Method for the controlled hardening of acid-setting binders and cements

The present inventions relates to a method for the controlled hardening of cid-setting cements and binders by first providing an acid-setting binder within a selected mold. Mixing with the binder an electrolyte to form a paste or slurry within the mold. Subjecting the paste or slurry mixture to electrolysis for a selected period of time or until the binder is sufficiently hardened whereby a hardened article is produced. Finally, removing the produced article from the mold.

FIELD OF THE INVENTION 
The present invention relates to binders and cements which require the 
addition of an acid component to permit hardening of the binder and more 
particularly to a method for the controlled setting of such binders and 
cements. 
BACKGROUND OF THE INVENTION 
Binders or cements which require acidic conditions to harden are well known 
in the prior art. For example, a zinc phosphate hydrate cement is produced 
by combining a metal oxide zinc powder with an acid such as phosphoric 
acid. When these two components are intermixed, rapid setting occurs and a 
high quality zinc hydrate cement will be formed. 
Other acid-setting binders exist in the art and many have found wide use in 
a variety of industries. Acid-setting binders are used as dental cements, 
molding compositions when combined with fillers and reinforcing materials 
and as grouts to solidify loose soil, gravel or other building material. 
Although these prior art cements have found wide acceptance, they possess 
certain drawbacks in practical application. Conventional methods require 
mixing of the cement components prior to placement within a suitable mold. 
As a result, the intermixed components tend to "set" prior to completion 
of the molding process. This premature hardening of the cement often 
results in a flawed product or clogged and damaged equipment. In addition, 
the rapid setting characteristic of the cement requires that all the 
mixing equipment be fully and completely cleaned immediately after 
transfer of the cement to the mold. 
If an acid-setting cement or binder is used to produce a hollow article, 
conventional casting techniques require a mold which can be tumbled, 
rotated or rocked to allow a layer of unreacted cement to be uniformly 
distributed within the mold's interior. Consequently, loading of the mixed 
cement components within such casting equipment requires great care in 
order to ensure that none of the cement will set prior to complete coating 
of the interior of the mold. 
Yet another disadvantage of acid-setting binders and cements is the need to 
prepare in advance the strong acid for addition to the metal oxide powder. 
Handling and preparation of such acids can be hazardous and requires great 
care on the part of the user. Further, articles produced with conventional 
acid-setting binders or cements are restricted by the type and number of 
filler materials which can be employed. Time constraints in terms of 
mixing and the need to avoid premature setting often make the inclusion of 
fillers within a finished product impractical or will yield a product of 
inferior quality. 
Even though acid-setting cements and binders have found wide acceptance 
within the construction and molding industries, the uncontrolled speed at 
which they set has restricted their expansion into other technologies. A 
need has therefor existed within the art for acid-setting cements or 
binders which can be controllably hardened and in so doing avoid the 
aforementioned problems. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
for the controlled hardening of an acid-setting cement or binder thereby 
preventing premature setting, flaws within the item being cast and clogged 
or damaged mixing equipment. 
Another object of the present invention is to provide a method for setting 
a binder whereby an acid reactant or catalyst is produced in situ thereby 
eliminating the need for the preparation of a separate acid solution for 
mixing with the metal oxide cement. 
A still further object of the present invention is to provide a method to 
control the hardening of an acid-setting cement whereby a hollow item can 
be cast without the need for equipment which spins or otherwise rotates. 
Yet another object of the present invention is to provide a method for 
producing a cast item having portions which are selectively hardened to 
yield a uniquely shaped item without the need for expensive and 
complicated casting equipment. 
Still another object of the present invention is to provide a method for 
the controlled hardening of an acid-setting cement whereby the mold itself 
is an operating electrochemical cell. 
A still further object of the present invention is to provide a method for 
hardening acid-setting cements and binders whereby the molded end product 
contains various aggregates and fillers. 
Yet another object of the present invention is to provide a method for the 
controlled hardening of an acid setting cement whereby the molded end 
product contains no flaws and meets the high tolerance requirement of 
precision casting techniques. 
