Effect of natural sisal fibers on the performance and mechanism of polymer cement waterproof coatings

To address the issues of cracking and shrinkage in polymer cement waterproof coatings (JS waterproof coatings). The present work investigates the incorporation of 1 wt% natural sisal fibers with different lengths (0.25 cm, 0.5 cm, 0.75 cm, and 1 cm) to modify JS waterproof coatings, aiming to reduce shrinkage-induced deformation and cracking. The effects of the fibers were examined through tests of tensile strength, bond strength, shrinkage, water absorption, contact angle, impermeability, performance at internal and external corners, and rheological behavior. Microstructural and hydration-related analyses were also conducted using scanning electron microscopy (SEM), hydration heat measurements, thermogravimetric analysis, emulsion film formation observations, and digital image correlation (DIC) strain analysis. Compared with the coating without sisal fibers, the coating containing 1 wt% sisal fibers with a length of 0.5 cm showed increases of 56% in tensile strength, 47% in elongation at break, and 27% in bond strength, this optimal formulation exhibited reductions of 94% in drying shrinkage, 71% in water absorption, and 81% in water penetration rate, together with a 15% increase in water contact angle. These changes improved the impermeability, waterproofing performance, and crack resistance of the coating. The improvement was mainly attributed to the ability of sisal fibers to bridge newly formed cracks and limit both crack initiation and crack propagation. During curing, the fibers also filled pores and made the coating denser. In addition, water released from the fibers promoted cement hydration at later ages, providing an internal curing effect. When the fiber length exceeded 0.75 cm, however, fiber agglomeration occurred and the coating performance declined. This study provides guidance for the design of high-performance SA emulsion-based JS waterproof coating.