Influence of single and hybrid activators on the performances of high-volume fly ash cement pastes

As a solid waste and supplementary cementitious material, the fly ash (FA) had garnered attention for its potential to reduce the consumption of Portland cement (PC) in concrete and to diminish the carbon emissions from manufacturing and use of PC over past several decades. To substitute the PC by high volume fly ash (HVFA) in concrete further increased the environmental and economic benefits of constructional materials. However, due to the slower pozzolanic reaction of fly ash and significantly low strength of HVFA materials at early curing duration, the application of HVFA concrete contained 70% to 80% of fly ash was limited. This study investigated the feasibility of eight types of single activators (calcium hydroxide, calcium oxide, sodium sulphate, sodium carbonate, sodium silicate, potassium silicate, sodium hydroxide and borax) with varying dosages, along with one type of hybrid activator involved sodium sulphate and calcium hydroxide, on activation and improvement of strength of HVFA cement pastes at short or long curing stage. The compressive strength test and morphological analyses were conducted to study the effectiveness of activation by various activators on HVFA pastes and development of physicochemical nature of materials at microstructure aspect. It was found in study that addition of sodium sulphate in HVFA pastes contained 70% of FA had the highest compressive strength at 28 curing days comparing to HVFA pastes with and without activation by other types of activators. Then, the effectiveness of activation on HVFA pastes contained 70% of FA was further enhanced by hybrid activator. The strength increment from HVFA pastes without activation to HVFA pastes activated by hybrid activator was 35.8% and it was higher than the increment 19.4% caused by sodium sulphate as single activator on HVFA pastes. The incorporation of additional calcium into HVFA pastes with sodium sulphate promoted the generation of more hydration products which was conducive to reduce pore volume and to form compact internal skeleton. In addition, the hybrid activator was also effective on activation of HVFA pastes contained 80% of FA, resulting in 96.3% of strength increment caused by activation of hybrid activator at 28 curing days. At 360 curing days, the strength increment caused by activation of hybrid activator on HVFA pastes contained 60%, 70% and 80% of FA were -3.8%, 2.5% and 23.6% respectively. This study identified the optimal single activator and demonstrated the feasibility of hybrid activators on activation at HVFA cement pastes at short and long stage. The outcomes served to the broader application of HVFA materials in constructional industry and supported the development of sustainable materials.