A still further object of the present invention is to provide a method for 
the controlled hardening of acid-setting cements whereby the components 
for producing the acid-setting cement are uniformly mixed prior to 
hardening so as to provide a high quality end product. 
Another object of the present invention is to provide a method for the 
controlled hardening of acid-setting cements regardless of ambient 
temperature. 
These and other objects are achieved by providing a method for the 
controlled hardening of acid-setting cements comprising the steps of first 
providing an acid-setting binder within a selected mold. Mixing with the 
binder an electrolyte to form a slurry or paste. Subjecting the mixture to 
electrolysis for a selective period of time or until the binder is 
sufficiently hardened whereby a hardened article is produced. Removing the 
produced article from the mold. 
The present invention also relates to a method for the controlled hardening 
and strengthening of loose or granular soil or gravel into a cohesive mass 
and comprising the steps of first providing an acid-setting grout or 
binder. Mixing with the grout or binder an electrolyte solution to form a 
slurry or paste. Injecting the slurry or paste into the granular soil or 
gravel to completely saturate the soil or gravel and subjecting the 
injected mixture to electrolysis for a selective period or time or until 
the grout or binder is sufficiently hardened whereby a cohesive, hardened 
mass is produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the preferred embodiment of the present invention the method 
for controlling the hardening of an acid-setting cement comprises the 
following steps: 
1. An acid-setting binder such as powdered zinc oxide is place within a 
selected mold or mixing container. 
2. The powered zinc oxide is intermixed with an alkali salt solution. For 
example, a sodium phosphate salt solution is added to the powered zinc 
oxide to create a uniformly distributed slurry or thick paste. 
3. The mixture of acid-setting binder and electrolyte is then placed within 
a mold and subjected to electrolysis for a selected period of time or 
until the binder is sufficiently hardened. The electrolysis may be brought 
about by simply placing a conventional anode and cathode within the slurry 
mixture and providing an electrical current therebetween to complete a 
circuit. 
4. After sufficient hardening, the molded article is removed from the mold 
and the mold is ready for the next operation. 
Applicant's have discovered that the electrochemical reaction which takes 
place within the slurry or paste yields an oxidation reaction at the anode 
causing the in situ production of an acid. This acid will then react with 
the intermixed metal oxide powder to yield the end product cement. 
More specifically, the powered zinc oxide and the sodium phosphate salt 
solution react in a two step reaction process by the (1) initial 
production of a phosphoric acid followed by (2) reaction of the phosphoric 
acid with the zinc oxide to yield the zinc phosphate cement product. This 
two step reaction process is given below: 
##STR1## 
A wide variety of acid-setting cements are contemplated within the scope of 
the present invention. For example, calcium hydroxide powder may be 
slurried with a sodium sulfate electrolyte solution. If subjected to 
electrolysis, a sulfuric acid is produced which then reacts with the 
calcium hydroxide to form a solid calcium sulfate hydrate cement also 
known as gypsum. Other "acid-setting" cements and binders are contemplated 
as being within the scope of the present invention. The primary 
requirement of the acid-setting binder according to the present invention 
is that it react with an acid produced during the electrochemical 
reaction. 
The present invention encompasses a variety of electrolytes for mixture 
with the selected binder. Generally speaking, any of the alkali metal 
salts are contemplated as being within the scope of the present invention. 
Sodium or potassium phosphate salts should work well with the zinc or 
calcium oxide powders. Applicant's have also found that alkali sulphate 
salts will produce upon electrolysis sulphuric acid that will set calcium 
hydroxide forming calcium sulphate hydrate. The present invention is not 
limited to any particular electrolyte and includes all electrolytes which 
upon electrolysis produce acids at the anode of the cell and in sufficient 
quantities to react with and cause the cement to set. 
