Cement kiln dust-based geopolymer: Microstructural investigation and performances optimization through response surface methodology

• Published in: Journal of sol-gel science and technology 2024-09, Vol.111 (3), p.703-717
• Main Authors: Mourak, Abdellah, Hajjaji, Mohamed
• Format: Article
• Language: English
• Subjects: Bend strength; Bricks; Calcite; Ceramics; Chemistry and Materials Science; Composites; Compressive strength; Crystallization; Curing; Dust; Fourier transforms; Fused silica; Geopolymers; Glass; High temperature; Inorganic Chemistry; Kilns; Materials Science; Mechanical properties; Microstructure; Mortars (material); Mullite; Nanotechnology; Natural Materials; Optical and Electronic Materials; original Paper; Physical properties; Quartz; Response surface methodology; Sodium hydroxide; Water absorption; Zeolites
• ISSN: 0928-0707; 1573-4846
• DOI: 10.1007/s10971-024-06479-6

Cement kiln dust (CKD), which consisted of mullite, quartz, fused silica, and calcite (49, 23, 11, and 10 mass%, respectively) was alkali-activated ([NaOH]: 4–12 M) and cured under different conditions (25 ≤ T ≤ 85 °C, time (t): up to 30 days). The microstructure of the cured samples was investigated using X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The mechanical/physical properties of the cured samples were related to the above factors using response surface methodology. The results showed that increasing the levels of these factors enhanced the reactivity of mullite and quartz, leading thus to the formation of hydrosodalite along with geopolymer. Conversely, a pseudo-amorphous phase along with a limited amount of geopolymer occurred with lower levels of the factors. Moreover, samples prepared with concentrated solutions of NaOH or cured at high temperatures were the object of the formation of zeolite ZK, and semi-crystallized hydroxysodalite, respectively. Tobermorite and hydroxysodalite were formed at long and short curing times, respectively. The mechanical/physical properties of the cured samples were well related to the operating factors through quadratic models, and their changes were discussed in relation to the microstructure characterization. The results of the desirability approach demonstrated that the optimal values for bending strength, compressive strength, porosity, and water absorption (10.4 MPa, 9.1 MPa, 16.4%, and 28%, respectively) are obtained at: [NaOH] = 10 M, T = 73 °C, and t  = 23 days. In light of these results, the CKD is suitable as a binder for mortar and bricks. Graphical Abstract Highlights With a high silica and alumina content (77% by mass), the studied CKD is suitable for geopolymer synthesis. Upon activation and curing of CKD, geopolymer, zeolite ZK, hydroxysodalite and tobermorite were formed. The influence of operating factors on cured samples properties was assessed using quadratic models. The optimal operating factors were NaOH concentration = 10 M, curing temperature = 73 °C, and curing time = 23 days. The CKD demonstrates the potential for manufacturing brick-and-mortar binders.