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 alkali-activated 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.