Direct Numerical and Large-Eddy Simulations of the turbulent flow through a vertical square duct at different Reynolds and Richardson numbers are presented for a fluid with low Prandtl number. Buoyancy substantially affects the flow field, Reynolds stresses and heat transfer. A strong coupling between secondary flow and buoyancy effects is found and a double vortex pattern develops with increasing Richardson number which transports hot fluid from the hot to the cold wall. The alterations in the balance of the individual Reynolds stress components leading to the development of this vortex pattern are highlighted. An analysis of the global scaling of secondary flow strength and other quantities identifies the onset of the mixed convection regime. (C) 2018 Elsevier Ltd. All rights reserved.