Determination of production parameters of alkali - activated blast furnace slag for cold weather conditions

In today's rapidly developing concrete industry, concrete produced with Portland cement (PC), one of the fundamental components of modern construction, faces significant challenges when curing is carried out under low ambient temperatures. Under such conditions, the dissolution rate of cement components is greatly reduced, and hydration reactions slow down considerably. As a result, the setting time of concrete is extended, leading to serious difficulties in terms of workability and strength development during concrete production. These unfavorable conditions have highlighted the need for alternative binder materials to PC. In this context, alkali activated slag (AAC) systems, which exhibit high mechanical performance without the need for external heating due to their high calcium content, have emerged as a potential alternative to PC and have become the focus of recent research. In this thesis study, the usability of AAC as a partial or full replacement for PC under curing conditions at low temperatures (2 °C) was investigated. The main objective of the study is to evaluate whether AAS can serve as an effective alternative binder by sustaining hydration even under low temperature conditions. For this purpose, alkali activation was carried out using ground granulated blast furnace slag, sodium silicate, and sodium hydroxide. A total of 18 different mixtures were prepared by varying the alkali modulus (1.2 and 1.5), alkali dosage (5%, 7%, and 9%), and PC replacement ratios (0%, 10%, and 20%). Initially, the setting times of the mixtures were determined. Subsequently, compressive strength and ultrasonic pulse velocity (UGH) tests were performed on samples at designated curing ages. In addition, microstructural investigations were carried out using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses. Increasing the alkali dosage improved early age strength; however, it led to a reduction in 90 day strength. Simultaneous increases in alkali modulus and PC replacement ratio further reduced strength due to shrinkage induced cracking. The most favorable mixture designs were obtained with 10% PC substitution at a modulus of 1.2 and with no PC at a modulus of 1.5. Despite low temperature curing, PC replacement significantly accelerated the setting time. Variations in modulus and alkali dosage notably affected the intensity of peaks observed in the FTIR spectrum. SEM analyses revealed that cracks, especially evident in 90 day samples, became more pronounced with the addition of PC. The findings suggest that AAC systems hold promise as alternative binders under temperatures below 5 °C, where PC exhibits limited hydration performance, provided that appropriate mixture designs are implemented. Keywords: Portland cement, Blast furnace slag, alkali activated slag, Low curing temperature, Compressive strength.