Fly ash-based one-part alkali activated mortars cured at room temperature: Effect of precursor pre-treatments

Abstract

One-part or “just add water” alkali activated materials (AAMs) have attracted a lot of attention thanks to the use of solid alkaline activators that makes these materials more suitable to commercialization compared to conventional AAMs (two-part). This is mainly because large quantities of caustic solutions should be handled for producing conventional AAMs. So, one-part AAMs have a great potential for in-situ applications. However, heat curing (<100 °C) has been demonstrated to be the best condition to obtain optimized performances of one-part AAMs. This study investigates how to obtain high strength one-part alkali mortars cured at room temperature, considering a newly developed mix design, precursor pre-treatments and curing time. The mechanical performances (i.e., elasticity modulus, compressive and flexural strength) of the developed materials were reported, as well as physical properties, in terms of water absorption, open porosity and pore size distribution and microstructure (by means of FEG-SEM observations coupled with elemental analysis by EDS and FT-IR measurements). Class F fly ash have been activated by potassium hydroxide (KOH) and anhydrous sodium metasilicate. It was found that high strength one-part AAMs can be achieved by activating coal fly ash with a mix of KOH and anhydrous sodium metasilicate at room temperature. In particular, room temperature-cured one-part mortars obtained by pre-treated fly ash exhibited mechanical performance similar to those obtained by heat-cured mortars (at 70 °C, tested after 7 days), reaching a compressive strength (Rc) greater than 60 MPa at 28 days of curing when mechanochemical activation of fly ash was applied.

Keywords

• One-part geopolymers
• Alkali activation
• Fly ash
• Precursor pre-treatment
• Mechanical properties
• Room-temperature curing