Optimization of the fundamental properties of the MgO-K2HPO4-SiO2 cement system and its microstructural evolution mechanism induced by 2-phosphonobutane-1,2,4-tricarboxylic acid

PBTCA effectively retards MSPPC hydration at an ultralow dosage of 0.12%. • PBTCA prolongs setting time and improves the fluidity of MSPPC pastes. • PBTCA reduces hydration heat release and delays early hydration reactions. • PBTCA provides effective retardation with less strength loss than borax. • A three-stage hydration regulation mechanism of PBTCA is proposed. Magnesium silicon potassium phosphate cement (MgO-K₂HPO₄-SiO₂, MSPPC) uses dipotassium hydrogen phosphate (DHP) as its phosphorus source and reactive silica fume (SF) as an essential component. Although MSPPC can be prepared without conventional retarders, its setting and fresh-state behavior require regulation to extend working time. To address the high dosage, strength loss, and health and environmental concerns of borax, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) was evaluated as a low-dosage organophosphonic retarder. Effects of 0.06%, 0.08%, 0.10%, and 0.12% PBTCA on workability, hydration, mechanical properties, and microstructure were investigated, and a three-stage hydration-regulation pathway was proposed. PBTCA prolonged setting and delayed early hydration in a dosage-dependent manner. At 0.12%, initial setting time increased from 7.0 to 46.2 min, fluidity from 70 to 148 mm, and the hydration-temperature peak shifted from 8.46 to 42.30 min with a 5.43 °C decrease. Compressive-strength loss at 0.12% decreased from 8.58% at 3 d to 6.52% at 56 d, while all PBTCA groups showed higher flexural strength, greatest at 0.06%. Microstructural analyses showed delayed early hydration-product development without new principal crystalline phases; at 56 d, the > 200 nm pore fraction decreased from 37.70% to 30.85% at 0.12% with little change in total porosity. Compared with 2%-4% borax, 0.12% PBTCA provided comparable setting control, higher fluidity, and better strength retention at a substantially lower dosage. Mechanistically, PBTCA modifies the early ionic environment through PBTCA-Mg²⁺ interactions, while its proposed association at MgO-solution interfaces retards further MgO dissolution and MKP formation without terminating hydration.