Alkali-activated slag binders for nuclear facility structures: Effects of EAF slag and thermal exposure

This study aims to fabricate alkali-activated binders from blast furnace (BFS) and electric arc furnace (EAF) slags and to evaluate the effects of EAFS in the BFS matrix. This work also considers the influence of thermal exposure, similar to those experienced in nuclear facility structures, on the fabricated slag binders. First, alkali activators and BFS:EAFS (100:0, 90:10, and 80:20) were blended. Following alkali activation and curing steps, the samples coded B100E0, B90E10, and B80E20 were successfully fabricated. The sample with the highest EAFS insertion, B80E20, was further selected and thermally exposed, conducted at 150 °C for 1 day (B80E20-1) and 80 °C for 7 days (B80E20-2). Density measurement, X-ray diffraction (XRD), Fourier transform infrared (FTIR), compressive strength, and gamma-ray spectroscopy were conducted to characterize the fabricated sample series. According to the density measurements, it was seen that increasing EAFS paved the way for increasing it from 2.1979 to 2.2431 g.cm^-3. XRD patterns revealed the amorphous structure of B100E0 resulting from the polymerized aluminosilicate gel formation, while crystalline phases, such as Ca and Fe-containing assemblages, were detected as a function of EAFS insertion. FTIR spectra further identified the structural units and vibrational modes, which particularly are O-H, H-O-H, and Si-O-T (T: Al or Si), depending on EAFS addition. Together with XRD and FTIR findings, one can deduce that polymerization of aluminosilicate structure and formation of gels are highly affected by EAFS contribution due to its limited reactivity under alkali activation. It is interesting to note that compressive strength decreased from 54 to 43 MPa in line with the increasing EAFS in the BFS matrix, suggesting that load-bearing capacity of the slag binders is subjected directly to structural alteration rather than bulk densification. Another key finding has been found for radiation protection performance of the slag binders. That is, introducing EAFS in the BFS matrix resulted in enhancing photon-matter interactions, leading to a 5% or more increase in linear attenuation coefficient at 1173 and 1332 keV. In addition to these, it is important to highlight that thermal exposure can cause detrimental results on the final mechanical performance, because dehydration and re-arrangement of the structure after treatment could lead to the formation of gel discontinuity and micro and macro cracks. Therefore, this investigation provides sufficient insights into the development of alkali-activated slag binders, specifically strong candidates to be used in nuclear facility structures, and presents that the effects of EAFS and thermal exposure can determine whether these materials are effectively utilized where needed.