Properties of limestone-calcined clay incorporated cement-based materials (LC3)

Abstract

From a civil engineering perspective, the development of materials has contributed the most to the formation of our modern world and the evolution of societies. Among all the building materials, concrete has shaped most of our surrounding environment. Therefore, developing a society is largely dependent on cement-based materials. These materials are readily available, low-cost, and enable complex and massive shapes to be built almost anywhere. Ordinary Portland Cement (OPC) is the most commonly manufactured type of cement and the key ingredient in concrete. However, OPC production and other construction-related activities have a significant impact on sustainability, especially when it comes to raw material consumption, worldwide carbon dioxide (CO2) emissions, and energy demand. Clinker formation during cement production has the largest contribution to CO2 emissions in the cement industry. Therefore, replacing part of the clinker in cement or part of the cement in concrete with supplementary cementitious materials (SCMs) is an effective strategy for emission mitigation and sustainable development. Moreover, SCMs are also expected to enhance mechanical properties. Over time, industrial byproducts such as fly ash, silica fume, and slag have been used as SCMs. However, limited supplies of high-quality by-products have prompted a search for practical alternatives to these materials. Clays, on the other hand, are abundant throughout the Earth's crust and, after calcination, provide pozollanic properties. Limestone, the primary source of OPC, is also globally available in sufficient quantities. Limestone is calcined at high temperatures to form clinker particles during the production of OPC. This study focuses on investigating the properties of cement-based mortars that incorporate limestone and calcined clay as SCMs. Limestone calcined-clay cement (LC3) is a novel composite material that aims to reduce the environmental impact of cement production while maintaining or improving performance. By examining factors such as compressive strength, flexural strength, and workability behavior, this study aims to provide valuable insights into the potential benefits and limitations of LC3 as a sustainable alternative in the construction industry. During this research, it is intended to reduce the amount of energy-intensive clinker used in traditional OPC. Moreover, by producing LC3 mortars and then comparing their flexural and compressive strengths with mortars of OPC, the efficiency analysis of LC3 is established. For this purpose, different clays are prepared from clay deposits in Istanbul, Turkey, and the mineral compositions of multiple clays are investigated throughout the x-ray diffraction (XRD) analysis. Afterwards, three different clays, namely kaolinite, illite, and montmorillonite, are picked based on the XRD results for further processing. To remove humidity and water from these natural clays, they were heated at 150 degrees Celsius (oC) for 12 hours. After they became completely dried, they were ground and sieved below 90 micrometers (µm). Calcined clay refers to clay minerals that have been subjected to high temperatures, typically in the range of 600 to 900 oC, in a process known as calcination. Calcination involves heating the clay to remove any chemically bound water and other volatile compounds, resulting in a transformation of the clay's structure and properties. During calcination, the clay minerals undergo several changes. The removal of water causes physical and chemical transformations, resulting in a significant reduction in the clay's plasticity and shrinkage. The process also leads to the development of pozzolanic properties in the clay, enhancing its reactivity with calcium hydroxide, a byproduct of cement hydration. Therefore, each of these three clays is heated at three different temperatures (i.e., 600, 700, and 800 oC) in order to achieve pozollanic properties by creating structural disorder. In other words, the hydroxyl groups in the clays are supposed to be eliminated by dehydration and dehydroxylation processes at high temperatures. For each temperature, three different durations of heat are applied, and each clay is heated in the oven for 1, 1.5, and 2 hours, respectively. It is mainly to observe the best calcination process and point out the best calcination temperature and period during which perfect pozollanic properties are obtained. To clarify, too high temperatures can lead to a reduction in the surface area of the clays due to recrystallization; however, clays do not exhibit pozollanic properties at very low temperatures either. It was observed that calcination of the clay at specific temperatures and durations removes the hydroxyl groups from its crystal structure, making it more reactive when combined with limestone and OPC in mortars. Furthermore, it is concluded that high-purity clays with high kaolinite content do not change color after being heated and tend to keep their whitish color even after being calcined at very high temperatures. However, low-grade cacined kaolinite clays with high impurities, especially those rich in iron (Fe) content, exhibited a reddish color, which indicates the oxygen-rich atmosphere of the oven. Afterward, the calcined clays were blended with non-calcined limestone powder at a ratio of 2:1 by mass, effectively replacing the content of OPC with this combined mixture. Two different mixes were prepared from each calcined clay in order to determine the maximum replacement level and investigate the application areas of each substitution level. The first mix included 30% calcined clay, 15% limestone, and 55% OPC, and the second mix included 40% calcined clay, 20% limestone, and 40% OPC. Additionally, as the reference mix, mortars of OPC were prepared with a ratio of 1:0.5:3, indicating the mass of cement, water, and sand, respectively, which does comply with EN 196-1 standards. Observations revealed that the workability of the LC3 mortars is lower compared to the OPC mortars. The lower workability of LC3 mortars can be attributed to several factors related to the composition and properties of the materials involved. For example, kaolinite, which is a commonly used clay mineral in LC3 formulations, has a higher surface area and greater water demand compared to the main component of OPC, which is typically clinker. As a result, LC3 mortars with kaolinite clay may require more water to achieve the same level of workability as OPC mortars. This increased water demand can affect the overall fluidity and cohesiveness of the mortar mixture. To improve the workability of LC3 mortars, various strategies can be employed. These include optimizing the water-to-binder ratio and using superplasticizers or other chemical admixtures to enhance flowability. Therefore, a specific amount of water-reducing admixture (WRA) was added to each LC3 mortar in order to release the trapped water and keep the workability in an appropriate range. The study revealed that in order to enhance the workability of LC3 mortars containing kaolinite clays, a higher content of water-reducing admixture (WRA) is necessary compared to mortars with illite and montmorillonite clays. Additionally, the requirement for WRA significantly decreased at higher temperatures and longer durations, particularly for mortars containing montmorillonite clay minerals. Furthermore, increasing the temperature led to improved workability for all types of clays examined. Lastly, each mix was tested and compared to the OPC control mixture in terms of flexural and compressive strengths after 2, 28, and 90 days, respectively. Although the 2 days early strengths of LC3 mortars with 30% kaolinitic clay reached up to 85% and 60% of those of OPC mortars in the flexural and compressive strengths, respectively, after 28 days of curing, they achieved comparable strengths to OPC. Moreover, it was concluded that kaolinitic clays have the highest potential to be used as SCMs compared to illite and montmorrilinite, and 30% calcined clay is a far more reasonable and feasible substitution than 40%. Overall, the efficiency assessment, the best calcination process, the best replacement level, and the most efficient clays were determined based on the results and data obtained from the experiments.