulombs, “very low” category) and the minimum water penetration depth (9.86 mm) compared with CM (1438.60 Coulombs, 21.37 mm depth). Microstructural analysis confirmed denser matrix formation and reduced porosity in the ESP and FSP blends, indicating enhanced durability. These findings demonstrate that the optimized use of ESP and FSP in SCC can effectively reduce the environmental burden, underscoring their potential as eco-friendly, supplementary cementitious materials.