Process for producing concrete structures of high strength

The strength of portland cement concrete structures which incorporate alkali metal or alkaline earth polysulfides is increased by heating the structures at temperatures above 110.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This application relates to a process for producing portland cement 
concrete of increased strength and to the structures produced by the 
process. 
Portland cement concrete has become the world's largest bulk building 
material. The concrete comprises an admixture of portland cement with sand 
and aggregate (gravel or crushed stone). It is generally formed by mixing 
portland cement, water, sand and gravel or crushed stone, and allowing the 
mixture to harden to form a dense structure. The character of the "paste" 
mixture of water and cement is generally held to determine the important 
engineering properties of the concrete. 
Construction concrete is generally formulated to contain from about 5 to 30 
weight percent cement based upon the dry weight of materials. Generally, 
higher cement content (in the upper range of the usual content) produces 
concrete of higher strength. 
Recently, shortages of cement in many parts of the world make particularly 
attractive the prospects of supplying concrete of low cement content which 
has high strength. 
2. Description of the Prior Art 
Polymer cements and polymer-impregnated concretes are known. The former 
usually contain a latex such as polyvinyl chloride, polyvinyl acetate, 
acrylonitrite-butadiene-styrene, etc., or an acrylic in solid or liquid 
form. 
U.S. Pat. No. 2,820,713 discloses mortar compositions containing minor 
amounts of methyl cellulose and alkaline earth polysulfides and/or bunte 
salts as adhesion improvers. 
U.S. Pat. Nos. 2,962,467 and 3,198,644 disclose similar mortar 
compositions. 
"Characteristics of Portland Cement from the GypsumSulfuric Acid Process" 
by Fredrich Wolfe and Joachim Hille (Silikattechnik, 1967, 1812), pages 
55-57, discusses the effect on certain portland cements by the addition of 
calcium sulfide (Ca.sub.2 S). 
SUMMARY OF THE INVENTION 
It has now been found that polysulfide-containing portland cement concrete 
structures of improved strength can be produced by heating the structures 
at temperatures above about 110.degree. C. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A companion application Pending case Ser. No. 938,668, discloses and claims 
portland concrete compositions containing low cement contents, i.e., from 
about 6 to 15% by weight cement relative to aggregate. The compositions 
contain from about 1.5 to 6.0 weight percent of alkali metal or alkaline 
earth metal polysulfide, preferably calcium polysulfide. 
The concrete article containing the polysulfide will be heated at a 
temperature in the range of 110.degree. to 180.degree. C., preferably 
115.degree. to 160.degree. C., for a time of at least 0.1 hour, preferably 
0.25 to 24 hours, and more preferably from about 0.5 to 4 hours. 
The concrete composition will contain aggregate, and based on the aggregate 
from about 6 to 25 weight percent portland cement and from about 1.5 to 20 
weight percent alkali metal or alkaline earth metal polysulfide. 
The alkali metal or alkaline earth metal polysulfide, preferably calcium 
polysulfide, is blended with the cement and aggregate at the time of 
mixing. The calcium polysulfides are articles of commerce and are 
conventionally prepared by the reaction of sulfur with lime water. Other 
polysulfides such as barium polysulfide are produced in a similar manner. 
Their preparation is described on page 63 of "A Text-Book of Organic 
Chemistry", Vol. III, Part I, edited by J. Newton Friend, Charles Griffin 
and Co., London, 1925. For agricultural purposes, they are conventionally 
sold as concentrated aqueous solutions, e.g, about 30 weight percent 
CaS.sub.x. 
Thus, in preparing the concrete, substitution of a calcium polysulfide 
(CaS.sub.x) solution containing the appropriate amount of water for the 
desired water/cement ratio in the concrete will be the most suitable 
method. Otherwise, preparation of the concrete mix is by conventional 
means. There are several methods for determining proper water/cement 
ratios, etc. These are described in "Portland Cement and Asphalt 
Concretes", Thomas D. Larson, McGraw-Hill Book Co., Inc. (New York, 1963), 
pages 85-99. 
The ratio of cement to polysulfide in the composition will preferably be in 
the range of about 4:1 to about 2:1. 
The aggregates used in preparing the concrete are preferably those defined 
in ANSI/ASTM-C-33-77 for fine and coarse aggregates. The ratio of 
coarse-to-fine aggregate will range from about 1:2 to 7:1, preferably 1:1 
to 4:1. 
The concrete article may be heated soon after initial set, but it is 
preferred that a normal curing period of at least 7, and preferably at 
least 20, days be employed before heating. 
Heating of the structure may be accomplished by any suitable means. Large 
articles thus may be heated by the use of radiant heat, or by direct 
application of flame to the surface, etc. Smaller articles, such as 
building blocks, stepping stones, pipe sections, etc., can be conveniently 
heated in ovens. 
The process by which the concrete articles are strengthened is not exactly 
understood, as cross-linking of polysulfides is not believed to occur, so 
some interaction with the portland cement or change in the sulfur 
structure is probably involved. 
Strength development is complete after heating and cooling, assuming that a 
sufficient curing period is provided before the heating treatment. 
In the case of large structures, where heating of the entire bulk is 
difficult or impossible, application of heat to the exterior will result 
in heating and consequent strengthening of the heated layer. This can be 
quite beneficial in many applications where stress is placed primarily on 
the outer parts of the structures.

EXAMPLES 
The following examples illustrate the invention. The examples are 
illustrative only and are non-limiting. 
EXAMPLE 1 
A mix was prepared with a commercially available "ready mix" concrete, 
containing about 3.2% calcium polysulfide relative to aggregate. The ready 
mix contained about 10.1% by weight Type I portland cement (based on 
aggregate). The ready mix was screened, and the following proportions of 
materials were obtained. The cement, of course, was included in the 
screening, accounting for the large amount of material passing 100 mesh. 
TABLE I 
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Screen Analysis of Ready Mix Concrete 
Mesh Size Wt. % Retained 
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1/4" 17.8 
10 14.7 
20 24.4 
40 15.1 
80 11.3 
100 0.1 
Passing 100 16.5 
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The mixes were prepared by mixing 91% weight ready mix with 9% of an 
aqueous 29% CaS.sub.x solution. 
The concrete was molded into 2-inch (5.1 cm) cubes. The cubes were 
wet-cured for one week at room temperature. Three of the cubes were placed 
in an oven and heated for two hours at 250.degree. F. (121.degree. C.). 
The cubes were removed from the oven and allowed to cool overnight. The 
three heated cubes and three unheated cubes were broken on a Baldwin press 
at a crushing rate of 0.5 in (1.27 cm) per minute, giving an average 
compressive strength in psi of 3366 for the headed samples and 2050 for 
the unheated samples. Unheated samples with this quantity of polysulfide 
develop compressive strengths of about 2700 psi after 28 days wet cure. 
These data show that a significant improvement in strength can be achieved 
by the process of this invention relative to the unheated 
polysulfide-containing compositions. 
Although many specific embodiments of the invention have been described in 
detail, it should be understood that the invention is to be given the 
broadest possible interpretation within the terms of the following claims.