Alkali-activated pastes by Basic Oxygen Furnace slag and fly ash: Mechanical and microstructural analysis

This study aims to investigate the mechanical behavior and microstructure formation of alkaliactivated pastes prepared from basic oxygen furnace (BOF) steel slag and fly ash (FA). In addition, the fluidity and setting time of five different proportions of FA and BOF (0/100, 75/25, 50/ 50, 25/75 and 100/0) were prepared using silicate and sodium hydroxide as activators. Compressive strength (1, 7 and 28 days), stress-strain curve, modulus of elasticity and fracture energy were evaluated in the hardened state. Infrared spectroscopy, scanning electron microscopy, X-ray diffraction and thermal analysis were used to analyze the microstructure. The best mini-slump results was 50/50 paste with 117.3 mm, 8.0 Pa and 2096.8 kg/m3 of spreading diameter, yield stress and density, respectively. The calcium present in BOF accelerated the setting time. Compressive strength of paste 50/50 presented higher, 52.3, 57.6 and 62.3 MPa at 1, 7 and 28 days and also the higher modulus of elasticity (25.9 GPa) and fracture energy (116.9 N/ m). Microstructural analyses indicate the increasing the BOF slag content, increases the availability of calcium, densifying the microstructure, due to the formation of C-(N)-A-S-H (hydrated calcium aluminosilicate) and C-A-S-H (hydrated calcium aluminosilicate) gels, which are also responsible for the mechanical strength growth up to 50 % BOF increment. However, in pastes with higher fly ash contents (75/25 and 100/0), the N-A-S-H (hydrated sodium aluminosilicate) gel type predominates. Therefore, the results obtained confirm the potential of using BOF slag as a calcium and Fa as a silica source in alkali-activated pastes.