Linking Microstructure Formation to Hydration Mechanisms in Magnesium Oxychloride Cement: Water–Thermal–Acoustic Analysis

ABSTRACT Understanding the early hydration of magnesium oxychloride cement (MOC) remains a fundamental challenge, as its rapid reaction, complex phase evolution, and microstructure formation are strongly coupled and difficult to capture using conventional single‐indicator techniques. To address this challenge, this study established a novel “water–thermal–acoustic” synergistic method by integrating in situ 1 H LF‐NMR, calorimetry, and ultrasonic pulse velocity measurements, enabling continuous and multidimensional monitoring of the early hydration process. This approach reveals MOC's early hydration as a “burst‐like,” three‐stage process, demonstrating that water consumption, heat release, and structural build‐up are intrinsically linked. Applying this framework, the mechanisms of three acidic admixtures—potassium dihydrogen phosphate (KP), citric acid (CA), and 1‐hydroxyethylidene‐1,1‐diphosphonic acid (HEDP)—were elucidated, revealing a retarding efficacy of HEDP > KP > CA. This is governed by two pathways: a physical “precipitation–coating” for KP versus a more comprehensive dual “adsorption–chelation” for CA and HEDP. Critically, this kinetic control is key to crystallization optimization. By regulating supersaturation dynamics, the admixtures drive the transformation of the microstructure from porous, irregular rods to a dense network of high‐aspect‐ratio fibrous crystals, without altering the final 5Mg(OH) 2 ·MgCl 2 ·8H 2 O composition. This work provides a mechanistic foundation for tailoring the microstructure in MOC, paving the way for the rational design of next‐generation, resilient inorganic composites.