Magnesium oxide (MgO) content in Portland cement clinker critically influences clinkering and long-term soundness. Limited MgO(≤2%, mass fraction) optimizes mineral formation by reducing liquid-phase viscosity during sintering, but excessive MgO forms low-reactivity periclase. Delayed hydration of periclase to brucite (Mg(OH)₂) causes destructive expansion. Thermodynamically, Mg²⁺ can react with Al³⁺ from C₃A/C₄AF to form magnesium-aluminum layered double hydroxide (Mg-Al LDH), which offers superior anion binding capacity. However, kinetic competition with formation of AFt(ettringite)/AFm(monosulfate) phases in sulfate-rich systems often suppresses LDH formation. This study investigates the formation pathway of Mg-Al LDH and the role of temperature in MgO-doped C₃A hydration (without sulfate), revealing novel Ca-Mg-Al hydrates(X-phase) and microstructural evolution mechanisms.

MgO was obtained by calcining basic magnesium carbonate (900 ℃, 2 h). C₃A-MgO composites (40 g MgO to 100 g C₃A) were synthesized from CaCO₃/Al₂O₃ mixtures (Ca/Al=3∶2), sintered at 1450 ℃ for 3 h, and air-quenched. XRD-Rietveld analysis quantified phases: 69.70% cubic C₃A, 25.09% periclase, 3.58% C₁₂A₇, and 1.63% free CaO. Hydration suspensions (w/s=10∶1, under N₂ atmosphere) were cured at 20 ℃ and 50 ℃ for 1 h-120 d. Hydration was stopped with isopropanol; solids were filtered, vacuum-dried (40 ℃), and characterized. Phase evolution was analyzed via XRD (Rietveld refinement) and in-situ high-temperature XRD (25-200 ℃). Morphology and elemental distribution were examined using FIB-SEM (Helios 5 UC) and EDS (Oxford UltimMax65). Platy particles were cross-sectioned by FIB etching (500 nm-2 μm depths) for compositional profiling.

At 20 ℃, C₃AH₆ forms rapidly, constituting 50% of solids within 1 h, and C₃A is fully consumed by 3 d. Mg-Al LDH emerges after 6 h, grows gradually to 45% and with approximately 7% periclase residue at 56 d. XRD analysis reveals a novel X-phase at 28 d, characterized by distinct diffraction peaks at 6.70° and 13.38° 2θ. In contrast, at 50 ℃, accelerated dissolution of periclase enables complete hydrolysis of MgO by 28 d. Mg-Al LDH dominates the system after 7 d, reaching 54% at 56 d and replacing C₃AH₆ as the primary phase. The X-phase forms within just 1 h and consistently increases with prolonged curing.

The morphology and composition of Mg-Al LDH exhibit significant temperature dependence. At 20 ℃, LDH crystallizes as well-defined hexagonal sheet with Mg/Al ratios ranging from 2.77 to 3.16 (EDS data). SEM images and EDS confirmed C₃AH₆ dissolution is driven by Mg²⁺, and Al³⁺ originated from C₃AH₆ supports Al³⁺ for LDH growth. At 50 ℃, however, LDH shows irregularly shaped flake with a lower, more homogeneous Mg/Al ratio (2.21 ± 0.11). This decrease of Mg/Al ratio suggests enhanced Al³⁺ availability from C₃AH₆ at elevated temperatures.

The formation pathway of the X-phase involves a three-component reaction: C₃AH₆+Mg₄Al₂(OH)₁₄·3H₂O+CH→X. X-phase early appearance at 50 ℃ (1 h vs. 28 d at 20℃) confirms temperature-dependent kinetics. In situ high-temperature XRD indicates its structural reversibility: heating above 50 ℃ induces peak shifts (e.g., new peaks at 8.82° and 10.64° 2θ), yet cooling restores the original pattern, confirming thermally reversible structure. EDS of X-phase surface identifies composition of CaMgAl(OH)₇. FIB-EDS cross-sectional profiling exposes a linear Ca/Mg ratio within X-phase inner lamellae (slope = -0.46), implying continuous Mg²⁺ substitution for Ca²⁺ during maturation. Under curing at 50 ℃ for 120 d, platy X-phase particles evolve into stacked lamellar up to 20 μm width and 2 μm thickness. These particles encapsulate residual C₃AH₆, demonstrating the role of C₃AH₆ as reactants.

Elevating curing temperature from 20 ℃ to 50 ℃ accelerates periclase dissolution, enabling Mg-Al LDH to dominate C₃A hydration after 7 d and reducing its Mg/Al ratio from 2.77 to 2.21. Mg²⁺ etches C₃AH₆ surfaces, facilitating LDH growth by the release of Al³⁺. A novel Ca-Mg-Al hydrate (X-phase) forms through reactions of C₃AH₆, Mg-Al LDH, and Ca(OH)₂, exhibiting thermally reversible phase transitions and a composition of this phase surface near CaMgAl(OH)₇. Internal Ca/Mg ratio within X-phase lamellar particles suggests ion exchange during maturation.