Improvement of the early and final compressive strength of fly ash-based geopolymer concrete at ambient conditions

• Fly ash based geopolymer concrete-silica fume is suitable for concrete in hot weather in the absence of external heat.
• Sodium hydroxide is the dominant factor in 7 days compressive strength of FGC-silica fume.
• Sodium hydroxide can be reduced for lower compressive strength needs.
• Portland cement replacement increases the early and final compressive strength in ambient curing conditions.
• Portland cement utilizes the free water leading to a reduction in the formation of microcracks.

Sustainable concrete has reduced CO2 emissions, is durable, and is expected to have less detrimental effect on future generations, due to the fact that it utilizes waste materials. However, the need for external heat limits construction applications. The effects of sodium hydroxide ratio, external heat amount, and partial Portland cement replacement on fly ash-based geopolymer concrete were investigated. The early compressive strength, density, absorption, and permeable voids were measured, and the microstructure of the fly ash-based geopolymer paste was observed and characterized. The activating solution was a combination of silica fume, sodium hydroxide, and water. Experimentation showed that application of external heat plays a major role in compressive strength. Results also show that early and final compressive strength gains, in case of absence of external heat, can be improved by using Portland cement as a partial replacement of fly ash. The Scanning Electron Microscopy (SEM) results showed that the addition of Portland cement utilized the free water from the geopolymerization reaction. It not only led to a reduction in the microcracks formation due to less shrinkage, but also provided extra alkalinity, such as calcium hydroxide, which helped accelerate the fly ash and the activating solution reaction. Additionally, the permeable void ratio is affected by the Portland cement replacement, showing a significant reduction when the Portland cement ratio is increased.