Characteristics and neutron imaging of capillary water absorption for metakaolin and steel fiber reinforced slag based-geopolymer mortars

Abstract

The manufacturing of cement consumes energy and natural resources of raw materials, and emits large amounts of CO2. Geopolymers are eco-friendly building materials that have the potential to replace cement-based building materials. We used neutron radiography to report new data on water absorption and distribution in metakaolin (MK) and steel fiber reinforced slag-based mortars for the first time. Due to contradictory studies on the influence of adding steel fibers to slag-based geopolymers on compressive strength and the limited research on water permeability, workability and sorptivity, new results were presented. Alkaline-activated MK and steel fiber reinforced slag -based geopolymer mortars were prepared. The neutron radiography (NR) technique was used to investigate capillary water absorption in the mortars. The fresh and mechanical strengths of the mortars, as well as their permeability, workability and sorptivity, were determined. The compressive, flexural, and splitting tensile strengths, as well as the water permeabilities, increased, whereas the workability decreased as the steel fiber percentage in slag-based mortars increased. The results showed that the addition of steel fiber at volume fractions of 1%, 2%, and 3% increased the −7 and 28 days compressive strengths for the slag-based mortars by 44.7%, 50.7%, and 57.1% and 41.9%, 50.2%, and 55.1%, respectively. The NR results revealed that the resistance to water penetration in reinforced slag-based mortars is greater than that of MK. The addition of steel fiber introduced inhomogeneity in the matrix of the reinforced samples and created water-accumulating regions. Water proceeded regularly in the Mk samples but irregularly in the slag-based mortars. The capillary absorption factors k for Mk and slag-based mortars with steel fiber at volume fractions of 1%, 2%, and 3% were found to be 0.37 cm min−1/2, 0.2 cm min−1/2, 0.17 cm.min−−1/2, 0.19 cm min−1/2, and 0.2 cm min−1/2, respectively. According to NR, the optimal percentage of steel fibers in the slag-based mortar impeding water flow is 1%–2%. Steel fiber used at a high proportion to slag-based mortars increased sorptivity. To best fit the results at high absorption times, the second Fick's law of diffusion can be used.