Mechanical and microstructural properties of metakaolin-based geopolymer cements from sodium waterglass and phosphoric acid solution as hardeners: A comparative study

Sodium waterglass with mass ratios SiO2/Na2O and H2O/Na2O equal to 1.5 and 10, respectively was prepared from commercial silica fume as a silica source. The phosphoric acid solution with molar concentration 10 M was prepared by dilution of commercial phosphoric acid in distilled water. The ATR-infrared spectrum of alkaline hardener shows the presence of SiQ0, SiQ1 and SiQ2 units suggesting a more depolymerized solution. While the acid hardener contains H2PO4− due to the deprotonation of commercial H3PO4 indicating that the molar concentration 10 M contained an appropriate amount of water necessary for the workability. The formation of H2PO4− is appropriate for the second step of geopolymerization. Metakaolin-based geopolymer cements were obtained by adding each fresh hardener to metakaolin. The results show that the compressive strength of phosphate-based geopolymer cement is 93.8 MPa while the one of alkali-based geopolymer cement is 63.8 MPa. The difference of the compressive strength could be related to the formation of berlinite (AlPO4) in the structure of phosphate-based geopolymer cement which acts as a filler and reinforces the structure and therefore the compressive strength of the specimen. However, the microstructures of both geopolymer cements are homogeneous and compact structure. It was typically found that phosphate-based geopolymer cement has a higher compressive strength compared to the one of alkali-based geopolymer cement. It is important to note that the hardening of metakaolin-based geopolymer cement from phosphoric acid solution required an energy gradient while metakaolin-based geopolymer cement from sodium waterglass hardens at room temperature like regular Portland cement. The purpose of this work was to compare the mechanical and microstructural properties of metakaolin-based geopolymer cements obtained in the same condition using sodium waterglass and phosphoric acid solution as hardeners. Due to their higher mechanical properties, phosphate-based geopolymer cement could be used for the construction of roads and bridges and geopolymer cements from alkaline medium could be used for building the houses.