Development of alkali-activated materials using biomass fly ash, phosphogypsum, and recycled concrete aggregates

Abstract

Cement production is responsible for 9–11% of total CO2 emissions worldwide, so it is essential to develop low-energy materials, avoiding using Port-land cement. Using waste materials in developing new cement-free binders, such as alkali-activated materials (AAMs), is a promising method for creating novel building materials. This study aims to develop a cement-free AAM binder based on high-carbon biomass fly ash (BFA), calcined phosphogypsum (CPG), and the low-alkalinity activator Na2CO3/Na2SiO3. The created AAM binder was used to develop a lightweight AAM concrete containing recycled concrete aggregate (RCA). The dissertation consists of an introduction, three chapters, general conclusions, recommendations, lists of references and the author’s publications on the dissertation topic, and a summary in Lithuanian. The first chapter reviews the state-of-the-art investigations on AAM, especially the application of BFA and CPG in AAM technologies. The literature states that BFA can be used as an aluminosilicate precursor with NaOH/Na2SiO3 in geopolymerisation technology. CPG participates as a calcium source, creating new minerals in the AAM and significantly enhancing its early strength. The second chapter describes the characteristics of the raw materials used, the mix design, and the sample preparation for each part of the research. It also describes the techniques and methods that were tested to fully evaluate the effect of each parameter on the rheological properties, physical-mechanical properties and reaction products of AAM samples based on BFA, CPG, and RCA. The third chapter presents the results and analysis of the research. It determines the effect of the Na2CO3/Na2SiO3 ratio and temperature on the properties and reaction products of BFA-based AAM. Also, it investigates the influence of the CPG content and Na2CO3/Na2SiO3 ratio on the properties and reaction products of BFA-CPG-based AAM. Furthermore, it reveals the effect of the RCA content on the physical-mechanical properties of AAM concretes based on BFA, CPG, and RCA. The lightweight AAM concrete can be applied to non-structural components with low strength requirements. Five scientific papers were published on the dissertation topic: three in scientific journals included in the Clarivate Analytics Web of Science database with an impact factor, and two in conference proceedings included in the Clarivate Analytics Web of Science and Scopus databases. Eight reports, including the dissertation results, were presented at national and international scientific conferences.