Method and apparatus for producing lightweight concrete structure

A cementitious structural member has a high volume of encapsulated captive air and has an unusually high compressive and shear strength.

This invention relates to cementitious structural members and a method for 
manufacturing the same. 
More particularly, the invention relates to a cementitious structural 
member which has a high volume of encapsulated captive air but which has 
an unusually high compressive and shear strength. 
The advantages of providing a concrete block (or concrete wall or other 
concrete structural member) which includes a high proportion of air pores 
are well known in the art. The air pores increase the insulative 
properties of the block, reduce the weight of the block, and reduce the 
quantity of cement and sand required to make the block. However, concrete 
blocks which have a high proportion of air pores and have a density of 
less than about sixty to seventy pounds per cubic foot have been 
impractical because the blocks crumble or structurally are very weak. The 
weakness of such blocks is believed to be attributable to the fact that 
when a large number of air bubbles are produced in concrete, the surface 
tension of the bubbles "controls" the position of the concrete and 
segregates small portions of concrete in the interstices between adjoining 
bubbles. When the concrete is so segregated, the concrete in one 
interstice has little or no contact with the concrete in an adjacent 
interstice, i.e., an interconnected skeletal concrete structure is not 
formed, or, if it is formed, it is weak. 
One prior art concrete block attempts to solve the foregoing problem by 
uniformly dispersing alumina powder throughout a concrete--gypsum slurry. 
The alumina powder participates in a chemical reaction which forms gas 
bubbles in the slurry. When the gas bubbles form, the concrete-gypsum 
slurry is relatively viscous and impedes or prevents individual bubbles 
from coalescing to form large bubbles. The slurry then hardens, trapping 
the gas bubbles. The gas bubbles in the resulting concrete-gypsum blocks 
are each of a relatively uniform size. However, even though the 
concrete-gypsum blocks have a somewhat stronger connective concrete 
skeleton, the blocks typically in use are relatively heavy and do not 
produce a building structure having significant compressive and shear 
strengths. Another disadvantage of the concrete--gypsum blocks is that 
their manufacture and installation is labor intensive and costly. 
Accordingly, it would be highly desirable to provide an improved 
cementitious block which would include a high proportion of air pores and 
provide a high insulation or "R-factor", which would be comparatively 
inexpensive in manufacture, which would have a density in the range of 
five pounds per cubic foot to about fifty pounds per cubic foot, and which 
would, after being removed from a mold, facilitate curing of the block by 
distributing water over a large surface area inside the block. It would 
also be desirable to provide a method for manufacturing the improved 
block. 
Therefore, it is a principal object of the invention to provide an improved 
cementitious block and method for making the same. 
A further object of the invention is to provide an improved cementitious 
block which contains a high proportion of air pores but still can be 
employed in a wall structure system having compressive strength which 
greatly exceeds that of a conventional wood frame construction. 
Another object of the invention is to provide a lightweight, strong cement 
block which can be produced quickly and at low cost.

Briefly, in accordance with my invention, I provide an improved viscous 
moldable cementitious slurry for forming a cementitious structural member. 
The slurry includes water; a cementitious binder; and, a plurality of 
bubbles. Substantially all of the bubbles each have a width in the range 
of about 1/128 to 1/4 of an inch. The slurry has a viscosity sufficient 
generally to prevent the coalescing of the bubbles to form larger bubbles. 
In a further embodiment of the invention, I provide an improved viscous 
moldable cementitious slurry for forming a cementitious structural member. 
The slurry includes water; a cementitious binder; a plurality of small air 
bubbles, the slurry having a viscosity sufficient generally to prevent the 
coalescing of the bubbles to form larger bubbles; and, a chemical 
composition or energy source for reducing surface tension to facilitate 
the coalescing of the bubbles to form larger bubbles when the viscosity of 
the slurry is reduced to a selected level. 
In another embodiment of the invention, I provide an improved rigid 
lightweight cementitious structural member. The member includes a rigid 
cementitious structure; and, a network of different sized pores some of 
which are interconnected and each having a width in the range of about 
1/128 to 1/4 of an inch. The structural member has a density in the range 
of five pounds per cubic foot to fifty pounds per cubic foot and can 
include a smooth outer surface having small pores with a width no greater 
than 1/32 of an inch. 
In still a further embodiment of the invention, I provide an improved rigid 
lightweight cementitious structural member. The structural member 
comprises a rigid cementitious structure; a network of pores in the 
structure, at least some of said pores being interconnected and each 
having a width in the range of about 1/128 to 1/4 inch and shaped and 
dimensioned to draw water into the structure by capillary action; and, 
water in the pores. 
In yet another embodiment of the invention, I provide a improved process 
for producing a lightweight cementitious structural member. The process 
includes the steps of mixing together a cementitious binder and water and 
supplying air to form a slurry entrained with small air bubbles and having 
a viscosity which prevents the movement of the bubbles through said 
slurry; pouring the slurry into a mold to set up and form a lightweight 
rigid structural member, and, before the slurry hardens, decreasing the 
viscosity of the slurry to permit the bubbles to coalesce to form 
increased size air bubbles, and increasing the viscosity of the slurry to 
capture the increased size air bubbles and generally prevent coalescing of 
the increased size air bubbles. 