Regardless of the acid-setting binder selected, in all cases the 
electrolysis reaction is the primary means to control the pH of the 
mixture and thereby produce an acid which will then set the binder. In the 
most general terms, electrochemical reactions are those chemical changes 
associated with the passage of electrons across the interface between a 
conductor and solution. Thus, an electrochemical cell usually contains a 
pair of metallic or carbon electrodes which are immersed within an 
electrolyte solution. When an electrical current is established between 
the electrodes, positively charged ions move towards the cathode electrode 
while negatively charged ions move toward the anode electrode. The 
electric current is carried through the electrolyte by this migration of 
ions and serves to cause chemical change. More importantly for the present 
invention, there are produced at the positive electrode or anode acidic 
compounds as well as liberated oxygen gas. 
Although an associated rise in temperature results from the electrochemical 
reaction, applicant's have found that the hardening of the acid-setting 
binder is due primarily to the generation of acid at the anode and not to 
incidental drying from the heat of the chemical reaction. Applicant's find 
it useful to routinely add electrolyte solution to the mixture during the 
electrochemical reaction therby ensuring a constant supply of generated 
acid as well as to keep the electrodes immersed within the electrolyte 
solution. In general, high concentrations of electrolyte solutions are 
preferred in order to provide maximum acid production at the anode. 
The rate at which the current is applied is not critical since it is the 
total current applied which determines the amount of acid generated. 
Applicant's have found that both metal and non-metal (carbon) electrodes 
work equally well so long as the selected electrode is relatively inert 
and does not react at a speed sufficient to change the composition of the 
binder-electrolyte mixture. 
Applicant's have further discovered that by raising the voltage during the 
electrolytic reaction, an increase in the rate of acid production is 
observed thereby increasing the rate of hardening of the metal oxide 
cement. Conversely, a lowering of the voltage during the electrolytic 
reaction tends to decrease the production of acid with an associated 
lowering of the rate of hardening of the metal oxide cement. As a result, 
the rate of hardening of the cement according to the present invention can 
be selectively controlled depending upon the particular application of the 
cement. 
A variety of aggregates or fillers may be added to the metal oxide and 
electrolyte paste or slurry prior to or during electrolysis. Preferred 
aggregates and fillers include sand, clay, ground ceramics and glasses. 
The flexural or tensile strength of the metal oxide and electrolyte slurry 
or paste can be increased by the addition of fiber, woven fabrics or other 
material which is not readily subject to attack by the acid. Preferred 
fibers are generally spun or extruded glasses or silicas or synthetic 
organic fibers and plastics. The present invention is not limited to a 
particular group of aggregates and fillers but is intended to encompass 
the variety of such additives as is generally known in the art. 
It is within the scope of the present invention to provide a mold for use 
with a acid-setting binder which permits the molding of hollow items from 
a slurry or paste without the prior art necessity for associated rocking 
or spinning means. For example, a metal or carbon anode can be made part 
of or integral with the exterior mold surface while the cathode is 
positioned within the interior portion of the mold. When a current is 
applied, the slurry or paste within the mold will selectively harden along 
the interior (anode) surface of the mold while slurry or paste adjacent 
the cathode will remain soft. The resultant article is a hardened shell 
reflecting the shape of the anode and containing unreacted paste. The 
unreacted slurry or paste can then be recovered for use in manufacturing 
additional articles. 
In yet another embodiment according to the present invention the anode is 
selectively moved within the mold to a variety of locations by robotics or 
other means followed by the application of current. As a result, a product 
can be tailor-made to contain localized portions of hardening which would 
be otherwise unavailable. 
It is additionally within the scope of the present invention to provide a 
method for controllably hardening and strengthening of loose and granular 
material such as soil or gravel into a cohesive mass. In this embodiment, 
the acid-setting binder and electrolyte slurry are uniformly pumped or 
otherwise injected and intermixed with the granular material. After 
positioning electrodes within the mass, a current is applied and the 
mixture is subjected to electrolysis. In this way, premature setting is 
avoided and the acid-setting binder and electrolyte are completely filled 
within the intergranular spaces of the soil or gravel prior to hardening. 
