Cement and nickel mining industry is facing challenges such as the depletion of natural resources and insufficient landfill disposals of ferronickel slag. These problems can be solved by upgrading the ferronickel slag in concrete production without loss of quality. In this research, the use of ferronickel slag as fine aggregate and cement replacement in concrete and mortar was investigated. The ferronickel slag was ground using a ball mill to achieve two levels of fineness. The performance of mortar was assessed under heat treatment at 75 °C as a comparison to that of the normal curing. The pozzolanic activity of ferronickel slag was determined by the Frattini test. The results obtained showed that the strength of concretes increased with increasing of ferronickel slag content in the concrete mixture up to 40 %; beyond that the strength of concrete decreased. A positive effect on the compressive strength of mortar was achieved by using the slag with a higher fineness. The use of heat treatment at 75 °C enhanced the compressive strength of mortar. Assessment of the pozzolanic activity by means of the Frattini test indicates the non-pozzolanic reaction of the slag after 28 days. The use of heat treatment partially hydrated unhydrated cement grains.