We have been buying a portland cement additive that is supposed to densify and rapidly set concrete. We have been using it in lining large concrete water tanks and water tunnels. Now we find that this additive is a 20 percent solution of calcium chloride (1 1/2 pounds calcium chloride to 1 gallon of water). The manufacturer's recommended procedure is as follows: 1 part fresh portland cement, 1 1/2 parts clean sharp sand, and their solution diluted with 10 to 15 parts water. The concrete does set up much faster with the additive and our men like it because it makes the mortar slippery and works well on vertical surfaces. We have had no problems with the tanks lined this way, but are we getting a better, more dense and harder liner?
A mixture of 1 part portland cement to 11/2 parts clean, sharp sand will produce a dense, hard liner without an additive, if mixed with the minimum amount of water compatible with the required workability. Calcium chloride in the amounts of 2 pounds per bag of cement will accelerate hardening and not affect later strength. But curing is most important. If the workmen like the slipperiness of the mortar, and the finished job has proved successful, there seems to be no reason to discontinue its use.