Carbonating and Hydraulic Mortars. The difference is not only the cement type. Aggregates are just as important.

In spite of the increasing availability of mortar less products, mortar has traditionally held walling units together and is still the choice of many builders.

Blended lime - Portland cement (PC) mortars are becoming more fashionable and lime mortars are certainly preferred for restorative work, however the requirements for proper carbonation when lime or the new eco-cement magnesian mortars are used are poorly understood, especially in the English speaking world.

This paper compares carbonating and hydraulic mortars, discusses the chemistry of the strength giving reactions involved and the impact of physical factors such as aggregate size and grading as well as moisture. The role of aggregates for proper carbonation are considered from a theoretical point of view and in terms of best practice from the past. The paper concludes that sands suitable for hydraulic mortars are not suitable for carbonating mortars and visa versa and points out deficiencies in the current standards and codes of practice that do not recognize this.

A new direction is suggested that combines the best practice from the past with that of the present.