Evaluation of low-carbon cementitious composites in marine environment for coastal protection and artificial reef substrate

This study investigates the long-term durability, microstructural evolution, and biological receptivity of three cementitious materials—Ordinary Portland Cement (OPC), Alkali-Activated Material (AAM), and Recreated Roman Cement (RRC)—under natural marine exposure for one year. A comprehensive bottom-up approach was employed, starting with microstructural characterization using XRD, MIP, microCT, TGA, SEM, and FTIR. These analyses were followed by evaluations of mechanical performance, carbon footprint, and biological growth potential. After one year, OPC paste samples exhibited ettringite, thaumasite, and gypsum formation, causing full-depth cracks. Despite this, OPC mortar maintained the highest compressive strength (70 MPa) among all binders. In contrast, RRC showed no ettringite or thaumasite formation; only increased hydrotalcite formation at its edges was observed. Nevertheless, RRC achieved a strength level comparable to OPC (55 MPa). Sodium hydroxide-activated blast furnace slag was used as the AAM composite for this study. The tested AAM formulation imparted the lowest compressive strength (35 MPa), attributed to the drying shrinkage crack formation during the initial sealed curing, which exacerbated upon seawater exposure. Notably, AAM showed increased calcium silicate hydrate (C-S-H) formation with time. Both RRC and AAM mortars exhibited lower environmental footprints, achieving reductions in global warming potential (GWP) of 50% and 52%, respectively, compared to OPC.