PUMPING OF PLASTIC AND GUN PLASTIC CEMENTS
Both plastic and gun plastic cements may be pumped through a plaster gun. Here is a description of the two types of cement and how either may be pumped.
Plastic cement was developed to provide a cement that is very workable (that is/ easy to spread on the wall, to retain water appreciably longer than regular cement in mixed plaster, to have superior curing characteristics and the other inherent advantages we find in a top quality plastic such as CALAVERAS PLASTIC.
Plastic cements are one type of portland cement that will vary considerably from one manufacturer to another in composition and handling qualities, according to the marketing philosophy of each particular company. Calaveras has produced a superior plastic by careful selection and control of all Ingredients. ALL of the materials in Calaveras Plastic comply with A.S.T.M. and/or Federal specifications.
Gun plastic cement differs from plastic cement only in one factor. That is, any cement manufacturer may add one to three pounds of short (7M grade) asbestos fiber per bag of plastic cement to convert his plastic to gun plastic cement. There isn't any other difference between the two types of cement. Many plasterers prefer to add asbestos fiber in their mixer on the job. When plasterers add asbestos, they have produced their own gun plastic. The reasons for adding asbestos fiber to plastic cement (in the cement plant or at the jobsite) to produce a gun plastic mix are as follows:
1. Asbestos fibers prevent segregation of sand from cement in the hopper of the plaster gun or in the hose that carries the plaster from the gun to the nozzle.
Segregation of sand is the cause of sand packs or stoppage of flow of mortar through the system and may be caused by:
a. The mix being too lean.
b. The sand being too coarse or poorly graded.
c. Too much mix water added in the mixer (which would wash cement paste off the particles of sand and cause the sand to drop and out pack the hose.)
d. Sometimes (BUT RARELY) too little mix water.
e. A piece of paper from the torn cement bag getting into the system and blocking the flow.
f. A nail or sliver of wood shoveled into the mixer with the sand and becoming wedged crosswise in the hose to start a sandblock.
g. A rock or lump of clay being shoveled into the mixer along with the sand.
h. A piece of foreign material such as gypsum board or other in the sand pile.
i. Loose hose connections, with resultant loss of cement paste and and water, and other causes.
2. Asbestos fibers lubricate the gun and hose as well as the grains of sand in the mix and facilitates the flow of material through the hose.
In order to have the proper amount of fiber in he mortar, a good rule of thumb is to add one loose handful (or the equivalent of a one-pound coffee can) of asbestos fiber per bag of Calaveras plastic in the mixer. In you are using a two-bag mixer, as should be done on a gun job, two handfuls of fiber should be added to the two bags of cement. If you prefer to soak the asbestos prior to use, add about one full cup of wet fiber per bag of cement. Soaked asbestos compacts and takes up less volume than dry fiber.
I suggest the following pattern of feeding the mixer. Pour a small amount of water in the mixer, add part of the sand, the necessary asbestos fiber, the plastic cement, then the balance of the sand. As always is the case in the plastering Industry, allow adequate mixing time to develop good entrainment of air, to get full benefit from the cement no matter what type of cement you are using. In gun application the work pattern generally forces the contractor to mix and dump batches fast. To give the air-entraining agent In plastic OR gun plastic cement time to beat in the amount of air that it has been designed to do, about four or five minutes mixing time is recommended. This time isn't always allotted. If the batch is dumped earlier, the contractor isn't getting his money's worth from the cement.
Using a given brand and type of cement, it's normal to use less sand per bag of cement when pumping through a gun than would be the case in applying that same cement by trowel. This is so because plaster guns simply won't handle as much sand as application with a hawk and trowel will permit. The maximum amount of sand per bag of cement that a gun will pump varies with the gradation of the sand, the mechanical condition of the plaster gun (an extremely important factor), the distance or height to which the plaster is to be pumped, the presence or lack of constricting bends in the hose (a good gun man keeps the hose as straight as possible and makes all necessary bends sweep In as large a curve as is practical), and the care with which the amount of mix water is controlled.
The crew always should clear the gun and hose with a sponge before starting the plaster pump for the day and at the end of the day.
Some contractors have benefited from running a small amount of asbestos fiber In water through the gun and hose prior to dumping the first batch of cement plaster.
During hot weather, a precaution that should be taken over the lunch period or any other extended period of downtime is to place the nozzle in a bucket of water. Sometimes that simple act prevents trouble.