Metakaolin-based geopolymers possess excellent corrosion and high-temperature resistance, which are advantageous compared to ordinary Portland cement. The addition of slag in metakaolin-based geopolymers is a promising approach to improve their mechanical properties. Thus, this study investigated the effect of slag content on the strength and shrinkage properties of metakaolin-based geopolymers. Increasing the slag content and Na2O content was beneficial to the reaction of alkali-activated metakaolin-based geopolymers, thereby improving their compressive strength and density. After 56 days of aging, a maximum compressive strength of 86.1 MPa was achieved for a metakaolin-based geopolymer with a slag content of 50 mass%. When the Na2O content was 12%, the compressive strength of the metakaolin geopolymers with a slag content of 30% was 42.36% higher than those with a Na2O content of 8%. However, as the slag and alkali contents increased, the reaction rate of the metakaolin-based geopolymers increased, which significantly decreased the porosity, increased the shrinkage, and decreased the volumetric stability of the system. In this paper, in-depth study of the volume stability of alkali-activated metakaolin-based geopolymers plays an important role in further understanding, controlling, and utilizing the deformation behavior of geopolymers.

This study explores the effects of the interactions among Na2O content, metakaolin content and activator modulus on the compressive strength and autogenous shrinkage of alkali-activated slag–metakaolin (AASM) materials. The Box–Behnken RSM design was used to create an experimental scheme, establish a model, and optimize the mix proportions. Fourier transform infrared spectroscopy, scanning electron microscopy, and Mercury intrusion experiments were used to analyze the compositions, microstructures, and pore structures of the hydration products of the AASM, respectively. Results showed that the interactions between the activator modulus and metakaolin content, as well as Na2O content and metakaolin content, are the key factors affecting the compressive strength and autogenous shrinkage, respectively, of the AASM. Under the optimal conditions of Na2O content of 6%, sodium silicate modulus of 1.5, and metakaolin/slag ratio of 1: 3, the relative errors in the model verification test for compressive strength and autogenous shrinkage are 0% and 0.18%, respectively. In the water glass modulus and metakaolin content interaction, Ca2+, A13+, and Si4+ ions in the composite system react with several [SiO4]4− groups to form C-A-S-H and N-A-S-H gels, which fill each other to make the composite structure denser. When Na2O interacts with metakaolin, the OH− and Na+ in the solution react with A13+ and Si4+ to generate additional N-A-S-H, thereby reducing the compressive strength of the composite system, mitigating autogenous shrinkage, and increasing volume stability.

To investigate the effects of slag and Na2O content on the macroscopic properties and pore structure characteristics of alkali-activated metakaolin–slag (AAMS) composite cementitious materials, this study used X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM-EDS), and mercury-pressure (MIP) tests for characterization and analyzed the hydration product compositions, microstructures, and pore structure characteristics of AAMS composite cementitious materials. The relationships between the fractal dimension and the pore structure parameters, compressive strengths, and drying shrinkage rates of AAMS composite cementitious materials were investigated with the thermodynamic fractal model. The results showed that at the age of 28 d, the compressive strength and drying shrinkage of the AAMS composite binder increased by 20.57% and 215.11%, respectively, when the slag content increased from 0 to 50%. When the Na2O content increased from 8% to 12%, the compressive strength and drying shrinkage of the AAMS composite increased by 24.37% and 129.40%, respectively. The compressive strengths of AAMS composite cementitious materials increased with increasing slag content and Na2O content, but the drying shrinkage of the system increased, and the volume stability worsened. Microscopic analyses showed that with increases in the slag and Na2O contents, the hydration products of AAMS composite cementitious materials increased, and C-A-S-H and N-A-S-H filled each other so that the internal structures of AAMS composite cementitious materials were denser, and the porosities were significantly reduced. By comparing and analyzing the Menger sponge model and the fractal model based on the thermodynamic relationships, it was found that the fractal model based on the thermodynamic relationship better reflected the pore size distribution over the whole pore size determination range, and the correlation coefficients R2 were above 0.99, indicating that the fractal dimension calculated by the fractal model based on the thermodynamic relationship provided a comprehensive evaluation index for the pore structure characteristics of AAMS composite cementitious materials, and the fractal dimension correlated well with the pore structure parameters, compressive strengths, and drying shrinkage rates of cementitious materials.