Abstract
Fourier Transform Infrared (FTIR) spectroscopy is an important analytical tool used to identify the extent of geopolymerisation through the formation of new aluminosilicate structures. Kaolin, which can be used to produce geopolymer cement, is abundant in Nigeria. Hence, this study investigated the structural transformation of kaolin from Kankara into metakaolin and the geopolymerization mechanism of metakaolin-based geopolymer concrete using FTIR spectroscopy. Raw Kankara kaolin, calcined metakaolin, and selected geopolymer concrete samples produced with varying sodium hydroxide (NaOH) molarities (8 M, 10 M, and 12 M) and alkaline liquid ratios (Na2SiO3/NaOH) (ALr) were characterised to evaluate the chemical and structural changes occurring during calcination and alkaline activation. Concrete samples were produced at the varying mix parameters, cured and subjected to the FTIR test. Results from the FTIR analysis of the raw kaolin identified the characteristic kaolinite hydroxyl bands within the 3619-3690 cm⁻¹ region, confirming the presence of crystalline kaolinite. Following calcination, the transmittance within this region increased from 74.83% to 94.48%. At the same time, quartz-related absorption bands disappeared, and the principal Si-O stretching band shifted from 1002.7 cm⁻¹ to 1028.7 cm⁻¹, confirming successful dehydroxylation and the formation of reactive metakaolin. However, minor residual kaolinite was still detected. After alkaline activation, the principal Si-O-T (T = Si or Al) vibration shifted to approximately 984 cm⁻¹, indicating the formation of aluminosilicate geopolymer gel and the transformation of simple Si-O and Al-O bonds into a three-dimensional network. New absorption bands associated with physically bound water and minor surface carbonation further confirmed the formation of hydrated geopolymeric products. Comparative FTIR analysis showed that increasing the alkaline liquid ratio increased the transmittance around the Si-O-T band from approximately 63% to 75%, indicating enhanced geopolymerisation, denser aluminosilicate networks, which could lead to better strength. Similarly, increasing the concentration of NaOH to 10 M from 8 M improved polymerisation, whereas a further increase to 12 M reduced the degree of polymerisation due to premature gel precipitation, restricted dissolution of aluminosilicate species, and poorer network development, possibly leading to lower strength. The FTIR results established a direct relationship between spectral evolution, geopolymerization efficiency, and mechanical performance, demonstrating that optimum alkaline activator composition promotes the formation of a highly polymerised geopolymer matrix with superior strength. The study confirms FTIR spectroscopy as a reliable technique for monitoring structural transformation, assessing the extent of geopolymerization, and optimising the production of metakaolin-based geopolymer concrete.

Key words: Metakaolin, Geopolymer, Fourier Transform Infrared Spectroscopy (FTIR), Geopolymerization, Structural Transformation, Aluminosilicate-Gel