Improving the compressive strength of concrete by using nano fly ash and examining the interfacial transition zone (ITZ)

The purpose of this research is to reduce cement consumption by replacing nano fly ash (NFA) in the concrete mixture and increase the compressive strength of concrete by strengthening the interfacial transition zone (ITZ). Thus, four proportions of NFA with values of 0.1, 0.5, 1 and 5% by weight were selected in the concrete mixture and the compressive strength of concrete was evaluated within 7 and 28 days. A scanning electron microscope (SEM) equipped with an X-ray energy diffraction spectrometer (EDS) was used to detect the compounds resulting from the reaction between particles. Based on the results, in the sample containing 1% NFA (optimal sample), compared to the samples without NFA, the 7-day compressive strength of concrete increases by 97% and the 28-day compressive strength increases by 61%; which shows the NFA has a better performance in increasing the strength of concrete at early ages. Also, the amount of water absorption in the sample containing 1% NFA decreased to 33%, therefore, the permeability of concrete against destructive agents is improved. These results show that the interfacial transition zone (ITZ), which is the weakest link in the concrete chain and the limiting factor of concrete strength, is strengthened in the presence of NFA, and leads to an increase in durability and compressive strength of concrete.