Muhammad Riaz Ahmad

1. Global municipal solid waste (MSW) generation reached approximately 2.24 billion tons in 2020 and is projected to rise to 3.88 billion tons by 2050 [1]. Incineration is increasingly adopted as a...

The search for sustainable alternatives to ordinary Portland cement has intensified interest in alkali-activated materials (AAMs), yet the supply of conventional precursors like fly ash is declining. This study addresses this challenge by successfully transforming municipal solid waste incinerator bottom ash (IBA), a problematic residue, into a valuable resource through a simple water immersion treatment. The effect of untreated and treated (10–40% FA replacement) on reaction kinetics, strength development, and microstructural evolution was investigated using isothermal calorimetry, MIP, XRD, TG/DTG, FTIR, and SEM–EDS. The 10-day pretreatment effectively removes soluble chlorides (‑88%) and oxidizes residual metallic aluminum (−96.9 wt.%), mitigating the deleterious expansion and alkali consumption that typically hinder IBA’s use. Results show that untreated IBA has higher total porosity (15–30%), compromised reaction kinetics, and reduced 28-day compressive strength (nearly 50% at 40 wt.% replacement). In contrast, water treated IBA (WTIBA) mixtures exhibit superior microstructure, modified aluminosilicate gel composition (lower Si/Ca and higher Si/Al) and maintain superior compressive strengths at similar replacement level. This work demonstrates a promising processing route for upcycling IBA, enabling up to 20% fly ash replacement without compromising 28-day compressive strength in alkali-activated materials.

Purpose: Calcium silicate hydrate (C–S–H) governs the mechanical behaviour of cement paste, yet the nanoscale mechanisms by which seawater ions modify its cohesion and strength remain insufficiently understood. This study investigates how interfacial distance and cation substitution (Na ⁺ , K ⁺ , Mg ²⁺ ) affect the shear cohesion and tensile properties of C–S–H. Methods: Non-equilibrium molecular dynamics simulations were performed to evaluate shear and tensile responses of C–S–H layers containing different seawater ions at cation/Si ratios of 0.1–0.3. Perfect-contact and expanded interfacial distances (up to 1 nm) were considered to quantify the roles of water confinement and mobility. Results: Pure C–S–H showed the highest shear and tensile strengths, with performance decreasing in the order: CSH ≈ CSH_Mg > CSH_Na > CSH_K. Shear strength dropped sharply with increasing interfacial distance and vanished beyond ~ 0.32 nm when confined water transitioned from bilayer to trilayer and became mobile. Stress relaxation occurred via stick–slip interlayer sliding while solid layers remained rigid. Tensile tests showed consistent trends, with pure C–S–H exhibiting the greatest strength, stiffness, and ductility. Conclusion: Cation chemistry and interfacial distance strongly control nanoscale cohesion in C–S–H. Increased water mobility at larger interfacial distances weakens shear and tensile responses, linking ion type, water structure, and confinement to mechanical degradation. These insights provide guidance for designing durable cementitious materials in marine environments.