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

## TL;DR

This study develops a new mortar formulation for 3D printing using biomass fly ash and a plasticizer to improve flow and structural stability.

## Contribution

A novel combination of potassium-rich biomass fly ash and an air-entraining plasticizer is introduced to optimize 3D printable mortar properties.

## Key findings

- Biomass fly ash increases yield stress and thixotropy, improving structural stability after extrusion.
- The APA plasticizer modulates viscosity and extends setting time for controlled processing.
- A formulation with 1.5% APA and 2% BFAK achieves a maximum printable height of 35 cm.

## 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.