**1. Introduction**

Climate change disasters have made reducing greenhouse gases more important than ever, and cutting CO2 emissions from energy-intensive industries has become the main focus of net-zero carbon emissions. Among Taiwan's biggest energy consumers are thermal power generation, petrochemicals and cement manufacturing industries, including the second-largest petrochemical industrial zone in the world. Globally, it has been investing heavily in reducing this energy-intensive carbon emission problem in recent years, with the aim to achieve net zero carbon emissions eventually [1,2]. Petrochemical companies have used circular fluidized bed technology to reduce wastewater, reduce coal combustion, improve combustion efficiency and reduce carbon emissions. The derived wastes included fly ash (also known as mixed gypsum) and bottom ash (also known as by-product lime), which were then hydrated to produce hydrated by-product lime (also known as waste gypsum) for reuse to achieve the circular economy effect. The suitability of waste gypsum, which is

**Citation:** Lin, W.-T.; Korniejenko, K.; Mierzwi ´nski, D.; Łach, M.; Cheng, A.; Lin, K.-L. Feasibility Study of Waste Gypsum as a Full Replacement for Fine Aggregates of Controlled Low-Strength Material. *Mater. Proc.* **2023**, *13*, 19. https://doi.org/ 10.3390/materproc2023013019

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 14 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

an industrial by-product of secondary treatment, as a component material for replacing construction concrete was also a subject worthy of in-depth study and research. Using such materials as replacement for construction materials would be the most promising solution for waste resource recovery and disposal on a large scale [3].

In recent years, researchers worldwide have focused on recycling and disposing of the ash generated by circulating fluidized beds. These studies covered the basic material properties, hydration mechanisms, activation methods and applications of circulating fluidized bed combustion ashes [4]. They also provided an overview of the prospects of applying it in cementitious composites, alkali-activated materials or geopolymers [5]. The application of civil construction materials also included soil conditioners, cement substitutes, lightweight aggregates, road construction materials, controlled low-strength material (CLSM), roller compacted concrete and other technologies [6–9].

In this study, the waste gypsum provided by the plastic chemical manufacturer has been exposed to the air for more than 5 years in an interior. This has been carried out to stabilize it so it can be used as a complete replacement for fine aggregates in cement mortar. Ordinary Portland cement was used as the cementitious material, fly ash as the supplementary cementitious material and waste gypsum as the inert filler (fine aggregates). Testing for compressive strength, volume stability, water/sea immersion and scanning electron microscope (SEM) observations were used to determine the feasibility of the CLSM application.
