Formulation of a novel lightweight foamed composite based on by-product gypsum / Zhang Longjian

Zhang , Longjian (2024) Formulation of a novel lightweight foamed composite based on by-product gypsum / Zhang Longjian. PhD thesis, Universiti Malaya.

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Abstract

By-product gypsum is a solid waste generated from various industrial processes and contains mostly calcium sulfate. The storage and utilization of by-product gypsum has become a bottleneck restricting the development of industries that produce it. One common form of by-product gypsum is phosphogypsum (PG), generated from the production of phosphoric acid. Over 120 million tons of PG are produced annually worldwide, but it has a very low utilization rate. The effective utilization of this solid waste poses significant environmental challenges. At the same time, the construction industry is seeking sustainable and cost-effective alternative building materials to reduce reliance on natural resources. To address these issues, this study aims to develop a novel lightweight foamed composite using by-product gypsum as a primary resource, exploring its potential as a viable alternative to brick for non-load bearing applications in the construction industry. In this study, PG was initially subjected to calcination at different temperatures ranging from 100°C to 220°C. X-ray diffraction analyses confirmed complete dehydration occurred at 180°C, transforming PG to the hemihydrate form and the calcined PG had comparable strength as commercial building gypsum. Then, composite mixtures were prepared by incorporating calcined PG as the primary binder material, along with the addition of other materials such as ground granulated blast furnace slag (GGBS), recycled waste glass (RWG), and various types of fiber, namely polyvinyl alcohol fiber (PVA), basalt fiber, and glass fiber. The mixture variables included hemihydrate PG (HPG) content (60%-100%), GGBS replacement levels (0%-25%), RWG variation levels (0%-24%), and fiber additions (0%-1.35% by volume). The incorporation of 25% GGBS and 15% OPC as supplementary binder materials, along with 24% RWG as addition further improved water resistance of the PG-based composite without compromising strength. Fiber additions, especially 1% PVA fiber, enhanced flexural strength and limited cracks in PG-based composite at elevated temperatures up to 850°C. Besides that, foamed composite mixtures containing hydrogen peroxide as a foaming agent and calcium stearate as a foam stabilizer to study their effects. The inclusion of 1.5% hydrogen peroxide and 20% calcium sterate led to 55% lower bulk density but 66.2% lower thermal conductivity insulation and 45.5% better sound absorption, compared to the matrix without hydrogen peroxide and calcium sterate addition, due to increased number of pores and the more uniform distribution of those pores. Lastly, a novel lightweight brick developed from the optimized mixture containing 60% HPG, 15% OPC, 25% GGBS, 24% RWG, 1.0% PVA fiber, 1.5% hydrogen peroxide, and 20% calcium stearate exhibited bulk density (890 kg/m3) and compressive strength (5.2 MPa) was produced, meeting non-load bearing application along with lower thermal conductivity (0.238 W/m·K). Additionally, the developed brick presented no environmental threat, as it complied with the specified limit for leaching of heavy metals and adhered to safety standards for radionuclide content. This study thus systematically provides a better understanding of utilizing solid waste PG to produce a novel lightweight foamed composite, promoting the development of eco-friendly building material.

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