Material, process, and design optimization of local earthen soil reinforced with natural fiber waste and nanoclay for 3DP of functional structures

The construction industry urgently needs sustainable, locally sourced materials with enhanced performance and compatibility for 3D printing (3DP). Earthen soil, though abundant and eco-friendly, often lacks the rheological and mechanical properties necessary for optimum extrusion and buildability. This study develops a 3D printable composite using Qatari earthen soil, bio-waste coconut fibers (CF), and nanoclay (NC), aiming to overcome these limitations. Twelve different material compositions were evaluated to optimize flowability, structural build-up, and mechanical strength. Among these, Mix 7, containing 6 % CF and 0.2 % NC, demonstrated optimal performance, achieving 80 % flow retention after 60 min, dynamic yield stress of 3213.76 Pa, and plastic viscosity of 64.63 Pa s. It also exhibited the highest compressive and flexural strengths (12.43 MPa and 2.30 MPa in molded samples; 11.20 MPa and 2.00 MPa in 3DP samples). In contrast, mixtures with higher NC or fiber content, such as Mix 12, experienced brittleness, poor flowability, and structural failures due to fiber misalignment and particle aggregation. Using a design of experiment (DOE) approach, key printing parameters, specifically a layer height of 2.5 mm, printing speed of 30 mm/s, and extrusion multiplier of 2.0, were optimized to enable fabrication of free-form, acoustically functional wall elements via a knitting-concrete approach. Although the optimized parameters improved print quality and reduced cracking, curved regions still exhibited stress-induced failures, indicating a need for further geometrical refinement. This study proposes a material–process framework for sustainable 3DP using locally available resources, reinforcing the synergy between mix design, printing parameters, and functional performance.