As an alternative to combustion, the effect of adding a problematic, high sulphur content pet coke with different particle sizes on the thermal conductivity of Portland cement pastes and mortars has been investigated. Pet coke addition significantly decreased the thermal conductivity of both paste and mortar samples. The fine-grained (<0.5 mm particle size) fraction had the greatest effect on thermal conductivity at low addition levels, and use of this size fraction limited the strength reduction associated with pet coke addition. Thermal conductivities as low as 0.12 W/mK were obtained for paste samples containing 75 wt.% pet coke. This represented a 76% reduction in thermal conductivity compared to control samples. Factors contributing to the reduction in thermal conductivity and comparison of the effects of pet coke addition on mortars and pastes are discussed. (c) 2013 Elsevier Ltd. All rights reserved.