Mechanical and microstructural characterization of an alkali-activated slag/limestone fine aggregate concrete

Four limestone-based, alkali-activated slag fine aggregate concretes, two of which contained amorphous silica in the form of diatomaceous earth, were fabricated using different activating solutions (NaOH/waterglass or Na2CO3). Emphasis in this work was placed on using simple manufacturing methods and widely available materials, to ensure that these formulae are practical as construction materials in the developing world. Although cured only at room temperature, these fine aggregate concretes have good compressive strengths (∼45 MPa) and their tensile strengths increased from ∼2.6 MPa after 1 day of curing to ∼4 MPa after 28 day for the NaOH-activated formulae. Samples activated with Na2CO3 had negligible tensile strengths after 1 day, increasing to ∼2.5 MPa after 28 day. The main cementing phase was shown to be calcium–silicate–hydrates in all formulae; those activated with Na2CO3 also showed the presence of hydrotalcite. No evidence of geopolymeric phases was found, though incorporation of Na to form N–S–H that balance charges arising from Al substitution of Si in C–S–H is likely. Despite the short (∼120 s) pot life of the strongest formula, NaCl was shown to be an effective retarding agent, which reduced the strengths of different formulae, at worst, by less than 25% after 28 day of curing.