Utilizing Landfill Condition Waste Coal Ash to Manufacture Fine Lightweight Aggregates for Concrete Internal Curing Applications
Authors:   , Alqenai Yousif , Farnam Yaghoob 
Published: December 2024

This project studied the potential use of fine lightweight aggregates (FLWA) created from 'as received' landfill condition waste coal ash (LC-WCA), referred to as LC-WCA-FLWA, for internally curing concrete. The research involved successful LC-WCA-FLWA production, performance optimization, and assessing the internal curing performance. LC-WCA consists of an intricate blend of coal ash by-products with field debris and was characterized for its oxide content, glassy phases, particle size, and moisture content. This characterization, combined with thermodynamic modeling and analytical calculations, identified a sintering temperature of 1075°C with the addition of 2% sodium hydroxide (by mass) as a fluxing agent was necessary for successful LC-WCA-FLWA production. The engineering properties of LC-WCA-FLWA, including density, porosity, particle size distribution, crushing strength, water absorption, and desorption, were assessed demonstrating promising properties and meeting ASTM standards for use in concrete. The study then focused on optimizing the manufacturing process to produce high-performance LC-WCA-FLWA. The effects of the sintering mean residence time (MRT) on the engineering properties and morphological structure of LC-WCA-FLWA were investigated by adjusting the sintering configuration. By incrementally modifying the furnace angle and rotational speed, it was determined that an MRT of 15 minutes provided the best overall performance. A shorter MRT resulted in an underdeveloped core and shell structure with insufficient particle bonding, leading to reduced strength. Conversely, longer MRTs caused excessive sintering, thermal cracking, and diminished water absorption. During LC-WCA-FLWA production, technical challenges related to deposit layer and ring formation were encountered. Coating and pellets ([less than or equal to] 50 [mu]m in size) adhered to fissures on the inner surface of the ceramic pipe, creating a deposit layer. Continuous sintering caused pellet agglomeration on this layer, resulting in ring formation that obstructed production. Experimental investigation into various feeding parameters, such as rate, frequency, and mass, suggested that while adjusting these inputs might not completely eliminate ring formation, it could significantly reduce its occurrence and improve LC-WCA-FLWA production. The internal curing properties of LC-WCA-FLWA were compared with commercial expanded slate (ES-FLWA). The results showed that LC-WCA-FLWA adhered to ASTM C330 grading for optimal spatial distribution in concrete, exhibited 15% higher absorption capacity, and demonstrated superior desorption behavior for supplying internal water. Concrete samples were then prepared using the manufactured LC-WCA-FLWA to evaluate internal curing performance. ES-FLWA and fine normal-weight aggregates (NWA) were also used to prepare control samples for comparative evaluation. Both fresh and hardened properties of the concrete samples were assessed in accordance with ASTM standards. The fresh properties, including slump, air content, and density, indicated satisfactory performance when pre-saturated FLWA was used. Additionally, the hardened properties, such as compressive strength, flexural strength, shrinkage, water absorption rate, microstructure development, heat of hydration, and calcium hydroxide progression, were evaluated. Concrete made with LC-WCA-FLWA demonstrated superior internal curing performance, meeting ASTM and environmental requirements and outperforming control samples made with ES-FLWA and NWA. The findings conclude that using pre-saturated LC-WCA-FLWA for internal curing enhances concrete durability and extends its service life.