Comparative effects of tannic acid and ZnO nanoparticles on hydration, microstructure, and antimicrobial performance of Portland cement.

Despite the well-documented antimicrobial performance of ZnO nanoparticles, their incorporation into Ordinary Portland cement (OPC) is often accompanied by unfavorable effects, including delayed setting and deterioration of early-age mechanical properties. This investigation evaluates tannic acid (TA), a bio-derived organic compound, as a sustainable alternative cement additive to nano-ZnO. OPC pastes were modified with varying dosages of TA or nano-ZnO, and their influence on setting characteristics, workability, hydration progress, chemically combined water, free lime content, compressive strength, pore structure, phase composition, and microstructural development was systematically investigated. The results show that TA accelerates hydration at low dosage (0.25%) but exhibits a retarding effect at higher content (1%), whereas nano-ZnO consistently retards hydration regardless of concentration. At 3 days of curing, OPC containing 0.25% nano-ZnO attained a compressive strength of 78.3 MPa, while a pronounced reduction to 7 MPa was observed at 1% addition. In contrast, TA-modified systems demonstrated more stable mechanical performance, achieving 68.3 MPa at 0.25% and maintaining approximately 50 MPa at 1%. BET/BJH pore structure analysis revealed that both additives at 0.25% induced significant pore refinement, reducing the mean pore diameter from 67.92 nm to 48.52 nm for TA and 45.57 nm for nano-ZnO. Mineralogical and microstructural analyses (XRD, FTIR, SEM/EDX) indicated that TA promotes silicate dissolution and enhances the formation of portlandite, stratlingite, and tobermorite-like CSH phases. Conversely, nano-ZnO was found to inhibit hydration through the formation of Zn(OH)