Traditionally, stone walls can be built either with the dry-wall technique or by using mortar to bind the stones together, resulting in a more permanent structure. The basic ingredients of mortar are cement, lime, sand and water. The most common types of mortar are types S, N, M and O, and these vary only in the proportions of their compositions. There are some basic guidelines to use to find the best ingredients for mixing your mortar.
Its main constituents are calcium and silicon, so cement provides strength for the mortar. Portland cement is recommended and comes in several varieties. Types I and III are the best for mortar, with Type III setting faster but providing slightly less strength in the long run.
The basic ingredients for mortar are always the same, but depending on the qualities and dimensions of the wall being built, different proportions of each ingredient are used in order to ensure the perfect balance of strength and flexibility for your specific needs. The ratio of cement to lime is greatest in Type M and S mortars, resulting in a very strong mortar suitable for heavy-duty structures such as retaining walls.
Use hydrated lime, which gives smoothness and flexibility to your mortar because of its water-retention abilities and sets faster than non-hydraulic lime. Lime will help create a soft and supple mortar that won't be too stiff to work with. Type O mortar uses a higher ratio of lime than the other types, making it best for indoor walls that will not need to bear any weight load.
Sand adds and maintains the volume of the mortar, and always has a higher ratio than the other ingredients. Coarse sand produces the strongest mortar, but having a gradation of sizes is the best option. Also, siliceous sand tends to perform better than calcareous sand. It is important that the sand be clean of impurities, such as clay and any organic material, otherwise it will have more trouble adhering to the lime and cement.
Water provides the moisture to make the mortar spreadable, and then reacts with the cement so that the mortar sets. Hot water works best for mixing; use water that is pure of any contaminants. Drinking water will do the job. You should use just enough water for the mixture to attain a paste-like consistency that is stiff and workable but not crumbly.
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