Abstract
Geopolymers have distinct advantages such as having energy‐saving properties, being an environmentally protective material, and having high mechanical strength and durability. However, the shrinkage of the geopolymer materials is one of the major problems to affect its practical application. In this study, blast furnace slag‐based geopolymer pastes were prepared using sodium silicate and sodium hydroxide as activators to investigate the effect of the activator concentration and solid/liquid ratio on strength and shrinkage properties. For a better understanding of the reaction mechanism and microstructure of the geopolymer pastes, a multitechnique approach including scanning electron microscopy, X‐ray diffraction, and Fourier transform infrared spectra was carried out. The results showed that the geopolymers compressive strength increased significantly as the activator concentration increased. The increase in activator concentration first increased the flexural strength and then decreased and reached the maximum when the activator concentration was 40%. A higher activator concentration, as well as a lower solid/liquid ratio, generally led to serious geopolymers drying shrinkage. These findings are expected to be ascribed from the changes in the content of the alkali‐activated product (i.e., hydrate calcium aluminosilicate), which depends on the activator concentration. The increase in C‐A‐S‐H gel (hydrate calcium aluminosilicate) compacts paste densifiers but causes shrinkage fracture concerns. These results provide an appropriate proportion for alkali‐activated slag geopolymer pastes with better mechanical strength and antidry‐shrinkage cracking properties, which are beneficial for the further applications of geopolymer materials.
Highlights
Geopolymers are formed by the reaction of alkali hydroxide or silicate solution with solid aluminosilicate to form a three-dimensional network structure consisting of silica tetrahedrons and aluminum oxide tetrahedrons [1,2,3]
We investigated the effects of the activator concentration and solid/liquid ratio on the mechanical strength development and shrinkage behavior of slag-based geopolymer pastes to provide an experimental basis and references for the performance optimization of geopolymers
Is activator is cooled to room temperature (24 h) and mixes with slags with solid/liquid ratios of 1.5 : 1, 1.25 : 1 and 1 : 1. After 5 min of stirring (2 min at a low speed and 3 min at high speed), the mixtures are cast into a mould with a size of Ø50 × 50 mm for compressive strength tests and a mould with a size of 40 × 40 × 160 mm for testing the flexural strength and drying shrinkage. e specimens are cured in a standard curing box (20°C and 95% RH) for 24 h and demoulded. en, the specimens are stored under ambient conditions until testing (3, 7, and 28 d)
Summary
Geopolymers are formed by the reaction of alkali hydroxide or silicate solution with solid aluminosilicate to form a three-dimensional network structure consisting of silica tetrahedrons and aluminum oxide tetrahedrons [1,2,3]. The results showed that the activator concentration, sodium silicate modulus, alkali metal ions, and solid/liquid ratio influenced the geopolymers mineral composition, microstructure, and mechanical properties [12,13,14,15,16,17]. Ridtirud et al [25] found that the curing temperature and solid/liquid ratio played key roles in determining the drying shrinkage of fly ash-based geopolymers. Ling et al [27] thought the drying shrinkage of fly ash-based geopolymer pastes increased with decreasing module and activator concentration, but the total porosity is slightly lower. We investigated the effects of the activator concentration and solid/liquid ratio on the mechanical strength development and shrinkage behavior of slag-based geopolymer pastes to provide an experimental basis and references for the performance optimization of geopolymers
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