The present study investigates the gradual replacement of Ca²⁺ with Mg²⁺ ions in brushite (CaHPO₄·2H₂O). To date, this approach has not been systematically explored and may prove beneficial for the production of Ca_1−xMg_xHPO₄·nH₂O materials with tailored properties which are suitable for environmental and medical applications. For their production, solutions of sodium dihydrogen orthophosphate dehydrate, NaH₂PO₄·2H₂O, calcium nitrate tetrahydrate, Ca(NO₃)₂·4H₂O, magnesium nitrate hexahydrate, Mg(NO₃)₂·6H₂O and ammonium hydroxide solution, NH₄OH, were used. At low Mg/Ca molar ratios (up to 0.25) in the starting solution, partial replacement of Ca with Mg takes place (Mg doping) but no struvite is produced as discrete phase. When the Mg/Ca molar ratio increases gradually to 1.5, in addition to Mg-doped brushite, struvite, NH₄MgPO₄·6H₂O, precipitates. The microstructure of the materials produced for different degrees of Ca replacement with Mg has been analyzed in depth with the use of powdered XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), thermogravimetric (TG) analysis and SEM (scanning electron microscopy). The results of this study prove that the Mg/Ca ratio in the starting solution can be monitored in such a way that materials with tailored composition are obtained.

Over the past few decades, researchers have focused on developing new production methods for geopolymers to improve their properties for use in multiple applications as a functional material. This study introduces a new geopolymer system based on wollastonite and metakaolin as precursors. The role of wollastonite was also explored alongside metakaolin in geopolymers. Geopolymers were synthesized by adding wollastonite to metakaolin in different ratios: 0 wt.%, 12.5 wt.%, 25 wt.%, and 50 wt.%. The alkaline activator was then mixed with the powder, wollastonite, and metakaolin to prepare the geopolymers. In addition to mechanical tests, the hardened geopolymers were characterized using XRD, TGA, and SEM techniques. The findings revealed that adding wollastonite in amounts of 0 wt.%–12.5 wt.% did not affect the strength of the geopolymers. Increasing wollastonite between 25 wt.% and 50 wt.% significantly increased the geopolymers’ flexural and compressive strength from 3 MPa to 12.3 MPa and from 23 MPa to 54 MPa, respectively. The use of wollastonite as a precursor also led to fundamental changes in the microstructural structure of the geopolymer matrix: a new crystal phase, (Ca₅(SiO₄)₂(OH)₂), calciochondrodite was formed, and the Si-Al-Na crystal phase disappeared, leading to significant changes in the amorphous phase.

This preliminary study introduces the incorporation and chemical stabilization of sulfur waste into a geopolymer matrix and explores the concept of material production for further environmental and engineering solutions. In this study, a novel synthesis procedure for sulfur-based geopolymers was introduced, and the role of sulfur in geopolymers and its optimal content to obtain a stable structure were explored. Geopolymers were synthesized by dissolving sulfur in an alkaline activator in different proportions. The alkaline solution was then mixed with metakaolin to synthesize the geopolymer matrix. Adding sulfur in amounts from 0 wt.% to 5 wt.%, compared with metakaolin, led to an increase in the compressive strength of the geopolymers from 22.5 MPa to 29.9 MPa. When sulfur was between 5 wt.% and 15 wt.%, a decrease in the compressive strength was observed to 15.7 MPa, which can be explained by defects and voids in the geopolymer’s microstructure due to the solubility of excess sulfur. Because of the incorporation of sulfur into the geopolymers, a compact and dense microstructure was formed, as reported in the SEM analysis. An XRD analysis showed that, besides quartz and analcime, a new phase, A_l2·H₁₀·O₁₇·S₃, was also formed as a result of sulfur dissolution in the alkaline activator of the geopolymers.

This preliminary study introduces the incorporation and chemical stabilization of sulfur waste into geopolymers matrix and explores the concept of material production for further environmental and engineering solutions. Geopolymers were synthesized by dissolving sulfur in the alkaline activator of sodium hydroxide and sodium silicate in different proportions. The Alkaline solution was then mixed with metakaolin to synthesize the geopolymer matrix. Adding sulfur from 0 wt.% to 5 wt.%, compared with metakaolin, was found to lead to an increase in the compressive strength of the geopolymer from 22.5 MPa to 29.9MPa. When sulfur is between 5 wt.% and 15 wt.%, a decrease in compressive strength was observed to 15.7 MPa, which can be explained by defects and voids in the geopolymer microstructure through the solubility of excess sulfur and thus the formation of defects and voids in the geopolymer microstructure. Because of the incorporation of sulfur into the geo-polymer, a compact and dense microstructure is formed as reported in the SEM analysis. XRD analysis shows that besides quartz and Analcime, a new phase, Al2·H10·O17·S3, is also formed as a result of sulfur dissolution in alkaline activator of geopolymer.