Durability of mortar with calcined marl as supplementary cementing material

Calcined marl – ordinary ‘blue' clay containing calcium carbonate – is an effective pozzolan in cementitious materials in terms of strength if calcined at the correct temperature. Marl is unsuitable for the clay product industry, and might thus be the high-volume available alternative pozzolan globally that the cement and concrete industry is looking for in order to reduce its carbon footprint. Mortars are made where calcined marl replaces cement by volume at levels of 0, 20, 35, 50 and 65 vol.%. Excellent strengths at both 1 and 28 d relative to reference are obtained for up to 50 vol.% replacement, and the strength continues to increase up to 1 year. The capillary porosity volume increases along with the electrical resistivity, indicating a pore refinement or segmentation, as well as perhaps reduced ionic strength in the pore-water. These are properties that might reduce the propagation rate of rebar corrosion once initiated by carbonation, as carbonation rate increased with cement replacement. The chloride ingress is significantly reduced when cement is replaced by calcined marl. Sulfate resistance is also tested at both 5 and 20°C, but no significant changes are observed after 9 months' exposure.