The effect of unburned carbon in palm oil fuel ash on fluidity of cement pastes containing superplasticizer

The aim of this study was to clarify the influence of unburned carbon in palm oil fuel ash (POFA) on the fluidities of cement pastes containing a polycarboxylate-based superplasticizer (SP). The POFA was ground in a ball mill to produce ground POFA (GPOFA). Unburned carbon, which was the major part of unburned residue in GPOFA, was removed by heating at 500 °C for 1 h, producing treated POFA (TPOFA). Neither glassy phase crystallization nor agglomeration of GPOFA particles occurred during the heat treatment. Cement pastes containing GPOFA and TPOFA had lower fluidities than that of ordinary Portland cement (OPC) paste. Cement pastes containing TPOFA had higher fluidities than cement pastes containing GPOFA. Unburned carbon absorbed more SP than did the other particles in the cement pastes because of the carbon’s large specific surface area. Because of their irregular shape and porosity, GPOFA particles absorbed more SP than did OPC particles. Therefore, the higher the content of unburned carbon and GPOFA particles in the cement paste, the greater the quantity of SP needed to be added.