Mechanical properties and stress-strain behavior of alkali-activated concrete incorporating sintered MSWI fly ash

Technical Summary: Mechanical Properties and Stress-Strain Behavior of Alkali-Activated Concrete Incorporating Sintered MSWI Fly Ash

Problem Statement
The global generation of municipal solid waste (MSW) is escalating, with incineration serving as a primary treatment method in many regions, particularly China. While bottom ash is frequently recycled, MSWI fly ash is classified as hazardous waste (HW18) in China due to high concentrations of heavy metals, dioxins, and chlorides. Current harmless treatment methods, such as solidification/stabilization (S/S) with cement, pose long-term environmental risks, while washing and heating treatments have limitations. Although sintering is an effective method for immobilizing heavy metals, the utilization of sintered MSWI fly ash in construction materials remains under-explored. Specifically, there is a lack of research on optimizing mix proportions for alkali-activated concrete using sintered MSWI fly ash as a partial cement replacement, and insufficient data exists regarding its mechanical behavior, failure modes, and stress-strain constitutive models under uniaxial compression.

Methodology
This study investigated the mechanical properties and hydration mechanisms of alkali-activated concrete incorporating sintered MSWI fly ash and metakaolin (MK).

• Materials: Raw MSWI fly ash was obtained from a plant in Guangdong and pre-treated via high-temperature sintering (1050°C for 180 min) to reduce toxicity. The binder system consisted of P·O 42.5 cement, sintered MSWI fly ash, and MK, activated by a sodium silicate and NaOH solution.
• Experimental Design: A Box–Behnken design (BBD) combined with Response Surface Methodology (RSM) was employed to optimize the mix proportions. Four factors were analyzed at three levels: alkali equivalent (1–3%), water-binder ratio (0.40–0.50), sintered MSWI fly ash content (10–30%), and MK content (10–30%).
• Testing: Specimens were cured for 28 days. Mechanical properties, including splitting tensile and compressive strength, were measured. Uniaxial compression tests were conducted using electro-hydraulic servo devices equipped with digital image correlation (DIC) to monitor strain distribution and crack propagation.
• Microstructural Analysis: The hydration mechanism was elucidated using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Thermogravimetric (TG-DTG) analysis.
• Constitutive Modeling: Based on comprehensive factor experiments varying water-binder ratios and fly ash content, a modified stress-strain constitutive model was developed and validated against experimental data.

Key Results

• Optimal Mix Proportion: Multi-objective optimization identified the optimal mix as 1% alkali equivalent, 0.42 water-binder ratio, 20% sintered MSWI fly ash content, and 20% MK content. This combination yielded a predicted splitting tensile strength of 4.69 MPa and compressive strength of 63.92 MPa. Experimental verification confirmed these values with errors less than 5%.
• Mechanical Properties:
  • Strength: Increasing the alkali equivalent generally reduced splitting tensile strength due to constrained solubility of raw materials at high concentrations. While increasing sintered MSWI fly ash content slightly reduced compressive strength, the concrete maintained strengths above 60 MPa even at 30% replacement.
  • Failure Mode: Under uniaxial compression, the concrete exhibited a shear-type failure. The incorporation of sintered MSWI fly ash and MK delayed crack development, increased the peak stress, and enhanced the elastic modulus, ductility, and energy dissipation capacity compared to ordinary concrete.
  • Microstructure: SEM analysis revealed that 1% alkali equivalent produced a compact structure with flocculent gel, whereas higher alkali levels led to plate-like Ca(OH)2 and increased porosity. Sintered MSWI fly ash acted as a filler at optimal levels but caused aggregation and porosity at 30% content.
• Hydration Mechanism: XRD and FTIR analyses identified the formation of C-S-H, C-A-S-H, Friedel's salt, and riversideite. The presence of Friedel's salt indicated the effective solidification of chloride ions. TG-DTG analysis showed that higher alkali equivalents increased Ca(OH)2 content, while higher water-binder ratios reduced the quantity of hydration products (C-(A)-S-H), negatively impacting strength.
• Constitutive Model: A modified constitutive model was established, utilizing the CEB-FIP model for the ascending stage and a modified Zhenhai Guo model for the descending stage. The model incorporated water-binder ratio and sintered MSWI fly ash content as variables, demonstrating a high correlation ($R^2 > 0.99$) with experimental stress-strain curves. Additionally, a new formula for the elastic modulus was proposed to account for the specific variables of this concrete type.

Significance and Claims
The authors claim that this research provides a critical pathway for the harmless utilization of MSWI fly ash in the construction of "waste-free cities." By demonstrating that sintered MSWI fly ash can be effectively used in alkali-activated concrete without compromising mechanical performance, the study addresses both environmental management challenges and carbon reduction goals. The development of a specific stress-strain constitutive model and elastic modulus formula fills a gap in the engineering application of this material, offering a theoretical basis for its wider adoption in structural design. The study concludes that the optimized mix not only facilitates the reuse of hazardous waste but also enhances the ductility and energy dissipation of the resulting concrete, making it suitable for engineering applications.