The cement densities that you are given for a job are very specific. The blends and densities requested are chosen carefully and are the result of a lot of laboratory testing.
For your cement to perform as expected, the batch that you field mix must be as close as possible to the slurry mixed in the laboratory tests.
The dry cement powder must be wetted evenly on its first pass through the mixing head or the unwetted powder particles will clump together.
If there is clumping in the slurry, even if it’s not visible to the eye, the slurry density has been compromised.
These clumps are tough to break up even if the slurry is recirculated through the mixer.
Air entrainment doesn’t overly affect the strength of the slurry blend, but it will reduce the accuracy of your density measurement.
Some air will always get trapped in the slurry as part of the mixing process, but the less, the better.
There's nothing good about raising a cloud of dust during mixing.
Dry blending occurs when the ratio of mix water to cement powder is too low. The result is an unusually thick and dry slurry that will not flow through the ports in the bottom of the diffuser and into the mixing tub.
The diffuser will quickly plug up and fill to the top. Cement dust and even slurry may be pouring from the diffuser air vent at this time. This is a considerable mess to clean up with cement powder wasted and valuable pumping time lost.
Dry blending can occur while building up density under manual control. The operator must be careful to start the mixing with an ample rate of mix water before introducing cement powder.
One of the easiest ways to guard against dry blending is to keep the cement metering valve at less than 50% open. This may limit the speed at which you can cement, but if you mix too fast, you won't be able to recover if you start to dry blend.
The mixing head, cement metering valve, and the diffuser work together to reduce powder clumps in the slurry, air bubbles and escaping dust.
Become a member to get immediate access to the rest of this lesson, and all the other great content on LunchBox Sessions.