Effect of activator concentration on the strength, ITZ and drying shrinkage of fly ash/slag geopolymer concrete

• Fly ash/slag geopolymer concrete was studied by varying 10–16 M activator.
• The total heat released during paste formation was not significantly affected beyond optimum concentration.
• ITZ displayed satisfactory aggregate-paste interface at optimized concentration.
• Impact resistance of concrete increased with age and its compressive strength.
• The drying shrinkage was very small compared with the OPC concrete.

Geopolymerization of fly ash and slag in a single mix was studied at ambient condition with the help of an Isothermal conduction calorimetry. Increasing activator concentration in the mix led to its shortening of the induction time as well as peak position shifting in the calorimetric curves which showed more workability loss and faster setting. Geopolymer concrete made with fly ash/slag composite mix and various activator concentrations was evaluated for its mechanical properties, age hardening and drying shrinkage. The optimum compressive strength of concrete mix was obtained at 14 M activator concentration. The presence of spongy amorphous geopolymer pastes extended onto the aggregate surface as viewed under Field emission scanning electron microscope (FESEM) images supported satisfactory aggregate-paste interface in the concrete. The compressive strength, elastic modulus and impact strength increased and the Poisson’s ratio decreased with increasing activator concentration. Under repeated drop weight impact, the specimens failed between 95 and 224 hammer blows depending on the activator concentrations. Geopolymer concrete gained an early strength compared with the OPC concrete as predicted by the equation specified in ACI 209R-92. The drying shrinkage of samples was ∼0.1% only at the age of 6 months. It is concluded that fly ash/slag geopolymer concrete can be satisfactorily produced at room temperature with a desired compressive strength similar to that of OPC concrete.

FESEM image on ITZ of fly ash/slag geopolymer concrete.Figure optionsDownload as PowerPoint slide