CA1197273A

CA1197273A - Dry mix for hydration of plaster

Info

Publication number: CA1197273A
Application number: CA000425032A
Authority: CA
Inventors: Edmund J. Janicki
Original assignee: National Gypsum Co
Current assignee: National Gypsum Properties LLC
Priority date: 1982-08-26
Filing date: 1983-03-31
Publication date: 1985-11-26

Abstract

DRY MIX FOR HYDRATION OF PLASTER ABSTRACT OF THE DISCLOSURE An apparently dry mixture of hydrophobic pyrogenic silica and water, admixed with calcined gypsum in a ratio of water to calcined gypsum sufficient to hydrate the gypsum, with little if any excess water, avoiding need for energy, normally required, for drying the set gypsum product formed by the hydration.

Description

This invention relates to a novel plaster and water mixture and particularly to a method of adding water to calcined gypsum in an apparent dry form, providing thorough distribution of water throughout the mixture. The dry fo-rm of water is provided by mixing extremely fine droplets of wa~er with about 1 part, per lO0 parts water, of hydrophobic pyxogenic silica.
It is well known to add water to calcined gypsum to cause rehydration of the gypsum, producing a hardened set gypsum product, such as gypsum wallboard or plaster casts. The drying of gypsum wallboard and plaster casts is becoming rapidly more costly with increases in fuel costs, and a method by which drying can be avoided or minimized is highly desired.
A process is taught in U.S. Patent 4,328,178 in which water is combined with a porous inert filler, and this wet porous filler is then thoroughly mixed with what is referred to as commercial flower of gypsum, which is apparently hemihydrate9 CaS04-1/2H20. Enough water is present in the filler to combine with the hemi-hydrate and form dihydrate, CaS04 2H20, which is the set and hardened form of gypsum. The water to hemihydrate ratio in the examples ranges from .35:1 to .40:1, a ratio much higher than what is necessary to convert hemihydrate to dihydrate.
A fine powder which, although not porous, is capable of combining with 5 to 10 times its own weight of water and retain a dry powdory appearance~ is disclosed Z~3 in U.S. Patent 3,393,155. Th:;s powder has an average equivalent particle diameter below about 50 millimicrons, and has a surface which has been treated to introduce hydrophobic hydrocarbon groups thereon.
The present invention contemplates combining water with this very fine hydrophobic powder and then combining this water-containing apparently dry powder with hemihydrate in a substantially stoichiometric ratio, for water to hemihydrate, of about .14:1~ or possibly a slightly g~eater than stoichiometric ratio of about .15:1 to .20:1.
It is an object of the present invention to provide a method of combining water very uniforrnly with hemihydrate, in approximately stoichiometric quantities, and thereby producing set gypsum products which do not require drying.
EXAMPLE I

A preferred fine hydrophobic powder for use in the present invention is a hydrophobic pyrvgenic silica, as disclosed in U.S. Patent 4,328,178, and as sold by Tulco, Inc. under the trademark TULLANOX 500.
The pyrogenic silicas are produce~ for exarnple, by oxidation or hydrolysis type reactions of silicon cornpounds in a flame, as described in V7S. Pat. 2,990,249.
The pyrogenic silica is then made hydrophobic by reacting therewith various organosilanes, forming a surface treatment thereon. The organosilane may be a halo alkyl silane such as dimethyl dichlorosilane.
-- 2 ~

The hydrophobic pyrogenic silica is necessarily extremely fine9 and may have an average equivalent particle diameter below S0 millimicrons. Prior to the use of this hydrophobic pyrogenic silica in a gypsum composition, it is combined with several times its weight of water This combining of water with the fine hydrophobic pyrogenic silica powder requires apparatus for bre~k; ng the water up into very minute droplets. Any high shear emulsifying technique may be used. As the water is broken up into minute droplets, the droplets must be coated with the even finer particles of hydrophobic pyrogenic silica.
A typical water/pyrogenic silica emulsion will contain approximately % b~ Volume % by Weight Air 61 0.2 Water 34 99.1 Hydrophobic pyrogenic silica 5 0.7 The stability of the em~llsion is increased with an increase in the ratio of hydrophobic pyrogenic silica to water. The smaller the water droplets are, the more hydrophobic pyrogenic silica there will be bound to the water and the more stable will be the emuls-ion.
The percentage of water by weight in a stable emulsion may be varied from about 90% to about 99.2%.
Because this water in hydrophobic pyrogenic silica emulsion is seemingly a dry powder, it may be blended with calcined gypsum using dry blending techniques.

2~3 The water present iIl the emulsion is free uncombined water readily available for reaction with calcined gypsum. Only a stoichiometrically sufficient amount of emulsified water is required to satisfy the calcined gypsum and set the gypsumO A 5% to 10% excess may9 however, b~ desired to assure complete rehydration and humidification of the formed and set product.
Additi~es ~ay be used as requ;red for set control, density control, reinforcement or to~provide other desirable properties.
For stoichiometric hydration of calcined gypsum, an amount of emulsion containing from about 13.5 lbs to about 15 lbs water is combined with 100 lbs of calcined gypsum. This ratio will vary with variations-in the purity of the gypsum.
This stoichiometric hydration process may be employed in casting gypsum blocks or other shapes or in producing paper-faced gypsum wallboard. The products thus produced will not require drying prior to storage, shipment or ~se, and hence the invention results in con-siderable fuel savings.
Having completed a detailed disclosure of the preferred embodiments of my invention so that those skilled in the art may practice the same, I contemplate that variations may be made without departing from the essence of the invention or the scope of the appealed claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of combining water and calcined gypsum comprising the steps of, first, combining the water with an ultrafine hydrophobic dry powder in the form of very fine water droplets each surrounded by said ultrafine hydrophobic dry powder to form a seemingly dry emulsion powder, and, second, using a dry blending method to thoroughly blend said emulsion powder with calcined gypsum.

2. The method of claim 1 wherein said water in said emulsion powder is blended with a substantially stoichiometric amount of calcined gypsum to provide sub-stantially uniform set gypsum.

3. The method of claim 1 wherein said water in said emulsion powder is blended with calcined gypsum in an amount of water about 5% to 10% in excess of the stoichiometric amount to provide set gypsum with a slight excess of water to provide a substantially normally humidified set gypsum product.

4. The method of claim 1 wherein said dry powder is pyrogenic silica.

5. The method of claim 4 wherein said water is combined with said hydrophobic pyrogenic silica in a weight ratio of from about 90:10 to 99.2:0.8.

6. The method of claim 4 wherein said dry powder has an average equivalent particle diameter below 50 millimicrons.

7. The method of claim 6 wherein said water is combined with said dry powder in a weight ratio of from about 90:10 to 99.2:0.8.

8. The method of claim 7 wherein said dry powder and water combination is combined with calcined gypsum in a weight ratio to provide about 13.5 lbs to about 15 lbs of water per 100 lbs of calcined gypsum.