The first mortars were simply clay-muds, then lime and sand. The basic ingredient in modern mortar is Portland cement, which is also the active component of concrete.
Supporting Masonry Work
Two things all masonry materials have in common is that they’re heavy and that they’re rigid, and therefore subject to cracking. If the surface on which a masonry structure is placed can move, that masonry will eventually crack. All masonry must be built on stable ground, either soil that hasn’t been disturbed before or fill that’s been mechanically compacted. In most cases, footings also have to be below the frost depth, that is, the deepest that the ground is expected to freeze in the winter. When earth freezes, the water in it expands as it turns to ice, and that can generate enough force to lift or heave a building, cracking the rigid masonry material. On top of that stable ground, masonry walls are built on footings. In most cases, footings are concrete structures, although beds of compacted gravel or crushed stone can work as well. Always wider than the wall, footings spread the weight of the masonry above over a greater area, so it doesn’t sink into the ground unevenly and crack the wall. The width of a footing varies with the weight it will support and the bearing capacity of the soil it rests on. Most footings are about half as deep as they are wide, which provides enough depth for the footing to act like a beam in supporting the load. Concrete slabs, as well as stone or brick walkways, must also be supported on solid ground, but rather than having a concrete footing, they rest on a base of crushed stone. Like a footing, crushed stone helps distribute the weight of, say, a car wheel, to the ground below. The stone also provides drainage, and a more or less even surface for the masonry above to rest on. It also allows the concrete, stone, or brick above it to move laterally ever so slightly. All masonry expands and contracts with temperature changes, and if it’s restrained from moving, it will crack.
What is Concrete, Anyway?
The concrete used for slabs and footings is a mixture of gravel, sand, and Portland cement. Portland cement is a combination of clay and limestone fired at a very high temperature and then ground to a fine powder. Water is added on-site. The Portland cement and water combine and surround the sand and gravel. At the same time, the water reacts chemically with the Portland cement and the whole slurry begins to harden. It’s important to add just the minimum amount of water needed to make a workable mix—excess water weakens the finished concrete. Concrete takes about 28 days to fully harden, although it usually becomes walkable within 12 to 24 hours. Depending on the purpose of the concrete, additives may be included in the mix. Fiber reduces the chances of cracking. Plasticizers allow less water to be added while still resulting in concrete of a workable consistency. Retarders slow the hardening process, something that can be useful in hot weather when concrete can set too quickly to finish well. Conversely, accelerators speed concrete’s hardening, which is useful in cold weather when concrete sets slowly. For big jobs, pros order concrete from a batch plant and it’s delivered in ready-mix trucks. A fully loaded ready-mix truck contains about 10 cubic yards of wet concrete, about 20 tons of material. Jobs requiring less than one cubic yard of concrete are usually done with bagged mixes. Mortar is a mix of Portland cement and sand and is used for bedding masonry units such as concrete block, brick, and stone. Traditional mortar also includes lime, which makes the mortar stickier, so it stays on the ends of each brick or block as it’s being placed. Mortars used before the end of the 19th century were composed of only lime and sand, which made a very soft mortar. That played well with the soft bricks of the time. Today’s mortars use additives other than lime to achieve good working characteristics and are sold in several formulations intended for laying brick, block, or stone. Pros usually have sand and bagged mortar delivered in bulk, but as with concrete, bagged mixes are available for smaller jobs.
Can a DIYer Do Masonry Work?
Most masonry work is a job for professionals. Big jobs require heavy equipment, specialized tools, and skills honed over years. That said, there are still a number of masonry tasks that fall into a DIYer’s wheelhouse. Walkways, for example, can be a DIY project. And there are a number of masonry repairs that a handy person with a few tools and the right products can do well. For example, repointing failing mortar is within the capabilities of most people. Likewise, patching concrete is a simple job and resurfacing a pitted and worn slab or sidewalk. Precast concrete blocks open up some great possibilities such as fire pits and patios.
