Characteristics of quicklime activated GGBFS based cementless high-performance concrete

Abstract

Concrete is the second most consumed material globally, after water, and is primarily composed of ordinary Portland cement (OPC), aggregates, and water. While cement acts as a binder that provides strength, aggregates fill the space between the cement hydrates, enhancing the mechanical properties. The production of cement relies on raw materials such as limestone and clay, which are finely ground and then subjected to high temperatures-up to 1400°C in a process known as calcination. This stage, especially the calcination of limestone, is extremely energy-intensive and is a major contributor to global carbon emissions. In fact, cement manufacturing is responsible for approximately 7-8% of CO2 emissions worldwide. As the demand for concrete grows with rapid urbanization, the need to reduce its environmental impact has become critical. In response, researchers have explored alternative low- carbon binders. Limestone Calcined Clay Cement (LC3) has shown promise by reducing CO2 emissions by up to 30 % while maintaining strength. Geopolymer and alkali-activated materials also offer sustainable alternatives, emitting 80 % less CO2, although they come with challenges in cost and handling. Quicklime (CaO) activated binders provide a cost-effective, easy-to-handle solution, but their potential remains underexplored. This study examines the use of quicklime activated binders in advanced concrete applications, including ultra-high performance concrete (UHPC) and high-strength concrete. First, the study aimed to achieve high strength in ground granulated blast furnace slag (GGBFS) based cementless UHPC, while investigating the role of calcium formate and calcium chloride as accelerators using advanced characterization and microstructural analysis. The UHPC specimens were evaluated using various techniques, including heat of hydration, thermogravimetric analysis, X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy, and porosity measurements. The results showed that both accelerators improved the UHPC properties, with calcium formate being more effective than calcium chloride, as evidenced by higher heat of hydration, improved microstructure, and lower porosity. Furthermore, the thermodynamic modeling revealed insightful information on the reaction mechanisms with C-(N-)A-S-H and hydrotalcite as the primary hydration products formed inside the matrix. Notably, the UHPC specimens with calcium chloride exhibited a very low pH value, which could increase the risk of corrosion in reinforced structures. Therefore, the use of calcium formate as an accelerator can lead to the development of cementless UHPC with superior properties. Second, the research examines how curing temperature affects the development of quicklime (CaO) activated cementless GGBFS high-strength concrete (HSC). Compressive strength tests were conducted on samples cured at 50 °C, 70 °C, and 90 °C for 12 hours, with additional evaluation after 3 and 28 days of water curing. TGA, FT–IR, NMR, MIP, and BET analyses further investigated the microstructural evolution. Findings show that higher temperatures significantly enhance pozzolanic reactions, leading to initial strengths of up to 95.6 MPa and maintaining high levels after 28 days. Elevated temperatures enhance the formation of denser and more stable hydration products, predominantly C–S–H, confirmed by spectroscopic analysis, which relates with increased binder reactivity to enhanced compressive strength. Additionally, improved porosity refinement at higher temperatures correlates with increased strength. This research highlights the dual benefit of using cementless high-strength concrete: achieving substantial strength and reducing CO2 emissions, supporting its potential for high-strength applications with reduced environmental impact. Third, the study explored the use of cellulose microfibers (CMFs) as an internal curing agent in both cementless mortar and UHPC made with GGBFS. In the cementless mortar, CMFs were added up to 2 wt% of the binder, leading to enhanced hydration, as shown by increased cumulative heat release and improved microstructural characteristics. Thermal analyses (TGA) revealed more weight loss at higher CMF concentrations and indicated formation of more calcium silicate hydrate (C-S-H) gel. In UHPC, CMFs were incorporated in saturated conditions at up to 1.5 wt%, where they improved internal curing and hydration near the fiber surfaces. Despite their flexibility, CMFs did not compromise compressive or tensile strength but slightly improved them at certain dosages. Additionally, the use of CMFs in both applications contributed to reducing CO2 emissions by up to 77.8%, highlighting their potential for enhancing sustainability in concrete materials. These studies are believed to provide an enhanced understanding from the application perspective of lime activated GGBFS based cementless binder especially in UHPC and HSC.