Effect of Biobased Natural Kapok Fiber on Properties of Sustainable Mortars Containing Slag and Fly Ash as a Partial Replacement of Portland Cement

Cementitious materials are among the most widely utilized construction materials worldwide, owing to their versatility, cost efficiency, and structural performance. However, the environmental challenges associated with ordinary Portland cement (OPC) production, including significant CO 2 emissions, have driven researchers to explore alternative, sustainable solutions. This research investigates the potential of incorporating biobased natural kapok (KP) fiber, a lightweight natural fiber, into cementitious mortars of Ground Granulated Blast‐Furnace Slag (GGBS)– and fly ash–based blends, where its application remains limited. The study evaluated the effects of KP fiber incorporation on the strength properties, pore structures, thermal insulation performance, and microstructural morphology of these mixtures. Results demonstrated that adding KP fibers at 1% by cement weight improved thermal insulation by up to 19.64% and by 10.97%, respectively, in fly ash and GGBS mortars. Moreover, 15.76% and 10.97% flexural enhancements were determined in thermally insulated GGBS and fly ash and mortars, respectively. Linear regression analysis between pore structure characteristics and the determined strength/conductivity values highlighted the significant role of KP fibers in modifying the cement matrix. These findings highlight the potential of KP fiber as a sustainable additive for advancing environmentally friendly construction materials.