There is much confusion between masonry cements and portland/lime blends in the masonry market today. The one question that is always asked is, “Which one is better?” The only way to answer that is, the products are different but when handled appropriately produce quality masonry construction. Masonry cements and portland/lime blends have different ingredients, manufacture processes, and are covered under different specifications. However, the two have similarities which include: bond, workability, weather resistance, water permeance, efflorescence potential, water retention, and board life.
Masonry cements are manufactured in a cement plant using portland clinker, plasticizers, and an air entraining agent all interground together. Some items which can be used as plasticizers are limestone, clay, raw feed, and lime hydrate. These materials when finely ground have the ability to adsorb water thus increasing workability. Air-entraining agents protect from freeze-thaw deterioration and provide additional workability. Air entraining agents produce tiny, microscopic air bubbles in mortar. These air bubbles act as ball bearings, increasing workability. The freeze-thaw protection is accomplished because any water that may freeze and expand will do so inside these bubbles, therefore not allowing stresses to build within the mortar joint.
Masonry cements in Types N, S, and M are covered by ASTM C91, Standard Specification for Masonry Cements. All manufactures of masonry cements must show that their product meets the physical requirements set forth in ASTM C91. ASTM C270, Standard Specification for Mortar for Unit Masonry, covers construction of masonry structures using mortars. Therefore, construction of masonry structures using mortar in accordance with ASTM C270, and masonry cement qualifying under ASTM C91, renders acceptable mortar provided you have quality workmanship.
Portland/lime blends are manufactured in a blending facility using portland cement and hydrated Type S lime. The lime provides for workability, much the same as plasticizers do for masonry cements. Lime is made up of hexagonal shaped hydroxide crystals. These crystals are thin, flat particles which slip and slide over one another, but never separate completely, acting as a lubricant. The particles of lime in a portland/lime blend are very fine, helping to increase its ability to retain water effectively, again to help with workability.
Portland/lime blends in Types N, S, and M are covered by ASTM C270 only. This is due to the fact that the two ingredients in a P/L blend, portland cement and lime, are each covered by their respective specifications: ASTM C150, Standard Specification for Portland Cement, and ASTM C207, Standard Specification for Hydrated Lime for Masonry Purposes. Therefore, construction of masonry structures using mortar in accordance with ASTM C270, and a P/L blend using ASTM C150 portland cement and ASTM C207 lime, renders acceptable mortar provided you have quality workmanship.
Despite the differences between masonry cements and P/L blends, they still are very similar materials where end results are concerned. One of the most important items when constructing a masonry wall is to insure its ability to keep the water out, often referred to as water permeance. Both masonry cement mortars and P/L blend mortars have the capability of providing for water permeance, given they are used effectively by the mason. The key to water permeance is full, complete bed and head joints. It is common for a mason who is comfortable using one of the products, to have problems using the other. The most common reason for a wall to lack water permeance is the mason was not familiar with the product being used and thus was unable to get full, complete head and bed joints. Another item which follows into this same idea is bond. Full head and bed joints provide for a stronger more complete bond, which is achieved by a mason who is familiar with the product they are using and knows how to use it effectively.
Many times, those opposed to masonry cements contend that the air bubbles in the mortar interfere with bonding and thus water permeance. Studies have been preformed to test bond with different cements, but many of these tests are preformed in “stored-in-air” conditions, which do not readily compare to real life situations. Tests preformed in a way that resembles actual field conditions show that masonry cements achieve high bond strengths, just like P/L blends do. This is due to the fact that all cement based products need water to hydrate all of the cement particles. When a specimen is stored in a laboratory without atmospheric moisture, the cement particles are not allowed to hydrate completely, decreasing bond and water permeance capabilities.
Another issue commonly misunderstood is efflorescence. Efflorescence is the white colored “stain” which can appear on masonry construction. Many people confuse this white colored stain for lime and blame the P/L blend. However, this “stain” occurs when water dissolves the soluble salts contained in all masonry materials, the water then evaporates, and the salts are left behind. This occurrence is common on newly constructed masonry structures, due to the inherent moisture trapped within the structure itself. Given time, it fades and does not reoccur unless moisture is introduced somewhere else in the system.
One last issue that comes up when talking about masonry cements versus P/L blends is the potential for autogenous healing of fissures in mortars when a P/L blend is used. The theory is that when a fissure develops in a P/L mortar, the lime in the mortar is capable of filling in the fissure. It is important to point out that it is the tiny micro-fissures that can be healed, not cracks. This happens because water is allowed to enter through the fissure, which dissolves some of the calcium in the lime rich mortar. Then carbon dioxide from the air reacts with the dissolved calcium producing calcium carbonate, which plugs the fissure. There is no doubt that this can occur, the only question is what happens to the void that is created when the calcium dissolves and moves to fill the fissure. One final thing, autogenous healing will not remedy cracks and other problems usually contributed to poor masonry techniques.
The controversy surrounding masonry cements and P/L blends will more than likely never go away. Just remember, the key to having a successful masonry project, is a well qualified mason on the job who is comfortable with the product he is using, not which type of mortar the designer specifies for the project.
Guide To Masonry Mortar
Types Of Mortar ( ASTM C-270)