Strength and Durability in Acid Media of Alkali Silicate‐Activated Metakaolin Geopolymers

The Mechanical and chemical properties of inorganic polymers based on metakaolin activated by alkaline blends of sodium silicate and sodium hydroxide were characterized. Twenty‐four formulations were elaborated with variable molar composition SiO 2 / Al 2 O 3 (2.6–3.0), Na 2 O / Al 2 O 3 (0.55–0.7), and H 2 O /Solids (0.53–0.57). The effect of the chemical composition on the compressive strength development was first investigated. Infrared spectroscopy showed that the condensed reaction products were formed by bonds of T – O – T ( T = Si and/or Al ) and water retained. Two formulations of highest strength were chosen to evaluate their chemical resistance after immersion for 10 days in 0.5 N HCl solutions at 60°C; the compressive strength and microstructural changes were characterized. Before the acid exposure the strengths reached up to ~80 MPa, and after acid attack, the loss of strength was a function of the SiO 2 / Al 2 O 3 ratio. Infrared spectroscopy and electron microscopy suggested that the deterioration was due to the destruction of the polymeric network with migration of Na and Al toward the solution and the incorporation of H 3 O + and ingress of Cl − ions in the corroded structure; however, the chemical resistance was good considering the aggressiveness of the acid solutions.