Dual Modified Calcined Oyster Shell Lime Grouts for Crack Repair in Humid Coastal Limestone Relics

Abstract Coastal humidity accelerates fissure deterioration of limestone artifacts, requiring grouts to combine substrate compatibility with environmental durability. Discarded oyster shells from Sanmen, Zhejiang were calcined as the sole binder, blended with silica fume and polycarboxylate superplasticizer, then modified with SiO₂ nanosol or hydroxypropyl methylcellulose at 0.2 to 0.6%. SEM and XRD revealed modification mechanisms, while grouting simulation and onsite repair at Hangzhou Yuewang Temple verified serviceability. The optimal 0.4% nanosol dosage coupled pozzolanic reaction with micro filling, generating calcium silicate hydrate gel and an acicular AFt skeleton, achieving nearly 11 and 10 MPa in 28 d compression and 1.8 MPa in flexure. At 0.4% dosage and low water to binder ratio, HPMC water retention cut shrinkage to 0.2 to 0.3% and raised 28 d bond strength from 0.2 to 0.65 MPa. Cracks deeper than 50 mm suit SSPSi systems, while 40 to 50 mm shallow cracks suit SSPSH systems.