3D-Printed Gypsum–Cement–Pozzolan Composites with Crumb Rubber: Strength and Durability

This research investigates the formation and behavior of sustainable crumb rubber-modified gypsum–cement–pozzolan (GCP) composites, with a view to their use in a broad concept for construction. GCP binders are gaining attention as a low-carbon replacement for Portland cement, and the addition of recycled rubber helps the achievement of circular economy goals and potentially increases durability. The present research evaluates the impact of crumb rubber (CR) on the mechanical strength, water absorption, dimensional stability, and freeze–thaw resistance of 3D-printed GCP-rubber composites. Composite blends of variable proportions of crumb rubber were prepared at constant binder ratios. Mechanical properties were defined by prism specimens (40 × 40 × 160 mm) by the flexural and compressive strengths, and deformation was determined by micrometers to measure longitudinal strain as a function of curing. Water absorption was determined prior to freeze–thaw cycling to define pore saturation. Durability was investigated using two approaches: (1) controlled freeze–thaw experiments on cube specimens, with XF1 grade performance achieved, and (2) ultrasonic pulse velocity (UPV) testing of specimens 3D-printed for assessing internal structural change after long-term frost exposure. Results showed that compressive strength decreased moderately (10–20%) with increasing rubber content from 17% up to 50%, while flexural strength improved up to 15%, showing the elastomeric action of CR. Water absorption was reduced by 5–8% in the rubber-modified blends due to the hydrophobic character of rubber. Deformation tests also confirmed minimum length variation (<0.02%) during curing. Freeze–thaw durability was enormously improved, and test specimens retained more than 95% of initial strength. UPV measurements detected only a relatively modest velocity drop (~50 m/s) after 36 days cycling with subsequent stabilization up to 200 days, demonstrating long-term internal structure with minimal progressive damage. In summary, the findings demonstrate that GCP composites with crumb rubber incorporated are printable, dimensionally stable, and capable of freeze–thaw degradation resistance. Despite a moderate loss of compressive strength, the balance of introduced durability and sustainability suggests their competence as viable materials for additive manufacturing in construction.