The characteristics of alkali-activated slag-fly ash incorporating the high volume wood bottom ash: Mechanical properties and microstructures

Abstract

The main objective of this study was to evaluate the mechanical properties and microstructure of alkali-activated material (AAM) incorporating the high volume of wood bottom ash (WBA) for the production of paste samples (AAWP). WBA was used to replace 0 %, 30 %, 50 %, and 70 % of the mass of a blend of ground granulated blast furnace slag (GGBFS) and class-F fly ash (FA) with a mass ratio of GGBFS/FA of 1:1. These paste mixtures were activated by an alkaline solution of sodium hydroxide (NaOH) 10 M and sodium silicate (Na2SiO3), with an alkali concentration Na2O of 4 % and varied silica modulus (Ms) ratios of 0.5, 0.75, and 1.0. A series of tests were performed following the relevant standards, including slump flow, setting time, compressive strength, and ultrasonic pulse velocity (UPV) to evaluate the mechanical properties of paste samples, while the microstructure of hardened paste was examined by X-ray diffraction (XRD), scanning electronic microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Thermogravimetric analysis (TGA). The compressive strength of AAWP ranged from 14.6 to 50.0 MPa at 56 days of curing age. The results revealed that using 30 wt% WBA significantly improved compressive strength and microstructure compared to the reference mixture. Meanwhile, the AAWP with 50 wt% WBA replacement performed the compressive strength of approximately 30 MPa, and the replacement by 70 wt% WBA exhibited a significant reduction in strength. Moreover, increasing the Ms ratio slightly increased the compressive strength and qualities of the paste samples in all levels of WBA replacement. The C-(A)-S-H/Calcite gel and the Ht-like phase were the main hydration products of the paste samples.