Developing beneficial applications for sewage sludge is a key challenge in many countries, given the amount of sludge generated and the disposal or recycling options currently available. Sewage sludge from the wastewater treatment plant at Koprivnica in Croatia has been treated using MID-MIX ® technology that involves the addition of Ca(OH) 2 and CaO to dewatered sludge. This produces a low-density non-hazardous white-grey powder. The treated sewage sludge has been heated to 800, 900 and 1000 °C to form MID-MIX ® treated sewage sludge ash with reduced organic content. MID-MIX ® treated sewage sludge and ash have been used to replace 10 and 20% of cement in mortar and concrete samples. The addition of MID-MIX ® treated sewage sludge causes a significant increase in void content which reduces the mechanical properties and durability of mortar and concrete samples. However, replacement of cement by MID-MIX ® ash did not significantly affect the strength, water penetration or gas permeability compared to control samples. Leaching tests on MID-MIX ® powder, MID-MIX ® ash and mortar samples produced low levels of heavy metal release. The research shows that thermally processed MID-MIX ® treated sewage sludge produces an ash product that has potential to be beneficially used in cementitious materials for selected applications.