Influence of Si:Al ratio on the microstructural and mechanical properties of a fine-limestone aggregate alkali-activated slag concrete

Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested; two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by Na 2 CO 3 . Each of these series was made up of three formulae containing different amounts of Al 2 O 3 . The compressive strengths of the series activated by waterglass/NaOH after 28 days were ≈65 ± 5.3 MPa, a 22% increase compared to previously reported formulae containing no additional Al 2 O 3 . Increasing the amount of Al 2 O 3 did not further increase strength, however. The Na 2 CO 3 -activated formulae had strengths of ≈35 ± 3 MPa after 28 days, representing no increase in strength over formulae not containing Al 2 O 3 previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously reported results for formulae not containing Al 2 O 3 and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy, showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume fraction of unreacted slag than formulae without Al 2 O 3 . The reason for the increase in strength of Al 2 O 3 -containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.