Dry shrinkage and micro-structure of alkali-activated fly ash/slag pastes incorporated with silane coupling agent modified MWCNTs

Abstract

In this study, functionalized Multi-Walled Carbon Nanotubes (MWCNTs) grafted with a silane coupling agent (MS) at concentrations of 0.1 %, 0.2 %, 0.3 %, and 0.4 % were used as additives to investigate their impact on the reaction kinetics, working performance, mechanical strength, and drying shrinkage of alkali-activated fly ash/slag (AAFS) pastes. Additionally, various micro-structure techniques, including X-ray Diffraction (XRD), Thermogravimetric Analysis (TG), Differential Scanning Calorimetry (DSC), Differential Thermal Analysis (DTA), nitrogen (N2) absorption, and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), were employed to examine the morphology, chemical composition, and mineralogy of AAFS pastes. The experimental findings indicate that all the modified AAFS pastes exhibit prolonged setting time and increased flowability compared to the control AAFS pastes, with the enhancement of these properties being proportional to the MS content. Specifically, with the addition of 0.3 wt% MS, the compressive strength and flexural strength of LGMS-0.3 % were 41.2 MPa and 6.4 MPa, respectively, at the 28-day curing age, representing a 15.7 % and 39.1 % increase compared to the control AAFS pastes. Simultaneously, significant reductions in mass loss and drying shrinkage of over 70 % and 90 % were observed, respectively, in comparison to the control AAFS pastes. Micro-structural analysis reveals that the inclusion of MS leads to an increase in the quantity of reaction products without affecting their type. Furthermore, the incorporation of MS decreases the total porosity and the proportion of capillary pores with diameters ranging from 2 nm to 50 nm, resulting in the refinement of the overall pore structure of AAFS pastes. Notably, the three-dimensional interpenetrating network formed by MWCNTs and the silane coupling agent positively influences the mitigation of drying shrinkage and the enhancement of the mechanical strength of AAFS pastes.