High cement content is often found in concrete mix designs to achieve the unique fresh-state behavior requirements of 3D Printable Concrete (3DPC), i.e., to ensure rapid stiffening of an extruded layer without collapsing under the stress applied by the following layers. Some materials with high water absorption, such as recycled concrete aggregates, have been incorporated in concrete mix designs to minimize environmental impact, nevertheless, the fine powder fraction that remains from the recycled aggregate processing still poses a challenge. In the case of 3DCP, few studies are available regarding mix designs using Recycled Concrete Powder (RCP) for 3D printing. In this context, this study presents the use of RCP as a filler to produce a printable mixture with low cement content. An RCP with 50 μm average particle size was obtained as a by-product from Recycled Concrete Aggregate production. Portland cement pastes were produced with 0%, 10%, 20%, 30%, 40% and 50% of cement mass replacement by RCP to evaluate its effects on the hydration reaction, rheology, and compressive strength. It was found that the studied RCP replacement was not detrimental for the hydration reaction of Portland cement during the initial hours, and at the same time it was capable of modifying the rheological parameters of the paste proportionally to the packing density of its solid fraction. The obtained results indicated the viability of 3DCP with up to 50% cement replacement by RCP. It was concluded that RCP presents good potential for decreasing the cement consumption of 3DPC, which in turn could decrease its associated environmental impact while providing a destination for a by-product from recycled concrete aggregate production.

In light of the effort for decarbonization of the energy sector, it is believed that common geopolymer binding materials such as fly ash may eventually become scarce and new geological aluminosilicate materials should be explored as alternative binders in geopolymer concrete. A novel, tension-hardening geopolymer concrete (THGC) that incorporates high amounts of semi-reactive quarry wastes (metagabbro) as a precursor, and coarse quarry sand (granite) was developed in this study using geopolymer formulations. The material was optimized based on the particle packing theory and was characterized in terms of mechanical, physical, and durability properties (that is, compressive, tensile, and flexural resistance; Young’s modulus; Poisson’s ratio; absorption; drying shrinkage; abrasion; coefficient of thermal expansion; and chloride-ion penetration, sulfate, and salt-scaling resistance). The developed THGC, with an air-dry density of 1940 kg/m3 (121 lb/ft3), incorporates short steel fibers at a volume ratio of 2%, and is highly ductile in both uniaxial tension and compression (uniaxial tensile strain capacity of 0.6% at an 80% post-peak residual tensile strength). Using digital image correlation (DIC), multiple crack formation was observed in the strain-hardening phase of the tension response. In compression, the material maintained its integrity beyond the peak load, having attained 1.8% compressive strain at 80% postpeak residual strength, whereas upon further reduction to 50% residual strength, the sustained axial and lateral strains were 2.5% and 3.5%, respectively. The material exhibited low permeability to chloride ions and significant abrasion resistance due to the high contents of metagabbro powder and granite sand. The enhanced properties of the material, combined with the complete elimination of ordinary portland cement from the mixture, hold promise for the development of sustainable and resilient structural materials with low CO2 emissions, while also enabling the innovative disposal of wastes as active binding components.

This paper assesses the suitability of geopolymers (GPs) for use as adhesives for ceramic tile fixing, including their compliance to the relevant EN 12004 specification. Two series prepared with different percentages of metakaolin (MK), blast-furnace slag (BFS), and limestone materials activated by an alkaline NaOH/ Na2SiO3 solution are investigated. Tested properties included the thixotropy, setting, compressive strength, open time, and adhesion bond strength under different exposure conditions (that is, dry, wet, heat, or freezing-and-thawing cycles). Compared to cement-based mortars containing adjusted proportions of cellulose and redispersible polymers, the GPs exhibited higher thixotropy, reflecting additional energy for spreading the material over the substrate, yet better maintenance of the alternating patterns of ripples and grooves at rest. The bond strengths tested under different exposure conditions were remarkably high for the MK-based GP, given the fine MK particle sizes that foster geopolymerization and crosslinking of solid bonds in the hardened structure. The BFS-based GP exhibited relatively lower bond strengths (compared to MK) due to coarser particles. Such results can be of interest to civil engineers and manufacturers of ready-to-use building materials that aim at reducing the portland cement footprint while assuring performance and sustainability of tiling applications.

The need to constantly improve the quality and properties of manufactured products leads to the development of hybrid materials that combine different elements, complementing one another. Fiber-reinforced mortar is one of those products, as the fibers are used to improve cementitious materials’ flexural weakness. Experimental data on different metallic fibers dispersed in mortar demonstrate the correlation between early-age rheological properties and long-term mechanical strength. Both quantities depend on the ratio of the solid volume fraction of the fiber to a critical solid volume fraction characteristic of the form factors of the fiber. It is demonstrated that both effects arise from the packing stress of the fibers in the mortar when their concentrations are close to their maximum packing fraction. Geometrical arguments are used to explain how this critical volume fraction is related to the fiber form factor. Then, it enables the building of master curves using geometrical arguments.

Successful implementation of extrusion-based three-dimensional (3-D) printing requires the development of print materials with adapted rheology. In this study, filtration characteristics coupled with rheological properties of mortar mixtures are investigated to characterize the extrudability of print materials and establish a “printability window” (that is, the acceptable range of material properties for successful extrusion and shape stability). The extrudability was measured as the maximum force needed for the ram extrusion of the material. The fluid filtration rate was assessed in terms of desorptivity of the fresh mixture under pressure. The yield stress, plastic viscosity, and desorptivity were varied by changing the water-cement ratio (w/c), high-range water-reducing admixture (HRWRA) dosage, and welan gum (WG) content. Regression analysis indicated that during extrusion-based printing, the yield stress and desorptivity values can exhibit a more significant effect on extrudability than plastic viscosity.