Co-assembly of superplasticizer and viscosity modifying agent in designing printable concrete mixtures

Abstract

This study investigates the combined effects of superplasticizer (SP) and hydroxypropyl methylcellulose (HPMC) on the rheological behavior and printability of extrusion-based 3D concrete printing. A series of indirect rheological tests, including slump, slump flow, and slug tests, as well as printability tests comprising extrudability and buildability tests, were conducted. The results identified three distinct rheological regimes governed by the competitive adsorption of SP and HPMC on cement particle surfaces. In the SP-dominated regime, dispersion reduces both yield stress and plastic viscosity, resulting in increased slump and slump flow. In contrast, the HPMC-dominated regime is characterized by hydrogen-bond-driven structuration, which decreases slump and slump flow. Between these regimes, a transitional co-assembly stage is observed, where competing mechanisms produce a decoupled response, in which slump increases due to SP-induced dispersion, while slump flow decreases due to HPMC-induced agglomeration. Optimal printability is achieved only within a narrow dosage range where both admixtures are balanced, corresponding to a defined printability window in terms of slump and slump flow. Based on these relationships, a simplified mix design approach is proposed, enabling adjustment of admixture dosages to achieve target printability. The method was validated through iterative refinement, successfully producing the printable mixtures within two iterations.