Influence of waste tire rubber particles size on the microstructural, mechanical, and acoustic insulation properties of 3D-printable cement mortars

3D printing technologies of construction materials are gaining ground in the building industry. As well documented in the literature, these advanced manufacturing methodologies aim to reduce the labor requirements and materials waste, enhancing the architectural flexibility which would enable more sophisticated designs for engineering and aesthetic purposes. In this framework, the development of functional and eco-sustainable printable materials represents an extremely attractive challenge for research. In this study, the use of recycled tire rubber particles to replace sand in a 3D printable Portland-based compound was investigated. Specifically, two groups of continuous size grading polymer aggregates (tire rubber powder and granules as fine and coarse fractions, respectively) were analyzed in terms of impact on printability properties and on the microstructural, elasto-mechanical, and acoustic insulation performance of the material. The experimental results demonstrated a remarkable influence of the rubber particle size on the compounds’ behaviour. The high specific surface of the finest polymeric fraction promotes adhesion with the cement matrix, minimizing interface defects, and improving the mechanical toughness of the material. However, more significant mechanical strength loss was found than the modified material with the coarse aggregates due to a greater reduction in density and increased in porosity degree. On the other hand, mortar with larger rubber particles tends to have a higher unit weight, better mechanical strength, and ductility but worse interface binding with cement paste. Regarding the acoustic insulation properties, proper balance between rubber powder and granules in the mixes allows to obtain superior performance compared to plain mortar but the effect of the aggregate size is strongly dependent to the sound frequency range investigated