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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. [ABSTRACT FROM AUTHOR]

This study investigates the influence of internal curing water on the compressive strength and microstructure of high-performance cementitious materials. For this, three high-performance fine-grained concrete (HPFC) and cement pastes were prepared. Two reference mixtures were investigated with total water-cement ratios (w/c) of 0.30 and 0.35. The third mixture was prepared with a basic w/c of 0.30 and the addition of 0.3% of superabsorbent polymer (SAP), resulting in a total w/c of 0.35. X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and compressive strength tests were performed. The incorporation of SAP resulted in a refinement of the porous structure of the paste, despite increasing the total porosity. In addition, the paste containing 0.3% SAP resulted in an intermediate calcium hydroxide content compared with the reference pastes. Thus, it was concluded that SAP internal curing water participates in the hydration reactions of the cementitious material. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study on the magnesium sulphate resistance of mortar specimens incorporating 0, 10 and 20% of metakaolin (MK). The evidence of the attack was evaluated through the content of calcium hydroxide (portlandite) and formation of magnesium hydroxide (brucite) by thermal analysis (thermogravimetric and derivative thermogravimetric analysis). The mechanical degradation of the mortar specimens was evaluated through splitting tensile tests after 200 days of exposition to the magnesium solution. The addition of metakaolin resulted in a reduction in the content of calcium hydroxide and in a smaller formation of brucite in comparison with reference mixture. A tensile strength loss of about 7% was observed for the metakaolin mortars submitted to the magnesium solution attack for 200 days. [ABSTRACT FROM AUTHOR]