Mortar and Grout: Batching and Mixing Mortar

Site batched mortar station with paddle-type mortar mixer and five-gallon buckets for proportioning materials.

Mixing mortar seems like a such a basic thing in masonry construction that it’s easy to take for granted. Yet, when mortar is not properly batched and mixed, things can quickly go wrong. Mortar is responsible for binding the masonry units into a cohesive component. As a result, it should bond well to the units, accommodate the dimensional variation of the units, be durable and contribute to weather resistance of the masonry assemblage.

Portland cement, masonry cement and mortar cements are typically used, along with Type S hydrated lime, to function as the binder in the mortar. The binder portion creates the compressive strength, bond strength, durability and causes the early setting of the mortar. The role of the aggregate in a mortar mix is to act as a filler and to reduce the overall shrinkage that would occur if the mortar was only made with binding materials. ASTM C270, Standard Specification for Mortar for Unit Masonr, requires that the volumetric ratio of aggregate to binder be not less than 2 ¼ and not more than 3 times the sum of the cement and lime volume. This ratio is derived from the typical void space in well-graded aggregates of approximately 1 part void in 2 to 3 parts aggregate.

All cements used in masonry mortar are not equal. Portland and mortar cement generally result in mortars that have a lower air content and therefore slightly higher adhesion to units and higher mortar compressive strength. As a result, some job specifications will prohibit the use of masonry cement in mortar. In some cases, this prohibition is justified but it can also be a leftover artifact from previous specifications. It is important to use the mortar specified in the construction documents or receive written approval for any alternate materials.

Contrary to what is seen on some jobsites, site-mixed mortar should not be batched by shovel count. The results are not consistent from day to day and from person to person. Instead a fixed volume container should be used – a box or a bucket will provide a repeatable measure of volume. Alternately, many masonry contractors are purchasing pre-mixed mortar in sacks or bulk bags that only require the addition of water.

For proper site mixing of mortar, the following steps should be used. First, ensure that the appropriate safety equipment is available and used. Eye protection and waterproof gloves are the bare minimum. Use a horizontal paddle type mortar mixer – concrete mixers should not be used. Measure out the recommended amount of water for the batch and pour half of it into the mixer. Slowly add the dry materials and allow the mortar to mix for one minute after all dry ingredients have been added. Add the additional water as needed to get to the proper consistency and mix for an additional 4 to 5 minutes ensuring a uniform consistency. Let the mortar mix sit in the mixer without agitation for another 4 to 5 minutes to allow the moisture to be evenly distributed throughout the mortar.

The appropriate mortar consistency is influenced by the type of unit used. Concrete masonry units will obviously be more absorptive than glass block and, as a result, glass block mortar will be a stiffer mix. The term “workability” describes the suitability of the mortar for the intended purpose and is not always easy to quantify. Suffice it to say, the mason holding the trowel is the best judge of this mortar property and will be quick to point out if it is lacking. To summarize, mix mortars systematically with the materials and proportions required and slightly adjust the amount of moisture added on site to achieve the best mortar mix for each day on the job.