Selection of the Best 3D Printing High-Performance Mortars Using Multi-Criteria Analysis

High-performance concrete for 3D printing has recently attracted significant attention due to its potential to create structural elements without the need for traditional reinforcement. While various formulations have been proposed by researchers, evaluations are often limited to mechanical performance and printability, while cost and environmental impact are generally overlooked. This study expands the analysis by also considering cost and environmental impact, aiming to identify the optimal mix using a multi-criteria decision-making analysis (MCDMA). In the first phase, several high-strength mortar formulations were developed and assessed based on mechanical strength, printability, environmental impact, and cost. In the second phase, the most promising mix from the initial evaluation was further modified by incorporating different types of fibers, including aramid, carbon, glass, cellulose, and polypropylene. Comprehensive testing—covering mechanical properties and printability—together with cost and a life cycle assessment were conducted to determine the most effective mortar formulations. One of the main findings is that adding 0.05% of 20 mm length cellulose fibers in weight to a mortar containing Cem I 42.5R can increase the compressive strength by more than 9% without affecting the cost or environmental impact, also allowing the obtainment of a mortar apt for 3D printing. This increase in the compression strength is presumably related to a lateral restriction in movements of the mortar, which makes it increase the maximal principal stresses, and thus, its strength.