In yet still a further embodiment of the invention, I provide an improved 
process for producing a lightweight cementitious structural member. The 
process includes the steps of mixing together a cementitious binder and 
water and supplying air to form a slurry entrained with small air bubbles; 
pouring the slurry into a mold to set up and form a lightweight rigid 
structural member, and, before the slurry hardens, permitting the bubbles 
to coalesce to form increased size air bubbles, and increasing the 
viscosity of the slurry to capture the increased size air bubbles and 
generally prevent coalescing of the increased size air bubbles. 
In a further embodiment of the invention, I provide an improved process for 
producing a lightweight cementitious structural member. The process 
includes the steps of mixing together a cementitious binder and water and 
supplying air to form a slurry entrained with small air bubbles; heating a 
mold to a temperature sufficient to break air bubbles in said slurry 
adjacent mold to form a smooth slurry surface adjacent the mold; and, 
pouring the slurry into the heated mold. 
In another embodiment of the invention, I provide an improved process for 
producing a lightweight cementitious structural member. The process 
includes the steps of mixing together a cementitious binder and water and 
supplying air to form a slurry entrained with small air bubbles; pouring 
the slurry into said heated mold; and, decreasing the surface tension of 
the bubbles to facilitate the coalescing of the bubbles to form increased 
size air bubbles. 
In still a further embodiment of the invention, I provide an improved 
process for producing a lightweight cementitious structural member. The 
process includes the steps of mixing together a cementitious binder and 
water and supplying air to form a slurry entrained with small air bubbles; 
fusing the slurry to decrease the viscosity of said slurry and to coalesce 
the air bubbles to form increased size air bubbles; increasing the 
viscosity of the slurry to capture the increased size air bubbles; 
permitting the slurry to set to form a lightweight porous structure 
including a network of pores which draw liquid into said structure by 
capillary action; and, applying water to the structure such that the 
network of pores draws water into the structure by capillary action. 
Turning now to the drawings, which describe the invention for the purpose 
of explaining the use thereof and not by way of limitation of the scope of 
the invention, and in which like characters refer to corresponding 
elements throughout the several views, FIG. 1 is a block diagram 
illustrating the process for manufacturing a cementitious block or other 
cementitious structural member in accordance with the principles of the 
invention. In FIG. 1, cement 106 and water 101 are combined in mixer 100 
for a short period, presently typically about three to five seconds, after 
which sand 102, foam 103, and surfactant 104 are added and the resulting 
mixture is agitated for about thirty seconds. The foam 103 supplies air 
for the slurry. Foam 103 is produced by blowing or mixing air into a 
foaming agent. The bubbles in foam 103 presently preferably are quite 
small and the foam has an appearance similar to that of shaving cream. The 
foam bubbles typically have a width of less than about one sixty-fourth of 
inch. The bubbles presently are of uniform size, but can be of differing 
size. If desired, gypsum 108 can be added to mixer 100 with cement 106 to 
accelerate the setup of blocks produced using the slurry 111. Lime 109 can 
be added to increase the viscosity of the resulting slurry 111. 
Surfactants can, if desired, be mixed in foam 103 prior to adding foam 103 
to mixer 100. 
Water 101 is presently preferably heated to a temperature in the range of 
eighty to one hundred eighty degrees F. The foam 103 is presently 
preferably heated to a temperature in the range of eighty to one hundred 
eighty degrees F. The mold 112 and/or mixer 100 presently preferably are 
heated to a temperature in the range of eighty to about one hundred eighty 
degrees F. Additional heat can be added to mixer 100 by blowing heated air 
into the mixer or by any other desired means. As would be appreciated by 
those of skill in the art, the sand, foaming agent, surfactants, gypsum, 
lime, and/or heat 107 can, if desired, be omitted from the process 
illustrated in FIG. 1. If desired, the water, sand, and other components 
admixed in mixer 100 need not be heated. 
While the proportion of cement to sand by weight can be in the range of 
10:1 to 10:30, the ratio of cement to sand by weight typically is about 
1:1. 
After the water 101, sand 102, foam 103, surfactant 104, cement 106, and 
heat 107 have been admixed in mixer 100 for thirty seconds, the resulting 
heated slurry 100 is poured into a mold 112. The resulting slurry 100 is 
presently preferably fairly stiff, i.e., has a viscosity greater than the 
primary critical viscosity. As used herein, the primary critical viscosity 
is the viscosity of a cementitious slurry at ambient pressure at sea level 
(the slurry having a selected temperature and a selected composition and 
components) above which air bubbles in slurry 100 can not move through the 
slurry and coalesce to form in a desired time period larger bubbles 
substantially all of which are in a selected width range. When the bubbles 
can not move through the slurry, coalescing of bubbles is impeded. When 
bubbles coalesce, they join and unite. A pair of bubbles can touch without 
coalescing. For example, when a pair of spherical bubbles touch and each 
retain their original spherical shape, they have not coalesced. If a pair 
of bubbles bump and cause each other to change shape without breaching the 
outer wall of either bubble, they have not coalesced. If a pair of bubbles 
bump into each other and a portion of the spherical wall of each bubble 
collapses and the bubbles join in a figure eight type configuration (with 
the original spherical wall of each bubble intact except at the circular 
line along which the spherical walls meet and join), then the bubbles have 
coalesced. When a pair of rubber balls are pressed gently against one 
another, they take on a figure eight type configuration. As used herein, 
the secondary critical viscosity is the viscosity of a cementitious slurry 
at ambient pressure at sea level (the slurry having a selected temperature 
and a selected composition and components) above which air bubbles can not 
move through the slurry. The secondary critical viscosity is greater than 
the primary critical viscosity, i.e., a slurry with the secondary critical 
viscosity is stiffer than a slurry with the primary critical viscosity. 
