Mechanical and microstructural properties of MK-FA-GGBFS-based self-compacting geopolymer concrete composites

Abstract

Self-compacting geopolymer concrete (SCGC) is an eco-efficient and sustainable concrete with a lower carbon footprint than conventional concrete. The combination and doping of composites present a challenge in investigating the mechanical and microstructural properties of SCGC. This experimental work investigated the effect of different dosing levels of metakaolin (MK), fly ash (FA), and ground granulated blast furnace slag (GGBFS) on the properties of SCGC composites from both macroscopic and microscopic aspects. The main objectives of this study are to determine the fresh properties of SCGC in the fresh state and its mechanical properties in the hardened state and to explain the reasons for the differentiation of macroscopic properties through mesoscopic and microscopic analyses. Results showed that both MK and GGBFS reduce the flowability of fresh geopolymer concrete, with MK having a greater negative effect than GGBFS, with maximum and minimum slump flows of 645 mm and 505 mm, respectively. With the increase of MK and GGBFS admixture, the compressive strength of SCGC continuously increased until reaching the maximum value of 51.76 MPa, which corresponded to maximum admixtures of 50% and 40% for MK and GGBFS, respectively. GGBFS had the greatest positive effect on the rapid development of the early mechanical properties of SCGC, with calcium oxide (CaO) playing a crucial role. The 3d/7d compressive strength of SCGC was 0.83/0.85 of 28d when the slag admixture reached 40%, respectively. Strength prediction models with excellent fit coefficients were developed, and the correlation coefficients for the three equations were 0.83, 0.98, and 0.99. A higher stress level of SCGC corresponds to a higher intensity and density of the acoustic emission signal. Thermal analysis results showed that the higher the CaO content in the SCGC specimen, the smaller the residual mass of the sample, i.e., the higher the number of gels and compressive strength. Scanning electron microscope images showed that a higher CaO content of SCGC corresponded to a denser matrix structure and better mechanical properties.