Alkali-activated slag-metakaolin pastes: strength, structural, and microstructural characterization

Abstract

Pastes of granulated blast furnace slag (GBFS) and metakaolin (MK) at GBFS/MK mass ratios of 100/0, 50/50, and 0/100 were activated with sodium silicate with modulus (Ms) SiO2/Na2O of 1, 1.5, and 2, and with 5, 10, and 15% Na2O, relative to the mass binder. Samples were cured at 20 °C for up to 720 days, compressive strength, structural, and microstructural characteristics were investigated. Binders of 100% GBFS developed the highest strength of 120 MPa with 5% Na2O, while those of 100% MK required 15% Na2O to develop high strength. The composites 50% GBFS/50% MK required 10% of Na2O to reach up to 88 MPa. In general, the best Ms was 1–1.5. Structural and microstructural examination of the formulation 50/50 indicated the combined formation of a matrix of reaction products of crystalline C–S–H, an Al-substituted alkali charge-balanced calcium silicate hydrate C–N–(A)–S–H–type gel and geopolymeric gel (N–A–S–H), while 100% GBFS binders showed a 9 Å tobermorite-type calcium silicate hydrate (Ca5Si6O16(OH)2) intermixed with an amorphous C–N–(A)–S–H gel.