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Type M mortar is the strongest, and Type K the weakest.These type letters are taken from the alternate letters of the words "Ma So N w Or K".
Hydraulic mortar was not available in ancient China, possibly due to a lack of volcanic ash.
Mortars are typically made from a mixture of sand, a binder, and water.
The most common binder since the early 20th century is Portland cement but the ancient binder lime mortar is still used in some new construction.
Gypsum mortar is not as durable as other mortars in damp conditions.
Gypsum mortar was essentially a mixture of plaster and sand and was quite soft.