Limestone calcined clay cement (LC3), a low-carbon alternative containing only 50 % clinker, demonstrates good performance in harsh environments. This study explores LC3 mortar incorporating waste coral sand for marine applications, focusing on chemical resistance exposed to seawater and 5 % Na₂SO₄ solution. Mechanical properties, phase composition, and microstructural evolution were analyzed over time, alongside mass and length variations. Results revealed that both coral sand and standard sand LC3 mortars exhibited exceptional resistance to sulfate and seawater attack, with no visible cracking or significant strength loss. However, coral sand mortar outperformed standard sand mortar due to its porous structure, which localized ettringite crystallization within internal pores, refining the pore network and enhancing durability. X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy analyses confirmed ettringite formation in coral sand pores without matrix damage, whereas Friedel’s salt dominated in seawater-exposed samples. The synergy between LC3’s low pH (suppressing expansive ettringite) and coral sand’s pore-filling mechanism highlights its potential for sustainable marine engineering. This work validates LC3-coral sand composites as a durable, eco-friendly solution for coastal infrastructure, leveraging local waste materials to reduce carbon footprint and improve service life.