Rheological Properties of Metakaolin-Based Geopolymers for Three-Dimensional Printing of Structures (2021-11)¶

The use of geopolymers as binders in three-dimensional (3D) printing processes has great potential due to their fast strength development, high durability, and lower environmental impact compared to portland cement matrixes. Metakaolin-based geopolymers are a viable solution for a Brazilian-based additive manufacturing application due to the widespread availability of kaolinitic clays, which minimizes transportation costs and reduces the associated CO2 emissions. Nevertheless, it is necessary to identify the rheological behavior of this type of binder to evaluate its suitability in an extrusion process. This work presents a calorimetric and rheological characterization of pastes and mortars produced with a metakaolin-based geopolymer as the binder, exploring the influence of the nature of the activator, the water-solids ratio, and the aggregate on the rheological parameters that describe a 3D printing process, and on their evolution in time. Two types of metakaolin-based geopolymers were characterized: one activated with a mixture of potassium hydroxide (KOH) and potassium silicate (K2SiO3), and one activated with a mixture of sodium hydroxide (NaOH) and sodium silicate (NaSiO3). The water-solids ratio of each geopolymer paste varied between 0.40 and 0.50, and natural sand was added in a 40% volume per volume percent (v/v) fixed proportion to produce mortars. The yield stress of each sample was measured after different resting times using a vane rheometer. Isothermal heat flow curves were acquired in the same time scale to connect the reaction kinetics to the rheological measurements. The water-solids ratio and the presence of aggregate are able to modify the initial yield stress and the thixotropic buildup of the matrix. The sodium-based activator is correlated with a rapid structural buildup because of faster precursor dissolution and gel formation. The presence of sand increased the shear stress values and generated stiffer systems compared to the pure geopolymer pastes.