Geopolymer binders are a promising alternative to ordinary Portland cement (OPC) because they can significantly reduce CO₂ emissions. However, to apply geopolymer in concrete, it is critical to understand the compatibility between the coarse aggregate and the geopolymer binder. Experimental studies were conducted to explore the effect of the size of the coarse aggregate on the mechanical properties and microstructure of a metakaolin-based geopolymer (MKGP) concrete and ordinary concrete. Three coarse aggregate size grades (5–10 mm, 10–16 mm, and 16–20 mm) were adopted to prepare the specimens. The microstructure of the concretes was investigated with scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and mercury intrusion porosimetry (MIP). Results showed an opposite coarse aggregate size effect between OPC and MKGP specimens in terms of compressive strength. SEM/EDS analysis indicated that the MKGP concrete has a weaker microstructure compared to OPC concrete induced by a higher porosity. The differences in mechanical properties and pore structure between the MKGP and OPC concrete are attributed to the greatly differing shrinkages triggered by the large surface area and penny-shaped particles of metakaolin. The findings in this work help tailor the mechanical properties and microstructure of MKGP concrete for future engineering applications.

In this study, we investigated the degradation process of metakaolin-based geopolymers exposed to high temperatures up to 1,000°C. The changes in mechanical properties, microstructure, and phase assemblage of the samples were studied, and the results demonstrated that at high temperatures the activator type [i.e., ${\mathrm{Na}}_2{\mathrm{SiO}}_3/\mathrm{NaOH}$ (Na/Na) and ${\mathrm{Na}}_2{\mathrm{SiO}}_3/\mathrm{KOH}$ (Na/K) solution] affected the degradation mechanisms of geopolymer mortars. (Na,K)-based geopolymer specimens demonstrated better thermal resistance at temperatures above 200°C. This was evident from the higher compressive strength, lower porosity, and less cracking tendency of this mixture. Compared to (Na,K)-based geopolymers, more mass loss may result in considerable drying shrinkage of (Na,Na)-based counterparts, which would further lead to the occurrence and development of cracks at $\sim 200°\mathrm{C}$ . Above 200°C, the degradation of the mechanical properties of geopolymers could be attributed to crack development and degradation of material properties. Furthermore, compared to their (Na-Na)-based counterparts, (Na-K)-based geopolymers achieved improved chemical stability and did not form new crystalline phases above 1,000°C. Moreover, higher temperature exposure (1,000°C) led to significant sintering of geopolymers, forming a dense and homogeneous matrix and, as a result, improved mechanical properties of specimens. Overall, it showed that when specimens were exposed to different high temperatures, the mutual promoted effects between ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ in geopolymers played a significant role in crack development, sintering, and new crystallization formation in the specimens.

This study investigates the effect of silica moduli (${\mathrm{SiO}}_2/{\mathrm{Na}}_2\mathrm{O}$ ratio) of alkaline activators on the degradation mechanisms of fly ash–based geopolymer (FAG) mortars that are exposed to elevated temperatures of up to 1,000°C. The evolution of the mechanical properties, microstructure, mineralogy, and atomic structure of the FAG with three different silica moduli after the thermal treatment are studied. The results show that silica modulus affects the compressive strength and crack development of geopolymer mortars. Geopolymer mortars with the highest silica modulus have the best thermal stability and volume stability after exposure to high temperatures, as confirmed by the latest cracking initiation and lowest cracking intensity. The mechanism of decreased strength in geopolymer mortars with low silica moduli (owing to crack development) is different from that of increased strength in geopolymer mortars with high silica moduli (owing to further geopolymerization) before 400°C. The crystalline phases of nepheline and albite were observed to appear in the specimen at 800°C and 1,000°C, respectively, which was possibly due to ${\mathrm{Na}}^{+}$ entering into the crystal lattice of mullite and promoting the change of mullite into new crystal phases. Heating processes cause varying degrees of destruction of FAG and form a highly porous and weak structure owing to the sintering process above 800°C. Silica moduli are not an important factor for the gel phase structure of FAG at 1,000°C, which may be related to the same content of ${\mathrm{Na}}_2\mathrm{O}$ .

Geopolymers have been employed as a replacement material for cement in construction because they are better for the environment than are other replacement material options. In recent decades, externally bonded carbon fiber–reinforced polymer (CFRP) laminates have been used to strengthen or repair structures and improve their durability. The efficiency and reliability of a fiber-reinforced polymer (FRP) application depends significantly on the effectiveness of the bond between the FRP and the concrete substrate. This study examines the effect of the size of the coarse aggregate (5–10, 10–16, and 16–20 mm) on the bond between CFRP and metakaolin-based geopolymer (MKGP) concrete. The failure mode, failure load, distribution of strain along the bond length, maximum interfacial shear bond stress, effective bond length, local slip at the maximum interfacial shear bond stress, and bond–slip relationship between the CFRP sheet and the concrete substrate are investigated. Results show that the failure loads and maximum interfacial shear bond stress of MKGP specimens decreased with an increase in the size of the coarse aggregate, which can be attributed to the decreased splitting tensile strength of MKGP concrete with increasing size of the coarse aggregate. It was shown that this size affected the bond capacity between the CFRP and the concrete substrate and increased the effective bond length of ordinary Portland cement (OPC) and MKGP concrete by up to 24.5% and 46.8%, respectively. The average local slip between CFRP and MKGP concrete at the maximum interfacial shear bond stress increased with the size of the coarse aggregate, due to the decreased splitting tensile strength of MKGP concrete. Experimental results were used to recalibrate prevalent models used to calculate the effective bond length. Models to calculate the effective bond length are proposed that consider the influence of the coarse aggregate size. Popovics’s equations were employed to develop a relationship between the local bond stress and local slip by incorporating the effect of the size of the coarse aggregate. The findings of this study may contribute to the tailoring of bond behavior between CFRP and geopolymer concrete for engineering applications.