Lahcen Bouna

The high environmental impact associated with ordinary Portland cement production has driven increasing interest in alternative low-carbon binder systems based on alkali-activated materials. In this context, geopolymers synthesized from metakaolin and supplemented with natural or industrial by-products represent a promising route toward more sustainable construction materials. In this study, the partial substitution of metakaolin (MK) with stainless steel slag (SSS, calcium rich) or volcanic ash (VA, silica-rich) in alkali-activated cements (AACs) synthesis was investigated by analyzing their physical, mechanical, and thermal properties. The structural evolution associated with alkali activation was assessed using X-ray diffraction (XRD) and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). Fourier transform infrared spectroscopy (FTIR) revealed a shift in the main Si–O–T (T = Si, Al) asymmetric stretching band toward lower wavenumbers (≈1000 cm⁻¹), indicating changes in the aluminosilicate network consistent with geopolymer formation. Scanning electron microscopy (SEM) was used to examine the microstructural features of the hardened matrices. The results showed that samples containing 50 wt.% MK and 50 wt.% VA achieved the highest mechanical performance, with compressive and flexural strengths of 46.29 MPa and 16.2 MPa at 7 days, increasing to 56.66 MPa and 17.58 MPa at 28 days of curing, respectively. In contrast, the samples containing 50 wt.% MK and 50 wt.% SSS displayed lower strength development, reaching compressive and flexural strengths of 27.7 MPa and 9.6 MPa at 7 days and 41.01 MPa and 13.68 MPa at 28 days. Additionally, thermal conductivity decreased with increasing porosity and decreasing bulk density, highlighting the potential of these AACs as structurally efficient materials with improved thermal insulation performance.

This study investigates the importance of washing process in the quantitative improvement of both porosity and specific surface area of geopolymer. To this end, geopolymer was synthesized using natural kaolinite clay via alkaline activation. Depending on both unwashed and washed geopolymers, the following characterization techniques were achieved: X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Thermal Gravimetric Analysis (TGA), specific surface area, pore distribution, pore volume and Scanning Electron Microscopy (SEM). After the washing process, both specific surface area (S_BET) and pore volume (Vp) of geopolymers have increased considerably: S_BET = 0.59 m²/g and Vp = 0.001 cm³/g (unwashed) versus S_BET = 78.80 m²/g and Vp = 0.104 cm³/g (washed) respectively. Hence, the washing process allows increasing of the surface reactivity of geopolymers through the elimination among others of excess of unreacted alkaline solution which is responsible of the efflorescence.