Effects of nano-SiO2 additives on carbon fiber-reinforced fly ash-slag geopolymer composites performance: Workability, mechanical properties, and microstructure

Fly ash and slag are commonly used precursors in alkali-activated concrete. However, they suffer from high brittleness, poor toughness, and susceptibility to cracking. To address these limitations, this experimental study investigates the effects of different contents of nano-silica (SiO2) additives on the workability, mechanical properties, and microstructure of carbon fiber-reinforced fly ash-slag geopolymer composites (CFSGs). The results indicate that owing to its large specific surface area, nano-SiO2 significantly increases the water demand of the geopolymer, thereby considerably decreasing the fluidity and shortening the setting time of the geopolymer. However, nano-SiO2 improves the porosity, water absorption, and mechanical properties of the CFSG. The optimal mechanical strength is obtained by using 2% nano-SiO2. In addition, appropriate nanodoping can relatively improve the bearing capacity and fracture toughness of the specimen. Compared with that of undoped CFSG, the peak load, fracture toughness, unstable fracture toughness, and elastic modulus of the 2%-SiO2-doped CFSG increased by 8.78, 5.0, 9.6, and 9.8%, respectively. The incorporation of nano-SiO2 increases the shrinkage of the geopolymer, with a more significant impact on early shrinkage. Moreover, nano-SiO2 improves the microstructure of the cement matrix and interface through the filling, volcanic ash, and crystal nucleus effects as well as interface regulation. This increases the bonding force between the matrix and carbon fibers (CFs), which results in good bonding between the CFs and geopolymer matrix, accelerated geopolymerization reaction, and denser geopolymer paste, thus improving the mechanical strength of the CFSG.