Fractal Characteristics of Geopolymer Mortar Containing Municipal Solid Waste Incineration Fly Ash and Its Correlations to Pore Structure and Strength

Compression and mercury intrusion porosimetry (MIP) tests were conducted to analyze the effect of municipal solid waste incineration fly ash (MSWIFA) content on the mechanical performance and pore structure of geopolymer mortar. The MSWIFA weight contents were 0%, 5%, 15%, 25%, and 35% and the pore diameter distribution, specific surface area, and pore volume were considered to assess the pore structure of the geopolymer mortars. The popular fractal model was used to investigate the fractal features of the geopolymer mortars. Additionally, mathematical models of fractal dimension with pore structural parameters and compressive strength were established. The results showed that the compressive strength of geopolymer mortars decreased while the total pore volume and total specific surface area of mortars increased with the increase in MSWIFA content. As the MSWIFA content increased, the harmless pores (pore diameter < 20 nm) were refined. Specifically, the pores with a diameter of 5–10 nm increased in number but the pores with a diameter of 10–20 nm decreased in number with the increase in MSWIFA content. The pore structure in the mortars showed scale-dependent fractal characteristics. All fractal curves were divided into four segments according to the pore diameter, namely, Region I (<20 nm), Region II (20–50 nm), Region III (50–200 nm), and Region IV (>200 nm). The surface fractal dimension (DS) in Region I and Region IV was between 2 and 3. However, the DS in Region II and Region III was greater than 3, indicating the pores in Region II and Region III were non-physical according to the surface geometry because of the presence of ink bottle pores which distorted the result of the MIP. The complexity of pores in Region I and Region IV was reduced by the addition of MSWIFA. The DS is a comprehensive parameter that well describes the spatial and morphological distribution of pores in geopolymer mortars and exhibited a good correlation with the specific surface area, pore volume, and compressive strength. A mathematical model based on the DS was established to predict the compressive strength of the geopolymer mortar containing MSWIFA.