REPOSITORY > RESULTS

Abstract

Alkali activation is a chemical process in which aluminosilicate-rich materials that dissolve in basic media at room temperature form binding phases by polycondensation. Alkali-activated materials (AAM), also called geopolymers, are inorganic aluminosilicate-based materials produced mainly from fly ashes and metallurgical slags or clays with high Al and Si content and high solubility in basic media. The alkali-activated materials (AAM) are promising substitutes for binders such as cement or other products in construction and industry. With this technology, it is possible to transform secondary materials, such as metallurgical byproducts like slags, into a new group of useful construction products. Steel slag-based alkali-activated materials have high mechanical strength, good fire resistance and high thermal resistance at elevated temperatures, and in the case of low density (lightweight foams), low thermal conductivity. Steel slags are by-products of high-temperature metallurgical processes, used primarily to separate metal from nonmetal. Various types of slags with high aluminosilicate content can be used for the alkali activation process.
In the present work, electric arc furnace steel slag (EAFS) and ladle furnace basic white slag (LS) from the Slovenian metallurgical industry were selected and investigated as raw materials for alkali activation:
1. Different particle sizes (d < 63, 63 < d < 90, and 90 < d < 125 μm) of slags were used to study the effect of fraction on alkali activation and resulting mechanical properties. Smaller particles result in the largest surface area and thus higher reactivity and higher compressive strength.
Strength development at room temperature and in a heat chamber at 50, 70, and 90 °C and the effects of curing time for 1, 3, 7, and 28 days. High compressive strengths can be achieved by curing at elevated temperature such as 70 °C for a few days or at room temperature for 28 days. The AA reaction rate at elevated temperatures is detected by FTIR at a frequency of 970 - 1160 cm-1, which is due to the vibration of the Si-O-Si and Si-O-Al bonds.
2. The early shrinkage behavior of AA slag-based pastes in terms of autogenous deformation according to standard test method for autogenous strain of cement paste and mortar ASTM C1698-19, and the effects of temperature (room, 40, 60 °C) and humidity (30 and 90% of RH) were studied. Deformation is caused by dissolution of aluminosilicates, formation of gels from nanozeolites, and bonding and polymerization of Al with Si present. Different amounts of the resulting products cause a proportional change in the microstructure. During the autogenous deformation process, a certain amount of liquid added during the process is retained in the material. The hydration leads to a restructuring of the microstructure, greater densification and more deformation. When curing at higher humidity, hydration does not increase, the pores are filled with liquid, which increases the volume of the gel, decreases the density and deformation is less than when maintaining at lower humidity, when part of the liquid evaporates from the pores.
3. The mechanical properties of alkali-activated lightweight samples were determined using foaming agents, additives, and fibers to optimize the activation reaction process and pore distribution to achieve low density and thermal conductivity. Uniformly distributed pores were found using hydrogen peroxide (0.5 wt%) and Na-perborate (2.5 wt%) with stabilizer (1 wt%, nonionic Triton x-100). The density was reduced to 0.4 g/cm3 and the thermal insulation was improved to ~100 mW / mK. Adequate compressive strength (1.6 MPa) is achieved by fiber reinforcement with polypropylene fibers (0.07 wt%).
The starting materials and samples were characterized by powder X-ray diffraction (XRD), X-ray fluorescence analysis (XRF), Fourier transform infrared spectroscopy (FTIR), specific surface area with adsorption of gas molecules (BET), Hg intrusion porosity (MIP), mechanical properties measurements, scanning electron microscopy (SEM) and heat flux measurement (HFM).