How much water do we need to prepare cement mortar for plastering? What should be the water-cement ratio for plastering?
These questions are flowing in my mind for several days.
Our common practice is, we don’t give much attention to the water content of plastering mortar. We just leave it to masons who involve in plastering. They mix water as they require.
But as construction professionals, we need to know about the water-cement ratio for plastering. That’s why I decided to write about it in this post.
Let’s first learn about the water-cement ratio.
What Is The Water-Cement Ratio?
The notion of water-cement ratio was first developed by Duff A. Abrams in 1918.
The concept of the water-cement ratio is the ratio of the weight of water to the weight of cement.
If we see this in mathematical term, it looks like,
Water cement ratio= Weight of water ÷ Weight of cement.
It is mainly developed for concrete mix.
There is a close relationship between the water-cement ratio and the strength of concrete.
The lower the water-cement ratio the higher the strength and durability of concrete.
Simply meaning, if we use less water in the concrete mix, we’ll get higher strength and durability of concrete.
The water is mainly required to complete the hydration process of cement. And theoretically, 0.35 water-cement ratio is enough to complete the hydration process.
That means, we only need 0.35 kilogram of water for every kilogram of cement.
However, a mix with a ratio of 0.35 may not mix thoroughly and may not have enough workability. So, we need to use a reasonable amount of water in the mix. So that it becomes workable.
Water-cement ratios of 0.45 to 0.60 are typically used for concrete mix.
For higher strength concrete, lower ratios can be used along with a plasticizer.
The Water-cement Ratio For Plastering
As I discussed above, the water-cement ratio is mainly developed for concrete.
Does it applicable to plastering mix? If it does, what should be the water-cement ratio for plastering mortar?
An experiment conducted by IS standard observed that the Abrams’ law of w/c is also applicable for cement mortar.
Based on the experimental results, it is observed that the minimum w/c ratio required to make the cement mortar workable is 0.5.
From the experimental results, compressive and split tensile strengths are maximum at w/c ratio of approximately 0.6 for cement mortar of 1:3, 1:4, 1:5 and 0.7 for 1:6, 1:7.
Now we are sure that the water-cement ratio is applicable for plastering mortar also.
But the question is does it practically work in the field? Can we calculate the required water content for plastering mortar based on the specified w/c ratios?
We can. But it depends on many things. Such as the shape, size and the surface condition of sand.
If we consider the practical situation then what should be the water-cement ratio for plastering mortar?
Before discussing this issue, let’s discuss how we calculate the required water for plastering mortar.
How To Calculate Required Water For Plastering Mortar
We assume the required quantity of water is equal to 10% of the total weight of dry ingredients of mortar.
Required water = 0.10 × (weight of cement + weight of sand).
Suppose, We’ll prepare mortar with one bag of cement with the ratio of 1:4.
So, the required sand volume is,
=1.25 × 4 [1 bag cement = 1.25 cubic feet.]
= 5 cubic feet.
The weight of sand is,
= 5 × 45.31 [density of plastering sand is somewhere 1500-1800 kg/m3 , for my calculation purpose I assume 1600 kg/m3. 1600 kg/m3 is equal to 45.31 kg/ft3]
= 226.55 kilogram.
We know, the weight of 1 bag cement is 50 kilogram.
We now can calculate the required water.
The required water is,
= 0.10 × (50 + 226.55)
= 0.20 × 276.55
= 27.65 kilogram.
Now let’s see what is the water-cement ratio in this calculation.
Weight of water = 27.65 kilogram.
Weight of cement = 50 kilogram
Water cement ratio = 27.65 ÷ 50 = 0.55
What Should Be The Water-Cement Ratio For Plastering?
From our discussion, we found that the mortar strength is high with the water-cement ratio 0.6 to 0.7 and minimum water-cement ratio for making the mortar workable is 0.5.
From our calculation, we found that the water-cement ratio should be 0.55.
My suggestion is, when you prepare cement mortar for plastering, start with the water-cement ratio of 0.5. If required add more water to make the mortar workable.
As too dry and too wet mortar is difficult to work with, you need to be careful mixing water to the cement mortar.
If required, prepare some mortar on your construction project test basis and check how much water you actually need for the plastering mortar.