Dissolution-precipitation reactions of amorphous multi-oxide silicates. Application of ligands in low-CO2 cementitious binders

Future sustainable development goals urge the cement sector to explore decarbonization solutions and expect to reach net-zero CO2 emissions by 2050. One of the commonly explored solution is to utilize inorganic wastes produced from industries and construction sites as alternative cementitious material. This has been shown to significantly reduce the final CO2 footprint and lower energy consumption compared to Portland cement production. However, some of the challenges that limit their application include the low reactivity and varying chemical composition of these wastes. This is overcome by utilizing a technology called alkali activation where powdered wastes are reacted with different alkalis instead of water to produce cementitious binders.
This dissertation focuses on utilizing two such inorganic wastes namely, mineral wool (MW) and ground granulated blast furnace slag (GBFS) as alternative cementitious material for alkali activation process. In Europe, around 2.54 MT/yr of MW waste is produced as construction and demolition waste and is mostly landfilled. Defining the MW material properties, understanding and evaluating the reactivity of the material in alkali pH conditions are missing in the current state of the art and are needed for their successful utilization. This is because MW chemical composition varies depending upon sourcing options based on location, manufacturing age, and processing conditions. The results on the material properties of the MW wastes from different demolition sites and manufacturing locations collected around Europe showed homogenous median chemical composition, fiber length, and width for their respective types. Additionally, MW which is an amorphous material dissolves better in alkali pH conditions and forms cementitious phases which promotes their potential in aspects of valorization in construction applications.
GBFS is an industrial by-product from the steel-making process and is produced around 300–360 MT/yr. Even though it is one of the most reactive slag, due to its slow reactivity with water, corrosive alkali solutions are still needed to convert these into cementitious binders. Production of these corrosive solutions also involves CO2 emission which contributes to the final CO2 footprint of GBFS cementitious binder made with these solutions. Production of low CO2 sustainable binders by alkali activation of GBFS with environmentally friendly solutions like alkali carbonates was attempted in this dissertation with the help of optimizing the mix design with ligands as new chemical admixtures. The results showed that with ligand the strength of carbonate-activated GBFS binder at 2 d is 20 times higher compared to without ligand. This was a novel and successful attempt that showcases the possibility of modifying and optimizing a system by understanding the underlying chemical reactions and aptly using appropriate admixtures.