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There are only two kinds of cement used in masonry, Lime and Portland.
Cement is like flour, the essential ingredient in baking that can be used to make bread, muffins, etc. Similarly, cement is the essential ingredient for brick mortar, concrete and paving stones. Many people confuse cement and concrete, but they are not the same: Cement is like the flour and concrete is like the bread. Cement is basically the glue of masonry.
So this brings us back to our original question: Which is better, lime cement or Portland? Each has its pros and cons.
Portland Pros Portland Cons
Very high strength Impermeable to water
Quick set time No flexibility
Sets underwater Very high strength
Lime Pros Lime Cons
Flexibility Low strength
Autogenous Healing Slow set time
Cleaner manufacturing process
Since each cement has different good and bad qualities, most common mortars combine both types of cement. There are 5 main types of mortar, suitable in different applications, each with different strengths, properties and applications.
Type M – 3200 PSI – 100% Portland
Type S – 2200 PSI – 67% Portland – 33% lime
Type N – 1400 PSI – 50% Portland – 50% lime
Type O – 1000 PSI – 33% Portland – 67% lime
Type K – 750 PSI – 100% lime
Bricks and wall stones are usually laid in Type N. This 50-50 mix combines the good qualities of the Portland (strength and fast set time) with the flexibility, permeability and workability of lime.
Type M (pure Portland) or Type S must be used for anything exposed horizontally to the elements, such as a stone patio, door threshold or wall coping. This is because water pooling and salt will quickly erode the surface (usually within one year) if the mortar is Type N or lower.
Types O and K are rarely used (Type O is usually for glass block and Type K is practice mortar).
Types of Lime
The element that gives the cement strength is the clay content. Portland cement is about 30% clay based, while lime is usually about 5% clay. However there are three basic strengths of lime mortar, depending on the amount of clay present. The closer the clay percentage gets to 30% the closer it is to a Portland cement since the strength will be the same.
So Which Type of Cement Is Better?
There is no right or wrong type of cement, but there are many right and wrong applications of each and confusion as to what ratio is right for each circumstance. Many people know that using pure Portland on natural stone is bad (and it is), but in our Canadian climate you have to lay a flagstone patio in pure Portland if you want it to last more than a couple of years. Lime and Portland cements are equally useful to the trade, and only knowledge and experience will tell you what the right cement or ratio is.