Volume stability of Portland-dolomite cement pastes cured in different conditions

For clarifying the effect of dolomite on the volume stability of Portland dolomite cement (PDC) at high reaction degree, the length change of PDC pastes with the replacement levels of 10-30 wt% dolomite and cured in different conditions was examined, in contrast with Portland limestone cement (PLC). The hydration products and microstructure were investigated using XRD, TGA, and SEM. Results indicate that at 40 and 60 ? the PDC and PLC show similar deformation patterns, with a small expansion. At 80 ?, however, the PDC are characterized by a higher expansion than the PLC, especially at high replacement levels. The reaction process of dolomite in PDC is dependent on the availability of aluminate phases. In the presence of free alumina, dolomite would react preferentially to form carboaluminates, hydrotalcite, and calcite. When the alumina is exhausted, dedolomitization reaction takes place producing brucite and calcite. The high expansion in PDC is mainly associated with the dedolomitization, which may result in the reinforcing frame volume and crystallization pressure due to the formation of brucite and calcite in confined space. However, the expansion of PDC is too small to cause damage to hardened pastes. Therefore, the incorporation of dolomite in PDC has no adverse effect on the volume stability of cement-based materials.