Mix design strategies for 3D printable bio-based cementitious composites using rice husk particles as multifunctional aggregates

This work compares two mix design strategies for 3D printable cementitious composites using rice husk particles as multifunctional aggregates. The first strategy involves directly replacing sand with rice husk while keeping the cement paste volume constant. The second strategy introduces rice husk as an extra component, thereby increasing the solids content while maintaining a constant sand-to-cement ratio. Different volume fractions of rice husk (10%, 15%, 20%, and 25%) were used. Replacing natural sand with 15% bio-aggregates leads to a 53% decrease in yield stress and a 62% decrease in structuration rate, due to the lubricating effect of pre-wetted bio-aggregates and a decrease in system interlock. Adding rice husk as extra component, however, increases both yield stress and structuration rate, which is related to the reduced paste volume and increase in internal friction. With 25% rice husk, the initial yield stress increases from 1.21 kPa to 5.55 kPa, and the structuration rate increases from 26.9 Pa/min to 105.2 Pa/min. The second strategy effectively improved buildability and reduced cement consumption, making it the most suitable choice for producing printable mixtures. Mechanical tests indicate that rice husk has limited effect on anisotropy and interlayer bond adhesion. However, at higher fractions, increased porosity and particle debonding at interfaces pose new challenges for enhancing composite performance. The results highlight the potential rice husk as a multifunctional aggregate in 3D-printed cementitious composites, providing insights for the development of sustainable construction materials with optimized fresh and mechanical properties. •Two mix design strategies with rice husk for 3D-printed composites are compared.•Adding rice husk as an extra component improves shape retention and buildability.•Substituting sand with rice husk lowers yield stress, affecting print quality.•Higher rice husk content increases porosity and impacts mechanical strength.•Optimal rice husk content ensures good print quality and performance.