The high environmental impact associated with ordinary Portland cement production has driven increasing interest in alternative low-carbon binder systems based on alkali-activated materials. In this context, geopolymers synthesized from metakaolin and supplemented with natural or industrial by-products represent a promising route toward more sustainable construction materials. In this study, the partial substitution of metakaolin (MK) with stainless steel slag (SSS, calcium rich) or volcanic ash (VA, silica-rich) in alkali-activated cements (AACs) synthesis was investigated by analyzing their physical, mechanical, and thermal properties. The structural evolution associated with alkali activation was assessed using X-ray diffraction (XRD) and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). Fourier transform infrared spectroscopy (FTIR) revealed a shift in the main Si–O–T (T = Si, Al) asymmetric stretching band toward lower wavenumbers (≈1000 cm⁻¹), indicating changes in the aluminosilicate network consistent with geopolymer formation. Scanning electron microscopy (SEM) was used to examine the microstructural features of the hardened matrices. The results showed that samples containing 50 wt.% MK and 50 wt.% VA achieved the highest mechanical performance, with compressive and flexural strengths of 46.29 MPa and 16.2 MPa at 7 days, increasing to 56.66 MPa and 17.58 MPa at 28 days of curing, respectively. In contrast, the samples containing 50 wt.% MK and 50 wt.% SSS displayed lower strength development, reaching compressive and flexural strengths of 27.7 MPa and 9.6 MPa at 7 days and 41.01 MPa and 13.68 MPa at 28 days. Additionally, thermal conductivity decreased with increasing porosity and decreasing bulk density, highlighting the potential of these AACs as structurally efficient materials with improved thermal insulation performance.

The use of 3D printing holds significant promise to transform the construction industry by enabling automation and customization, although key challenges remain—particularly the control of fresh-state rheology. This study presents a novel formulation that combines potassium-rich biomass fly ash (BFAK) with an air-entraining plasticizer (APA) to optimize the rheological behavior, hydration kinetics, and structural performance of mortars tailored for extrusion-based 3D printing. The results demonstrate that BFAK enhances the yield stress and thixotropy increases, contributing to improved structural stability after extrusion. In parallel, the APA adjusts the viscosity and facilitates material flow through the nozzle. Isothermal calorimetry reveals that BFAK modifies the hydration kinetics, increasing the intensity and delaying the occurrence of the main hydration peak due to the formation of secondary sulfate phases such as Aphthitalite [(K₃Na(SO₄)₂)]. This behavior leads to an extended setting time, which can be modulated by APA to ensure a controlled processing window. Flowability tests show that BFAK reduces the spread diameter, improving cohesion without causing excessive dispersion. Calibration cylinder tests confirm that the formulation with 1.5% APA and 2% BFAK achieves the maximum printable height (35 cm), reflecting superior buildability and load-bearing capacity. These findings underscore the novelty of combining BFAK and APA as a strategy to overcome current rheological limitations in digital construction. The synergistic effect between both additives provides tailored fresh-state properties and structural reliability, advancing the development of a sustainable SMC and printable cementitious materials.