To develop a more reactive pozzolan for supplementary cementitious materials (SCMs), the co-calcination of kaolinite and limestone was investigated for its contribution to hydration of blended cement. Kaolinite (with ~50 wt% quartz impurity) was calcined at 700 °C, and a mixture of kaolinite and limestone was calcined at 800 °C. These activated SCMs were added to ordinary Portland cement (OPC), replacing ca. 30 wt% of the OPC. The compressive strength of these blended cement paste samples was measured after 28 and 90 days, while the hydration products and microstructural development in these blended cement pastes were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results revealed that adding free lime to OPC, together with metakaolin, led to enhanced compressive strength. The compressive strength of this new blended cement paste reached 113% and 112% of the compressed strength of pure OPC paste after 28 and 90 days of hydration, respectively. Furthermore, this study showed that the improvement was due to the increased consumption of Portlandite (CH), the formation of calcium aluminosilicate hydrate (CASH), and the reduction of porosity in the sample containing free lime and metakaolin.

Corundum and transition alumina powders are commonly produced through the Bayer process—that is, by digestion of bauxite in a sodium hydroxide solution, followed by gibbsite precipitation, which contains significant amounts of Na ⁺ ions. During the de‐hydroxylation of gibbsite, Na ⁺ is associated with retardation of the structural phase transformations between transition alumina and corundum (α‐Al ₂ O ₃ ) phases, although the exact role of Na is unclear. The influence of Na ⁺ ions on the phase transformation of γ‐Al ₂ O ₃ to α‐alumina was investigated by varying the concentration of NaOH in aqueous γ‐Al ₂ O ₃ suspensions. The phase transformation behavior of γ‐Al ₂ O ₃ was monitored through thermal analysis, X‐ray diffraction, scanning electron microscope, X‐ray photoelectron spectrometer (XPS), and transmission electron microscope. As the Na concentration in γ‐alumina suspensions increased, the amount of α‐Al ₂ O ₃ decreased in samples that were heat treated at 1200°C for 2 h. XPS analysis indicated that washing transition alumina powders with NH ₄ OH solutions had decreased the surface concentration of Na ⁺ and facilitated the phase transformation to α‐Al ₂ O ₃ .

Geopolymer binders have been highlighted due to their low carbon emission during production and processing. While metakaolin and F-type fly ash are commonly used as raw materials for aluminosilicate-based geopolymers, the long heat-curing requirements for hardening and strength development still pose challenges. This paper investigates the possible use of a hybrid microwave curing technique to design a set-on-demand approach to reduce the duration of heat curing in metakaolin-based geopolymer. The experimental design was established for samples with three different molar ratios (MR; 1.3,1.5, and 1.7) containing metakaolin, fly ash, and silica fume. Samples were subjected to 3 different curing regimes: oven curing, microwave (MW) curing, and hybrid curing (a combination of optimized microwave and oven curing). The performance evaluation was based on compressive strength, dimensional stability, and alkali leaching (efflorescence). Implementing only MW curing resulted in a significant decrease in compressive strength compared to their counterpart oven-cured samples. The reduction of compressive strength was more pronounced at lower molar ratios. The design of a hybrid curing approach where a portion of oven curing was replaced by MW resulted in a higher strength development than those only cured with MW. Similarly, the efficiency of hybrid curing was more pronounced in samples having MR of 1.5 and 1.7. Using MW curing in the geopolymer binders did not affect the alkali leaching; however, it increased the material’s drying shrinkage. Results showed that replacing a portion of oven curing with microwave curing in a hybrid approach can increase the operation speed and the hardening rate without significantly decreasing compressive strength.

The addition of boron oxide is well‐known to dramatically extend the afterglow persistence (PersiL) from minutes to hours in Sr ₄ Al ₁₄ O ₂₅ (S ₄ A ₇ ) co‐doped with 1 at% Eu and 1 at% Dy (S ₄ A ₇ ED x B), while the longest duration occurs when 30–40 mol% B ₂ O ₃ is used. To elucidate the influence of B ₂ O ₃ on the S ₄ A ₇ ED x B structure supporting PersiL, the thermodynamics and kinetics of phase transformation from amorphous preceramics of S ₄ A ₇ ED x B are investigated in the present work. Powders of S ₄ A ₇ ED x B containing x = 0, 10, 20, 30, and 40 mol% B ₂ O ₃ were prepared by Pechini processing, and their phase evolution was analyzed at characteristic exothermic reaction temperatures. Differential thermal analysis and thermogravimetry at different heating rates, X‐ray diffraction, micro‐Raman, and scanning electron microscopy methods are used to reveal that S ₄ A ₇ forms along different reaction pathways involving SrAl ₂ O ₄ , γ‐Al ₂ O ₃ , and SrAl ₄ O ₇ , with the pathway determined primarily by the amount of boron oxide present. Evidence is presented correlating the energy barrier of different phase transformations to the relative stability of Al ³⁺ coordination polyhedral networks that form the scaffolding in each crystal phase. To lift the kinetic limitations for direct crystallization of S ₄ A ₇ , at least 30 mol% B ₂ O ₃ is required for sufficient liquid borate to enable both the Sr ²⁺ diffusion and the reorganization in topological connectivity in the Al ³⁺ polyhedral scaffolding.