Understanding the effects of hematite and brucite additions on fresh- and hardened-state properties of metakaolin-based geopolymer cements

Hematite and brucite are two common minerals present in some industrial aluminosilicates precursors (e.g., natural clay) that are used for synthesizing geopolymer cements, which may have a significant impact on the properties of corresponding geopolymer materials. The effects of hematite (i.e., Fe₂O₃) and brucite (i.e., Mg(OH)₂) additions on the fresh- and hardened-state properties of metakaolin (MK)-based geopolymer cements were investigated and reported herein. First, MK-based geopolymer cements were produced with three Si/Al molar ratios (i.e., 1.5, 1.75, 2.0) and supplemented with hematite or brucite powder using three Fe/Al and Mg/Al molar ratios (i.e., 0.015, 0.050, and 0.085). Rheology, setting time, permeable porosity, compressive strength, and resistance to sulfuric acid of MK-based geopolymer pastes were measured. Experimental results showed that, at Si/Al = 1.75 and 2.0, the hematite addition reduced the setting time whereas the brucite prolonged setting times, indicating an accelerated and delayed alkali-activation process of MK-based geopolymers, respectively. At Si/Al = 1.5, both the hematite and brucite addition accelerated the setting time but increased permeable porosity. Only subtle changes in porosity were observed at higher Si/Al ratios. The hematite and brucite additions reduced the bulk mechanical properties of MK-based geopolymer cements but, in general, improved sulfuric acid resistance. Geopolymer cement supplemented with brucite exhibited better sulfuric acid resistance than the counterpart supplemented with hematite. Microstructural and statistical analyses revealed that the paste near the agglomerates of hematite and brucite particles exhibited relatively lower and higher micromechanical properties (i.e., elastic modulus and hardness), respectively.