Development of one-part geopolymer using fly ash and ladle furnace slag for microwave absorption

Microwave-absorbing building materials are essential in modern times due to the proliferation of electronic technologies in urban environments. Cement composites are commonly used; however, their production involves high carbon emissions and energy consumption, leading to environmental issues. Besides, the abundance of industrial waste has become a critical challenge today. Hence, one-part geopolymers, which were synthesized by fly ash (FA) and ladle furnace slag (LFS), were employed to overcome these issues. The effects of mixing ratios such as alkali activator/aluminosilicate source (AA/AS), sodium metasilicate/sodium hydroxide (SM/SH), and water/binder (W/B) on the performances of one-part geopolymer were investigated using 33 full factorial design. An optimal one-part geopolymer was obtained at AA/AS of 0.2, SM/SH of 5.0, and W/B of 0.25. Analysis of variance indicated that AA/AS, W/B, and AA/AS-W/B ratios had the most significant impact on the physical and mechanical properties, while the SM/SH ratio was less influential. Microstructural and phase analyses revealed the presence of calcium-silicate-hydrate (C-S-H), contributing to structural integrity and strength development. Besides, the influences of frequency level, ageing time, and mixing ratios on dielectric properties and microwave absorption were evaluated. The dielectric properties of the geopolymers were reduced with increasing frequency levels (2 – 18 GHz), resulting in favorable microwave absorptions of 50.0% – 90.0%. Specifically, the microwave absorption increased and reached a plateau beyond 8 GHz. As the geopolymers aged 28 days, the dielectric properties diminished, yet microwave absorption rose by over 80.0%. With the optimal mixing ratios, the one-part geopolymers excelled in mechanical strength and microwave absorption. Additionally, the effects of thickness and antenna separation on microwave absorption were examined to further maximize the geopolymers’ capabilities. The geopolymers with a thickness of 100.0 mm and antenna separation of 20.0 mm improved microwave absorption from 60.0% to 80.0% due to the increased multiple reflections and energy dissipation within the structure. The C-S-H enhanced polarization effects, reducing the dielectric properties and boosting the microwave absorption of one-part geopolymers. Moreover, the impact of aggregate content on compressive strength and microwave absorption of the geopolymers was explored. River sand and gravel were used as aggregates to produce one-part geopolymer mortars and concretes. The geopolymer mortar with 60.0% aggregates and the geopolymer concrete with 75.0% aggregates obtained maximum strengths of 41.7 and 52.9 MPa, respectively. This was attributed to the mechanical interlocking and porefilling effects of the aggregates. Although the aggregate increased the dielectric properties, they promoted impedance matching, polarization effects, and wave dispersion, resulting in a ~90.0% microwave absorption. This research provided valuable insights for optimizing FA-LFS one-part geopolymers and enhanced the understanding within the geopolymer community. This work also offers a sustainable option for constructing building materials capable of absorbing microwaves, especially suitable for areas with extensive Wi-Fi and 5G networks.