The Mechanism of Water Reducers

In the last article, we discussed the mechanism of air-entraining concrete, now we introduce another admixture, water reducer. The water reducers can lower the water required to attain a given slump. In this case, they can be used to improve workability at same w/c ratio, increase strength at same workability, reduce cost at same w/c ratio and workability.

The water reducers can be divided into two groups: Normal or mid-range water reducers, and Superplasticizers which are high-range water reducers.

1.Nomal water reducers(mid-range 8%-12% reduction)

Now we are looking at the charged properties of water reducer molecules and cement particles:

The water reducer molecule with 1 +VE and 2 -VE (you can also use water reducer whose molecule has 2 +VE ad 1 -VE, the effect is the same as a matter of fact) will combine with the positive and negative charges on the surface of the cement particles. So the extra - VE will be left on the surface of the cement particles, making the surface of the cement particles negatively charged so that the cement particles can repel each other.

If the cement particles repel each other, the repulsion of like charges will push the cement grains apart so that the water trapped in the particles could release, increasing hydration and workability.(It is easy to understand, The release of trapped water means there’s more water available for hydration.)

2.Superplasticizers(high-range 12-40% reduction)

These kind of water reducers are more recent and more effective type of water reducer. It can make 12-40% reduction compared to 8-12% caused by normal water reducers. So it can be applied to produce flowing concrete in situations where placing in inaccessible locations (too many steel bars in a small area) or where very rapid placing is required. It is also used to produce very-high strength concrete, using normal workability but a very low w/c. Just regard it as a very effective replacement of water, then all of the properties make sense.

So how do superplasticizers work? They work the same way as water reducers, separating cement particles from each other to release water. But in addition to providing electrostatic repulsion, it can also separate particles by steric hindrance effect. On the sulfonated polymer and copolymer superplasticizers, for example, the electrostatic repulsion comes from the negatively charged side sulfonic group and side carboxyl group, both of which are attached on the main polymer chain. On the other hand, the steric hindrance effect comes from the neutral side long graft chain which is also attached on the main polymer chain. That`s the mechanics of superplasticizers.

As we can see from the figure above, the steric hindrance caused by the long graft chain may be stronger than the electrostatic repulsion as it is a kind of physical blockage.

Now let us talk about the properties of SP.

This figure shows the slope of Slump-Time curve for repeated re-dosage is getting higher ang higher, it means that the water reducing effect of SP becomes worse and worse with the number of times the water reducing agent is added. As for why we add more than once, it can be seen from the figure that the workability of cement decreases a lot only after the first 1h addition.

So, we can get the side effects:

We also care about the compatibility, what will influence the properties of SP? C3A content could effect the cement strength, it could also reduce the effectiveness of a given dosage or the SP as the C3A has highly reactive affinity to SP. Besides, the fineness also make a difference, the finer the cement, the more the surface area would need to water to react with, and the higher the dosage of a SP required to obtain a given workability.

What`s more, SP also has a saturated point. According to the figure as follow, we find that the flowability of cement will decrease with time and then reach a plateau. We al find that SP could make a role even is added after 1 hour.

The related calculation of SP

SP can be regarded as a mixture of water and solid, both of which would vary with the type of SP.
\[
Mass\ of\ solid = s×G_{sup}×V_{liq}
\\ Where:\ G_{sup} = specific gravity = \frac{\rho (kg/m^3)}{1000(kg/m^3)}
\\ V_{liq} = volume\ of\ liquid(L)
\]

Determine the solid content of 8 litres of melamine SP(Sp.gr. of 1.10 and solid content 22%)
$$
Solid\ content = 0.22 × 1.1 × 6 = 1.45 kg\ of\ solids
$$

\[
M_w = V_{liq}×s×G_{sup}/100×\frac{100-s}{s}
\\ V_w = V_{liq}×G_{sup}×\frac{100-s}{s}
\]

Determine the volume of water added from 8.25 litres of SP per cu.m of concrete with the specific gravity of 1.21 and a solid content of 40% to achieve a desired slump.
$$
V_w = 8.25×1.21×\frac{100-40}{100} = 6.0L/m^3
$$