The aim of this paper is to study the heat of hydration of blended cement containing ground palm oil fuel ash and treated ground palm oil fuel ash (without unburned carbon), with and without superplasticizer (SP). The palm oil fuel ash (POFA) was ground in a ball mill to produce ground POFA (GPOFA). Unburned carbon, which is a major part of unburned residue in GPOFA, was removed by heat treatment at 500 °C for 1 h, producing treated GPOFA (TGPOFA). This result shows that the total heat of hydration of blended cement pastes containing GPOFA and TGPOFA are lower than OPC paste, proving that GPOFA and TGPOFA are good pozzolanic materials which can be used in massive concrete for preventing volume change and micro-cracks (caused by thermal stresses). Also, GPOFA and TGPOFA have low pozzolanic reactivity at the early hydration. However, TGPOFA has better pozzolanic reactivity than GPOFA due to its higher content of glassy phases. The hydration peak is retarded when SP is added into cement paste, but the total heat of hydration at the age of 7 days is no different even with the addition of SP. © 2011 Elsevier Ltd. All rights reserved.