Effects of a Waterborne Coating on the Moisture Response, Surface-Layer Structure, and Mechanical Properties of Inorganic-Bonded Bamboo Composite

Inorganic-bonded bamboo composite (InorgBam) combines the lightweight, high-strength nature of bamboo bundles with the heat resistance, low smoke emission, and environmental compatibility of inorganic binders. However, the inherent surface pores and inter-bundle gaps in this material may facilitate water ingress, and the protective role of waterborne coatings, along with their influence on mechanical performance, remains unclear. This study investigated these issues through short-term water absorption tests, dynamic water contact angle measurements, industrial X-ray computed tomography (CT), and mechanical evaluations. After 48 h of immersion, the coated specimens absorbed only 9.55% water, representing a 59.0% reduction relative to the uncoated group (23.29%). The coated surface exhibited minimal change in contact angle, indicating restricted droplet spreading. CT imaging further revealed a relatively continuous coating layer that sealed surface pores, inter-bundle gaps, and local depressions, with no evidence of deep penetration into the interior. Following coating application, the parallel-to-grain tensile, compressive, and flexural strengths exhibited marginal increases of 3.37%, 2.94%, and 3.27%, respectively, while the dominant failure modes remained unchanged. Collectively, these findings demonstrate that the waterborne coating delays moisture ingress primarily through surface sealing and barrier effects, rather than internal pore-filling, thereby preserving the basic mechanical properties of InorgBam under unaged conditions.