Valorization of cockle shell powder as a sustainable supplementary cementitious material: Multimodal characterization and pozzolanic assessment

The investigation into sustainable supplementary cementitious materials (SCMs) has intensified in response to the urgent requirement to reduce the carbon footprint of Portland cement. The study investigated the potential of cockle shell powder (CSP), a biogenic waste abundant in calcium carbonate, as a reactive SCM in both its raw and calcined states. A multimodal characterization approach was employed, utilizing the modified Chapelle test, pH analysis, thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD), particle size distribution (PSD) and scanning electron microscopy (SEM). Results revealed that CSP exhibited negligible pozzolanic reactivity (−36.71 mg/g CaO fixation), acting mainly as a filler and lime donor due to its crystallite aragonite phase. Nevertheless, CSP’s fine particles (d50 ≈ 3.3 µm, SSA ≈ 13.15 m²/g) improved packing density and pore refinement in blends. Among the ternary blends containing CSP, FA, and cement, the formulation incorporating 10% CSP consistently demonstrated a strong synergistic interaction with FA. This combination resulted in noticeably higher pozzolanic reactivity values, particularly in blends with 20 and 30% FA, where the measured reactivity increased to approximately 77 mg/g and 110 mg/g, respectively. This indicates that a 10% CSP replacement level effectively enhances the overall reactivity of CSP–FA–cement systems. At higher dosages of CSP, dilution effects reduce reactivity. FA exhibited moderate pozzolanic reactivity (374.40 mg/g), while OPC functioned as a lime donor with reactivity of −407.44 mg/g. Calcination of CSP resulted in a much higher reactivity of 348.71 mg/g. In ternary blends containing cement, FA, and CCSP, the formulation with 20% CCSP and 10% FA content showed the highest lime fixation, reaching a reactivity of 190.87 mg/g. Complementary pH test results supported these findings where pH profiles showed CSP provided mild alkalinity (≈12.36) that assisted FA activation. TGA/DTG confirmed CSP’s high CaCO₃ stability with decomposition between 680–740 °C. XRD identified crystallite aragonite as the dominant phase in CSP and portlandite in CCSP, and PSD analysis highlights CSP’s ultra fineness compared to OPC and FA. The SEM analysis confirmed that CSP is porous, CCSP is denser, FA is reactive with spherical morphology, and OPC is angular and crystallite each exhibiting distinct microstructural features that influence hydration and strength development in cementitious systems. Overall, CSP cannot be classified as a conventional pozzolan but demonstrates potential as a filler-type SCM when used at low dosages, offering a biogenic alternative for a portion of the cement utilized in construction sustainability benefits through biowaste valorization and partial clinker substitution.