Note on Cement-Lime Mortars versus Masonry-Cement Mortars

At times in the past, and to some extent even to this day, controversy has existed within the masonry technical community over the comparative performance of cement-lime mortar and masonry-cement mortar. Each cementitious system has advantages and disadvantages. Each has demonstrated general suitability for use and also general cost-effectiveness for suppliers and users.

Masonry cement complies with the physical property requirements of ASTM C91. Because the standard specifications are based on properties rather than ingredients, specific formulations of masonry cement vary from manufacturer to manufacturer. Ingredients and formulations are not required to be disclosed, and generally are not. Masonry cement is generally delivered to the jobsite in prepackaged form. It consists of a mixture of portland or blended cement and plasticizing materials (such as hydrated lime or finely ground limestone) together with other materials introduced to enhance performance. These other materials generally include air-entraining and water-retention additives, intended to improve freeze-thaw durability, workability, and water retention.

The primary advantage of cement-lime mortar is its high tensile bond strength. Its disadvantages are the additional complexity of mixing three ingredients, and some lack of workability (stickiness) if not retempered. The first disadvantage can be overcome by single-bag or silo mixes. The second can sometimes be overcome by retempering.

The advantages of masonry-cement mortar are its relative simplicity of batching and its good workability. It has a “fluffy” consistency (because of its entrained air), which leads to good productivity. Its lower tensile bond strength is accounted for by lower allowable stresses in design codes. In part because of these lower bond strengths, and in part because of tradition, masonry cement is prohibited in structural masonry zones of high seismic risk in the United States.

Considerable anecdotal evidence, and some controlled experimental evidence indicates that other things being equal, walls laid with cement-lime mortar leak less than walls with masonry-cement mortar. In the author’s judgment, this is true. It is also true, however, that acceptably water-resistant walls can be constructed using either cementitious system, however the cementitious system is not the most important choice to make when specifying masonry. The proper type of wall (drainage vs. barrier) and proper drainage details, if applicable, are more important.

From the viewpoint of cement producers, masonry cement is probably a profitable “niche” product. A 70-lb bag of masonry cement typically contains about 40 percent or less (28 lb or less) of Portland cement or other cements, and about 40 lb of ground limestone. The rest is airentraining additives, and possibly additives for water-retention and plasticity. A 70-lb bag of masonry cement (28 lb cement, 40 lb limestone, and additives) commonly sells for the same price as a 94-lb bag of Portland cement.

Mortar cement was introduced in the 1990s to preserve the construction advantages and potential profitability of masonry cement, while at the same time increasing the tensile bond strength of the resulting mortar to values comparable to those of cement-lime mortar. Mortar cement is regarded by building codes as the equivalent of cement-lime mortar, and is permitted in all seismic zones of the United States.