Structural transition to well-ordered phases of NaOH-activated slag-metakaolin cements aged by 6 years

Abstract

The strength evolution and microstructural transition of reaction products of alkali-activated blast furnace slag (BFS) - metakaolin (MK) blends were investigated for up to six years of curing at 20 °C. The BFS-MK ratios were of 100–0, 80–20, 50–50, and 20–80, activated with 5, 10 and 15% wt. Na2O (relative to the mass of the precursors) using NaOH as the only activator. X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (FT-IR), thermal analysis and 27Al and 29Si nuclear magnetic resonance (NMR) indicated the formation of C,N-(A-)S-H gel, hydrotalcite, AFm phases of monocarboaluminate and calcite in 100% BFS binders. For pastes of BFS-MK (80–20, 50–50 and 20–80), the addition of MK gradually limited the formation of hydrotalcite and AFm phases, but enhanced the formation of crystalline type A and X zeolites, as well as sodalite syn and gobbinsite intermixed with C,N-(A-)S-H and N-A-S-H gel without producing microstructural alterations. This improved the compressive strength contrary to the reported for other NaOH-alkali activated materials.