Influence of substrate and sand characteristics on Roman cement mortar performance

•Effect of coarser sand on rheological properties decreases as excess paste increases.•Porous substrates increase mortar strength and decrease porosity, WAC and shrinkage.•The influence of substrate on mortar strength is reduced if the substrate is wetted.•Substrate and sand characteristics and sample thickness affect mortar properties.•No simple correlation between properties of sampled and standard laboratory mortar.

When formulating repair mortars standard test specimens should be used with caution as these cannot be considered representative of samples of mortars collected on site. This work reports an approach to repair mortar formulation which takes into account the influence of porous substrates, sand characteristics and mortar thickness on the properties of both fresh and hardened Roman cement mortars. It is shown that mortars cast on a dry absorbent substrate show modified properties such as increased strength and decreased water absorption coefficient, the degree of which is a function of sand grading and surface characteristics, sample thickness and substrate sorptivity.