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. 2025 Jul 30;18(15):3564.
doi: 10.3390/ma18153564.

Optimized Mortar Formulations for 3D Printing: A Rheological Study of Cementitious Pastes Incorporating Potassium-Rich Biomass Fly Ash Wastes

Raúl Vico Lujano  1   2 , Luis Pérez Villarejo  3 , Rui Miguel Novais  4 , Pilar Hidalgo Torrano  2 , João Batista Rodrigues Neto  5 , João A Labrincha  4
Affiliations

Optimized Mortar Formulations for 3D Printing: A Rheological Study of Cementitious Pastes Incorporating Potassium-Rich Biomass Fly Ash Wastes

Raúl Vico Lujano et al. Materials (Basel). .

Abstract

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.

Keywords: 3D printing; biomass fly ash; cementitious material; circular economy; rheology.

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Conflict of interest statement

Author Raúl Vico Lujano and Pilar Hidalgo Torrano were employed by the company Cementos La Cruz. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Integrated analysis of rheological and workability requirements for enhanced 3D printing of cementitious materials.
Figure 2
Figure 2
Particle size of raw materials: (a) Cumulative volume (%) and (b) volume distribution (%).
Figure 3
Figure 3
X-ray diffraction of BFAK.
Figure 4
Figure 4
Schematic workflow for rheological characterization and optimization of biomass fly ash on cementitious materials for additive manufacturing.
Figure 5
Figure 5
(a) Logarithmic shear-rate ramp; (b) logarithmic up and down thixotropy model.
Figure 6
Figure 6
Experimental setup for 3D printing of calibration cylinders: (a) Large-scale 3D printer at Cementos La Cruz, and (b) digital model of the calibration cylinder.
Figure 7
Figure 7
Time-dependent evolution of yield stress (τ0) and consistency (K) for cementitious mixtures: (a) APA yield stress; (b) BFAK yield stress; (c) combined APA and BFAK yield stress; (d) APA consistency; (e) BFAK consistency; and (f) combined APA and BFAK consistency.
Figure 8
Figure 8
SEM of BFAK particles at different magnifications.
Figure 9
Figure 9
Thixotropy area of individual and dual combinations.
Figure 10
Figure 10
Hydration kinetics and heat release profiles of cementitious systems: (a,d) BFAK-modified pastes, (b,e) APA-modified pastes, and (c,f) combined BFAK and APA formulations.
Figure 11
Figure 11
Mini-slump diameter of mortar mixtures at different water-to-WPC cement ratios and workability zone.
Figure 12
Figure 12
Correlation between cylinder height and structural collapse of 3D-printed mortars prepared with water/WPC = 0.4.
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