The water to binder ratio is recognized as being the parameter that most influences properties of cementitious materials, such as masonry mortars. This parameter is used with all types of hydraulic binders used in masonry, such as Portland cement, natural cement or hydraulic lime. It represents the weight of water on the weight of the binder in a given mortar. This ratio has, among other things, a great influence on the workability of the mortar in the fresh state, as well as on all of its properties at the hardened state, such as compressive strengths, shrinkage and,ultimately, the sustainability of the masonry project.

Mortars that use hydraulic binders harden due to the chemical reaction (hydration) between the particles of the binder and the water. In theory, for every kilogram of hydraulic binder, about 0.25 kg of water is required for particle hydration. This therefore represents a water to cement ratio 1:4 or 0.25. However, a mortar mixed with a ratio of 0.25 will not achieve the necessary workability for the installation of masonry units. To achieve this workability, an additional quantity of water is added. When laying masonry units, a water to binder ratio greater than 0.75 is often used.

When additional water is added but not consumedduring hydration, it evaporates and in turn renders a mortar with greater porosity. As the water to binder ratio increases, with the added porosity, the end result is a decrease in compressive strengths in the mortar.

On the other hand, a lower water to binder ratio will reduce the porosity and will lead to higher compressive strengths. With less water in the mix, the mortar becomes less workable and could potentially affect the bond between mortar and masonry unit.

In summary, the more water we add to the mortar mix, the lower the compressive strengths and the less water we add to the mix, the higher the compressive strengths.


The results indicated on the manufactures technical data sheets are always linked to a certain amount of water. In masonry, this amount of water is expressed indirectly by the flow. The Canadian Standards - CSA A179 - states that a bedding mortar should have a flow, when measure in a laboratory, of between 110% and 115%. The same bedding mortar will have different results on site with the expected range between 130% and 150%.

The difference is explained by the fact that in a laboratory, the workability of the mortar is not taken into consideration and that the cubes used for testing have an absorption rate of 0%. On project sites, the workability of mortar is very important and the absorption rate of masonry units is variable. When work is set to begin on site, the mason could modify the water to binder ratio that is detailed on the manufacturers technical data sheet.

Among the reasons to change the water to binder ratio could be: the absorption rate of the masonry unit, ambient temperature, wind,weight of the masonry unit, etc. As explained previously, modifying the amount of water - or the flow - will change the compressive strengths of the mortar.

Aware of the many possible variations, the Canadian Standard CSA A 179 always refers to the mortar minimum compressive strength when the acceptance of these is built on the properties. Therefore at 28 days a Type N mortar will have an on site minimum compressive strength of 3.5 MPa, while a Type S mortar will have an on site minimum compressive strength of 8.5 MPa after 28 days.


■ To properly interpret the results obtained from on site tests, it is essential to know the water to binder ratio of the mortar prepared;

■ The result analysis should also take into consideration the absorption rate of the installed masonry units because it influences, among other things, the porosity of hardened mortar.