What Is Mortar And How Is It Different From Concrete?
Updated: Dec 10, 2022
Concrete is the number one, most common, building material used on Earth. It is everywhere. Mortar, while similar to concrete, is a different mixture of essentially the same ingredients: Portland cement, water and aggregate. The mixes for each are adjusted to deliver the characteristics required for any particular application.
Mortar is used as the "glue" to bond masonry units to one another. While it can be mixed to be as strong as concrete, it generally doesn't require the strength of concrete. It's the bonding characteristics, primarily, that the mason is more concerned with than the strength. Mixing mortar with a higher water-to-cement ratio typically creates a product that bonds well. This is just the opposite when it comes to concrete mixes. Concrete that is mixed with a lower water-to-cement ratio will be stronger and more aligned with its intended purpose.
Another difference, that many masons would say is the main difference, between mortar and concrete is the addition of hydrated lime into the mix for mortar. Lime has been added to mortar for over 2,000 years. So we could say it's a pretty well proven technology. It's stood the test of time.
The addition of lime to the mortar mix provides the mason with many benefits:
Plasticity: The lime causes the mix to be more elastic, which helps the mason to evenly spread the mortar onto the bricks, stones or blocks with a trowel. Lime in the mix helps it hang on to more water than without the addition of lime. This improves the mixes ability to penetrate into the masonry units and bond better. By also slowing the drying process and bonding more thoroughly to the masonry units, the lime reduces the amount of cracking along the mortar joints. This in turn reduces the amount of water penetration possible once the mortar joints are set (hardened).
Strength: Mortar that is more elastic, more able to flex under pressure, is less likely to crack due to structural movement such as expansion or contraction.
Moisture Permeability: The lime in the mix increases the ability of cured mortar to allow moisture to pass from the inside of the wall to the outside. In essence, the lime mortar mix works like a wick, pulling moisture from the backside of the wall to the outside of the wall.
Self-sealing: The liime in the mix, when exposed to mositure and co2 in the air causes calcium carbonate crystals to be deposited where the water evaporates. So where cracks appear, and water penetrates the wall, the process of self-sealing immediately begins. If the cracks are relatively minor, such as hairline cracks along the mortar joints, the wall can heal them on it's own.
When it comes to using the mortar, the most important mix characteristic to the mason is it's flexibility, how elastic it is. Without the addition of lime or some similarly functioning additive masonry wouldn't be the profession it is today.