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.

An experimental investigation was carried out using the volumetric proportioning approach to achieve printable portland cement concrete mixtures. The types of aggregates investigated were rounded pea gravel and coarse and fine sand. The test matrix of potential concrete mixtures was prepared based on watercement ratios (w/c) of 0.46 to 0.48, sand-to-stone ratios (sa/st) of 1.18 to 1.91, and paste-aggregate ratios (p/a) of 0.74 to 0.81. The workability and early-age strength of fresh concrete were characterized by field-friendly flow-table and unconfined compressive strength (UCS) tests. Test results indicated that the w/c, sa/st, and p/a all significantly affect fresh concrete pumpability and early-age strength. The overall research results revealed that pumpability and buildability can be evaluated with these two tests. The results of these two tests together are used to define a printable region.

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 (i.e., compressive, tensile, flexural resistance, Young’s Modulus, Poisson’s ratio; absorption, drying shrinkage, abrasion, and coefficient of thermal expansion; chloride ion penetration, sulfate, and salt-scaling resistance). The developed THGC with an air-dry density of 1,940 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 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% post-peak 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 mix, hold promise for the development of sustainable and resilient structural materials with low CO₂j, emissions while also enabling the innovative disposal of wastes as active binding components.

This study clarifies the effects of moisture (expressed as percentage saturation degree of permeable pore voids, PSD) on water ingress properties of concrete and establishes a region where PSD does not affect the quantitative water absorption. Experimental measurements and finite element model (FEM) simulation results for ordinary portland cement (OPC) concretes preconditioned to equilibrium moisture formed plateaus between 21 and 58% PSD. Non-continuous finer capillary pores (ϕ10 nm [3.937 × 10–4 mil, thou] to ϕ100 nm [3.937 × 10–3 mil, thou]) constitute the empty pores within the plateau region before tests. Water sorptivity of OPC and slag cement concrete blocks at several degrees of surface moisture with internal moisture gradients validate the existence of the plateau within the PSD range. Measuring short-term water absorption within this plateau region eliminates the effects of initial surface moisture content on the measured properties and evaluates the continuity and connectivity of pores, which is the major indicator of the durability of concrete.

Thermal-cured alkali-activated binders (AABs) are a potential replacement for traditional portland cement (PC) in concrete, primarily for precast applications. To avoid this energy-intensive regime and encourage wider application, this study investigates the development of ambient-cured AABs by adding graphene oxide (GO) nanoparticles. The mechanical strength and durability characteristics are determined for alkali-activated slag (AAS) mortar specimens prepared using 4, 6, and 8 molar (4, 6, and 8 M) concentrations of sodium hydroxide in the alkaline activator. The different percentages of GO by weight of slag are 0.0, 0.03, 0.06, and 0.09%. The mechanical parameters considered are compressive, flexural, and splitting tensile strengths. The durability parameters investigated are the rapid chloride permeability test (RCPT), sorptivity, and acid resistance. The performance of ambient-cured AAS mortar specimens containing GO is compared with thermalcured AAS mortar specimens (without any GO inclusions) and the control cement mortar (PC) to evaluate the effect of GO on the mortar characteristics. The strength of AAS mortar is observed to be higher both with and without GO inclusions for the molarity of sodium hydroxide greater than 4 M. The mixture containing 0.06% GO with a 4 M activator is found to exhibit optimal mechanical and durability characteristics. Mineralogical, chemical, and microstructural investigations confirm that the addition of GO to the ambient-cured AAS accelerates the rate of hydration, even at a lower concentration of the activator (4 M) due to its high specific surface area and consequent formation of a greater number of nucleation sites. Hence, ambient-cured AAS mortar prepared using 4 M sodium hydroxide and 0.06% GO is recommended for practical use.