Portland Cement Shrinks
Many homeowners don’t realize that ordinary Portland cement shrinks when it dries. The shrinkage is very little, however it does shrink.
This means that if you patch a crack or a hole, a very tiny leak may form after the cement dries.
Hydraulic Cement EXPANDS
Hydraulic cements do the opposite. They expand as they cure.
The reason they expand is because the powders contain expansive clays such as bentonite and a few others.
Once you add water to these powders the clay starts to puff up ever so much. The best analogy is adding yeast to flour to make bread.
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Larger Size Locked In Place
When the bread is baked, the larger size is permanently locked into place.
The same thing happens with the hydraulic cement. And it happens fast usually within minutes. The Portland cement sets up and the larger volume of the mixture is locked in place permanently.
Cold Water Slows Reaction
If you need more working time when you are filling holes, use ice water to mix the hydraulic cement. This cold water slows somewhat the rapid setting time.
Where To Use
In new construction I use hydraulic cement to
- fill the void areas in foundations around water pipes
- electric wire sleeves or conduits
- foundation form holes
- deep cracks in foundations
- any hole in a foundation
To get the best bonding between the wet hydraulic cement, be sure to get rid of all dust from the surfaces.
Dampen Existing Concrete
You’ll also get the best bond if you slightly dampen the concrete that will be in contact with the fresh hydraulic cement.
Author’s Note: The following email comment was received from Art, a Cement Manufacturer’s Representative in Southern California.
I hope you take this in the spirit in which it is intended and that is to just help your site be better. Your definition of a hydraulic cement is misleading. According to ASTM (American Society for Testing & Materials), a hydraulic cement is a cement that creates a chemical reaction when it comes in contact with water (hydration) and will cure under water.
Being a hydraulic cement in itself has nothing to do with expansion, although some hydraulic cements will shrink less than others, but this is due to them being a sulfoaluminate based cement, not a Portland cement and has nothing to do with them being a hydraulic cement. Being a sulfoaluminate cement is also the reason you have the shortened working times.
Art / Cement Mfg. Rep.