Mortar Materials & Mixing
Portland cement, lime, mortar types M through O, working time, retempering limits, and the effects of weather on mortar performance.
- Identify the primary ingredients in masonry mortar
- Explain the purpose of lime in mortar
- Classify mortar types M, S, N, O, and K by strength and application
- State mortar working time and retempering limits
- Describe how hot weather, dry brick, and wind affect mortar
Leçon 1
Portland Cement, Lime & Sand
Mortar Components
Masonry mortar is a mixture of cite cementing materials, fine aggregate, and water. The two most common cementing materials are Portland cement and masonry cement. Understanding each ingredient's role is essential for mixing mortar that performs correctly.
Portland Cement and Masonry Cement
Portland cement or masonry cement is used in most mortars as the primary binding agent. Portland cement provides the compressive strength and durability of the mortar. When combined with water, it undergoes a chemical reaction called hydration that produces the hard, stone-like material holding masonry units together.
Masonry cement is a factory-blended product that combines Portland cement with plasticizers and air-entraining agents. It simplifies field mixing because the lime and additives are already included.
The Purpose of Lime
Lime is added to mortar to improve workability and water retention. Lime makes the mortar easier to spread, gives it a buttery consistency that masons prefer, and helps the mortar retain water so it does not dry out too quickly on the board or in the joint. Lime also improves the bond between mortar and masonry units by keeping the mortar plastic longer.
There are two types of lime used in masonry:
- Hydrated lime (Type S) - the most common, pre-slaked and ready to use
- Quicklime - must be slaked before use, rarely used in modern practice
Remember
Lime does NOT increase mortar strength - that is Portland cement's job. Lime improves workability and water retention. This distinction is a common exam question.
Sand
Sand is the fine aggregate that makes up the bulk of mortar volume. Mortar sand must be clean, well-graded, and free of organic matter. The ratio of sand to cite cementing materials affects mortar strength and workability - too much sand produces a harsh, weak mix; too little makes it sticky and hard to work.
The Purpose of a Mortar Joint
A mortar joint serves three critical functions: to bond units, compensate for size variations, and resist moisture. The joint bonds adjacent masonry units into a unified wall, fills the inevitable dimensional differences between individual bricks, and provides a weather-resistant seal when properly tooled.
Portland cement or masonry cement provides strength, while lime improves workability and water retention. The mortar joint bonds units together, compensates for size variations, and resists moisture penetration.