3D microstructure of magnesium potassium phosphate ceramics from X-ray tomography: new insights into the reaction mechanisms

Abstract

Magnesium potassium phosphate ceramics are chemically bonded ceramics employed as biomaterials, in nuclear waste encapsulation and for concrete repair. The microstructure dictates material performance and depends on the raw mix composition. Synchrotron X-ray computed microtomography was employed to describe the 3D microstructure and its time evolution during hardening and gain insights into the reaction mechanisms. Any excess water with respect to the stoichiometry of the reaction brought about an increase in porosity, but, notably, a reduction in the average pore size. Crystals filled the water 'pockets' in the ceramic volume by growing larger, although less densely packed, increasing the complexity of the pore shape. Platelet over elongated crystal habit was favoured. Such a change in shape is likely related to a change in reaction mechanism, as crystallization from a gel-like amorphous precursor is hindered and progressively substituted by a through-solution mechanism. It is proposed that the time evolution of the microstructure is dictated by the balance between crystallization from amorphous precursor, prevailing in relatively 'dense' systems (with stoichiometric water or in low excess), and water segregation, prevailing at higher water contents. The former mechanism was shown to produce an increase in porosity with time, because of the density mismatch between the amorphous and the crystalline phase.

Microstructural evolution and texture analysis of magnesium phosphate cement

“…Crystallization has been proposed to occur close to the surface of the MgO grains, on their surface, or also partly at random in the amorphous mass . Recently, reaction kinetics analysis and mCT indicated that more than one process may be active during the setting reaction, with each single contribution changing in time and in function of the mix formulation . Because MgO annealed at lower temperature possesses a higher specific surface area, a more reactive MgO should favor the development of more K‐struvite nuclei, as confirmed by higher initial crystallization rates .…”

“…7 Recently, reaction kinetics analysis and mCT indicated that more than one process may be active during the setting reaction, with each single contribution changing in time and in function of the mix formulation. 7,12,17 Because MgO annealed at lower temperature possesses a higher specific surface area, a more reactive MgO should favor the development of more K-struvite nuclei, as confirmed by higher initial crystallization rates. 7,12 According to the mechanism of microstructural evolution described above, it may be argued that with more crystals, more pores (with D > 2.7 μm) should form as it is, in fact, observed (8728, 5161 and 4734 in the VOI of the samples MPC_1500_15 days, MPC_1550_15 days and MPC_1600_15 days, respectively), and illustrated in Figure 2.…”

“…As recently probed for MPC formulations in a range of water‐to‐solid weight ratios using Synchrotron X‐ray microComputed Tomography (mCT), the amorphous‐to‐crystalline transformation, which is always incomplete, plays a central role in dictating the development of the microstructure and, ultimately, the performance of the cement, establishing the link between reaction mechanisms and properties.…”

“…12,15 Correlation of mechanical performance with amount of K-struvite, 8,14 indicated in the progressive development of a pervasive network of tabular/elongated K-struvite crystals the main toughening mechanism in MPCs. 8,16 As recently probed for MPC formulations in a range of water-to-solid weight ratios using Synchrotron X-ray microComputed Tomography (mCT), 17 the amorphousto-crystalline transformation, which is always incomplete, plays a central role in dictating the development of the microstructure and, ultimately, the performance of the cement, establishing the link between reaction mechanisms and properties.…”

“…In order to clarify the conflicting evidences on the development of porosity 1,17,18 and prove/disprove the models of microstructural evolution of MPCs, 1 in this work, the microstructure of MPC produced from MgO sintered at three different temperatures and its time-evolution have been described in a fully non-destructive fashion with mCT.…”