Durability and Microstructure Properties of Low-Carbon Concrete Incorporating Ferronickel Slag Sand and Fly Ash

Ferronickel slag (FNS) which is also known as electric arc furnace slag is a byproduct of the production of ferronickel alloy. The production of FNS at Societe Le Nickel (SLN) in New Caledonia is about 2 Mt per year with an existing stockpile of 25 Mt, which presents an excellent potential for concrete applications in the Pacific region. The possibility of using FNS from SLN as fine aggregate replacement in concrete is investigated. The low-carbon-concrete mix design includes 50% natural sand replacement by FNS sand and 25% ordinary portland cement substitution by fly ash. Microstructural analysis by scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDS) of the interface transition zone (ITZ) of FNS sand shows that the excess in Portlandite weakening the ITZ of natural aggregate is absent in FNS sand ITZ. As a result, the resistance against chemically aggressive ions diffusion, water absorption, sorptivity, bulk and surface resistivity, and volume of permeable voids are significantly improved compared with the reference concretes due to the pozzolanic effect of FNS strengthening the ITZ. The substitution of 50% natural sand by FNS sand allows offsetting the detrimental effect of using fly ash on the concrete resistance against carbonation. All results show that using FNS sand in concrete can improve the concrete performance.