In addition, the electrodes can serve after hardening as reinforcing or 
attachment points. 
In order to demonstrate the method according to the instant invention, the 
following examples are offered. It should be appreciated that these are 
merely examples to show the utility and effectiveness of some of the 
embodiments of the recent invention. Inclusion of these examples should 
not be interpreted in any manner as limiting to the scope of the present 
invention to the specific conditions set forth in the examples. 
EXAMPLE I 
A test electrolytic cell was set up in a glass beaker by adding 30 grams of 
monobasic sodium phosphate hydrate to 100 ml of distilled deionized water 
to form 2 molar solution. The initial pH of the solution was approximately 
4.5. A volume of 200 ml of zinc oxide powder was mixed into the solution 
to form a thick paste. Two stainless steel rods (8 mm in diameter) were 
partly immersed in the paste to form an electrolytic cell. The electrodes 
were held suspended in the solution with a plastic clamp that provided an 
electrode separation of 5 cm. A DC current of 1 ampere was set and allowed 
to run through the cell for two hours. The pH at the anode dropped to 1.0. 
Gases bubbled from both the anode and cathode indicating electrolysis was 
occuring. The temperature in the cell rose from an initial temperature of 
25.degree. C. to a final temperature of 45.degree. C. At the end of the 
test run (2 hours), the current was shut off and the electrodes were 
removed from the paste. The zinc oxide paste at the cathode was soft and 
the hole which had formed within the paste around the electrode collapsed 
upon itself. The paste at the anode was hardened and the adjacent hole 
remained open when the electrode was removed. The cell was permitted to 
cool for one hour. The hardened paste at the anode was removed by washing 
out the soft paste in the cell and separating the hardened portion around 
the anode. The sample of the hardened paste was examined using standard 
x-ray diffraction techniques. The hardened paste was found to consist of a 
mixture of zinc oxide and zinc phosphate hydrate cement (a synthetic 
hopeite). 
EXAMPLE II 
In a second experimental run, an identical test cell to that given in 
Example I was assembled but 8 mm diameter carbon rods were substituted for 
the stainless steel electrodes. A DC current of 0.5 amperes was passed 
through the cell for two hours. A cemented cylinder or hole formed around 
the anode similar to that observed in Example I. Examination of the 
cemented material using standard x-ray diffraction demonstrated that the 
cemented product produced was zinc phosphate hydrate cement. 
EXAMPLE III 
Approximately 100 ml of a saturated solution of sodium sulphate decahydrate 
(67% solution) was mixed with a 200 ml volume of finely divided calcium 
hydroxide to form approximately 150 ml of a stiff paste. Two 8 mm diameter 
carbon rods were inserted in the paste to act as electrodes. The 
electrodes were maintained at a separation distance of 5 cm. A current of 
1 ampere was passed through the paste for ten minutes. The initial pH of 
the paste was 12.4. After two minutes, the pH in the vicinity of the anode 
had dropped to 2.0. Gases bubbled from both the anode and the cathode 
indicating that electrolysis was occurring. After ten minutes of operation 
the temperature of the paste had risen from the starting temperature of 
25.degree. C. to 46.degree. C. Hardened paste was observed in the volume 
surrounding the anode. The current was shut off and the cell was allowed 
to cool for approximately 35 minutes. The hardened paste that formed at 
the anode was recovered by washing out the soft paste in the reaction 
cell. The hardened paste was examined using standard x-ray diffraction 
techniques and was found to be composed of calcium sulphate dihydrate 
(gypsum) along with some unreacted calcium hydroxide. 
While this invention has been described as having a preferred design, it is 
understood that it is capable of further modifications, uses and/or 
adaptations of the invention following in general the principle of the 
invention and including such departures from the present disclosure as 
come within the known or customary practice in the art to which to 
invention pertains and as may be applied to the central features 
hereinbefore set forth, and fall within the scope of the invention and of 
the limits of the appended claims.