Mortar for Masonry part 1: Cement

Cement-lime mortar used for brick and stone masonry is made up of cement, aggregate, lime and water. Aggregate forms the structural body of the mortar, cement is the glue that binds the aggregate, lime improves the workability of the uncured mortar, and water chemically combines with the cement to cause the mortar to harden as well as contributes to the plastic consistency of the wet mortar.

PORTLAND CEMENT
Portland cement, the most common cementitious material in mortar consists primarily of calcium silicates. Common sources of calcium include limestone, marble, and other minerals, and of silica, clay, sand, shale, and marl. Lesser quantities of compounds of iron, aluminum, magnesium, and sulfur are added to improve the hydration properties of the finished cement (hydration is explained below).

The raw materials are ground to a small particle size, blended, chemically combined in a large, high-temperature kiln, finely ground again, and mixed with a controlled amount of gypsum to produce portland cement. In its finished state, the cement particles are very small, ranging in size from 0.0004 to 0.0006 inches in diameter.

Portland cement is an hydraulic cement: It hardens by chemically combining with water, a process called hydration. The small size of the cement particles is important to ensuring the fullest possible hydration of the cement during the curing process.

OTHER CEMENT TYPES
Blended hydraulic cement is a mixture of portland cement with other cementitious materials such as blast furnace slag (itself an hydraulic cement), or any of a number of pozzolans (materials that react with the calcium hydroxide in wet portland cement to form other cementing compounds) such as fly ash, silica fume, or natural pozzolans derived from shales or clays.

The use of blended hydraulic cements reduces the quantity of portland cement in the mortar, reducing the energy required to manufacture the cement. Depending on their formulation, blended hydraulic cements can produce mortars with increased strength, lesser permeability, and other favorable properties in comparison to portland cement-lime mortars.

Masonry cements and mortar cements are proprietary blends of various cementitious materials, lime, and other ingredients. Unlike mortar made from portland cement or blended hydraulic cement (cement-lime mortars), mortar made from either masonry cement or mortar cement does not need the addition of lime as a separate ingredient. Both of these cement types offer the convenience and consistent quality of factory mixing, and can produce mortars with improved workability when wet, less shrinkage while drying, and other benefits. Masonry cements, which rely, in part, on air entraining (the generation of very small air bubbles within the mortar mix) for their workability, have a lower bond strength than cement-lime mortars, and may not be suitable for some applications, such as masonry subject to high wind or seismic forces. Mortar cements are produced to a different standard that produces mortars with bond strength comparable to cement-lime mortars.

Lime mortar is made with just lime, aggregate, and water--in this type of mortar, lime itself acts as the aggregate binder. In the absence of any hydraulic ingredients, lime mortar cures as atmospheric carbon dioxide gradually combines chemically with the mortar, a process called carbonation. Lime mortars are primarily used in the restoration of historic masonry where it is important that new materials used to repair such structures have physical properties close to those of the the original materials. Lime mortars with varying degrees of hydraulic properties and greater compressive strength can also be produced by adding varying amounts of other cementitious materials to the mortar mix.

	Hydraulic cement	Lime and other workability agents	Aggregate
Portland cement-lime mortar	portland cement	lime	sand
Mortar with blended hydraulic cement	blended hydraulic cement	lime	sand
Mortar with masonry cement	masonry cement		sand
Mortar with mortar cement	mortar cement		sand
Lime mortar	(none)	lime	sand