Mechanical and biological characterization of geopolymers for potential application as biomaterials

In this study three different geopolymer compositions have been investigated and characterized as potential biomaterials. Two geopolymer formulations are mainly composed of metakaolin, with some silica additions so that to reach Si/Al = 2.10 molar ratio, the third one contains a reduced amount of metakaolin and it is constitutes mainly of silica gel with composition: H24AlK7Si31O79 with Si/Al = 31. While in the first two formulations the powders were added of NaOH and sodium silicate in different percentages as activator and ligand, respectively, in the third one two different KOH addition methods (separately or jointly with potassium silicate solution) were performed. Room temperature consolidation was followed by thermal activation of composition Si/Al=31 at 60°C for 150 min and at 500°C for 180 min. The work presents exhaustive microstructural characterization (FT-IR, SEM/EDS, XRD) jointly with compression resistance tests and bioactivity studies on pressed powders of the two geopolymers. The materials were composed of amorphous aluminosilicates and a limited amount of zeolitic phases, found on the top surface. The compressive strength of the first two compositions was higher than 15 MPa and flexural strength around 2 MPa after 2 days of curing at room temperature. Compressive strength tests were carried out on Si/Al = 31 geopolymer on both activated sample series and demonstrated that when added separately the activator leads to more fragile specimens (0.90 MPa vs 1.95 MPa). To authors knowledge the effect of geopolymer preparation on mechanical properties of thermally activated Si/Al = 31 formulation has never been proved before. The bioactivity was successfully tested with the soaking of the samples in a simulated body fluid (SBF) for 3 weeks. The formation of a layer of hydroxyapatite on the surface of the materials was shown both by SEM micrograph and EDS analysis.