The effects of sodium citrate, sodium tripolyphosphate and the plant protein on properties of 3D printing gypsum-based materials: Retardation mechanism at ultralow content

Abstract

Retarders are indispensable admixtures for ensuring sufficient printability time of 3D printing gypsum-based materials. However, a well-established fact related to high retarder content that excessive retarder content can reduce the mechanical properties of gypsum pastes. Therefore, the crucial strategy for expanding the application of 3D printing gypsum-based materials is to achieve efficient retarder at ultralow content. This study systematically analyzed the mechanisms of sodium citrate, sodium tripolyphosphate, and the plant protein retarder on gypsum-based 3D printing materials at ultralow content by workability, mechanical properties, and microstructure test. The results demonstrated that the plant protein retarder exhibited optimal performance in enhancing the workability and mechanical properties of the paste. When the content of this retarder was 0.4 ‰, the initial setting time of the gypsum paste was extended from 7 min to 56 min, while increasing the 7-day compressive and flexural strengths of the gypsum specimens by 81.63 % and 47.19 %, respectively. Mechanistically, the three retarders differed in gypsum particle adsorption due to distinct anionic structures, regulating setting time differently. These retarders complexed with Ca^2 + to form insoluble compounds, reducing hemihydrate gypsum dissolution and transforming dihydrate gypsum crystals from elongated prisms to plate-like structures. Enhanced crystal interlocking reduced hardened paste porosity, ultimately improving mechanical strength.