Heat 113 is preferably, but not necessarily, applied to mold 112 to heat 
the mold to a temperature of from eighty degrees F to two hundred and ten 
degrees F. When mold 112 is heated, the heat in the mold breaks bubbles 
which are in the slurry adjacent the inner surfaces of the mold. This 
produces a relatively smooth, dense, continuous outer block surface which 
is adjacent the inner surfaces of the mold 112 and has small pores that 
presently are typically one thirty-second of an inch wide, or smaller. 
Heat from mold 112 hastens hardening of the slurry. Molds for producing 
blocks and other cementitious structures are well known in the art and 
will not be described herein. 
Chemical reactions taking place in mold 112 due to the presence of the 
cement 106 or other chemicals such as calcium chloride can produce heat. 
This heat, in combination with the heat provided by the heated mold 112, 
fuses slurry 111 soon after it is poured into mold 112, temporarily 
reducing the viscosity of slurry 111 to a point below the primary critical 
viscosity so that air bubbles in the slurry 111 can move, coalesce and 
form larger bubbles. The air bubbles in the slurry have a critical surface 
tension. The heat also functions in conjunction with surfactants to weaken 
surface tension and facilitate coalescing of bubbles. In the presently 
preferred embodiment of the invention, the bubbles in the foam 103 will 
not readily collapse and/or coalesce unless the slurry is heated to a 
temperature above 80 degrees, preferably to a temperature in the range of 
ninety degrees to one hundred and eighty degrees. It is anticipated, 
however, that bubbles in foam 103 could be collapsed without using heat 
and by using a appropriately strong surfactant, vibrations, ultrasound, 
and/or any other desired means. 
As used herein, the surface tension of a bubble in a cementitious slurry at 
ambient pressure at sea level (the slurry having a selected temperature, a 
selected composition and components, and having a certain viscosity below 
the primary critical viscosity) is the ability of the bubble to resist 
coalescing in the presence of a particular surfactant(s) or other chemical 
and/or in the presence of other means for causing a bubble to collapse or 
coalesce. As is well known, there are foaming agents which produce bubbles 
which are more resistant to collapse and coalescing either in the presence 
of a certain surfactants or when heat, vibration, ultrasound, or other 
means are applied to the cementitious slurry to cause bubble to collapse 
or coalesce with another bubble to form a new larger bubble. One foaming 
agent can produce bubbles more resistant to a particular surfactant than 
another foaming agent. After a foaming agent is selected, or after the 
means for introducing bubbles in a cementitious slurry is selected, then 
various surfactants or other bubble collapsing and coalescing means can be 
readily tested to determine if the desired width range of bubbles is 
obtained during the desired time period when the viscosity of the 
cementitious slurry is below the primary critical viscosity. As used 
herein, the critical surface tension of a bubble is the surface tension 
below which the bubble will coalesce with other bubbles in a cementitious 
slurry at ambient pressure at sea level (said slurry having a certain 
temperature, a selected composition and components, and having a certain 
viscosity below the primary critical viscosity) to produce increased size 
bubbles in a desired width range during a selected period (length) of time 
when the viscosity of the cementitious slurry is below the primary 
critical viscosity. 
When slurry 111 fuses and its viscosity is reduced below the primary 
critical viscosity, slurry 111 is also in the process of setting up and 
hardening. Accordingly, after temporarily decreasing, the viscosity of the 
slurry begins to increase and soon once again exceeds the primary critical 
viscosity. The viscosity of the slurry continues to increase until the 
slurry sets and forms a substantially rigid block. In the presently 
preferred embodiment of the invention, the viscosity of the slurry is 
below the primary critical viscosity for only about thirty to forty-five 
seconds. About four minutes after the slurry is poured into the mold, it 
has hardened and formed a "green" block which is sufficiently rigid to be 
removed from the mold. 
The "green" block is pushed upwardly out of the mold by a platform which 
comprises part of the bottom of the mold. Once this platform clears the 
top of the sides of the mold, the green block 114 is removed from the 
platform onto a conveyor or other block transport mechanism. As the block 
114 is transported away from the mold, it is further hydrated 115 by 
spraying the block with water or by dipping the block in a water bath. The 
pore network in the block 114 absorbs supplemental water into the block by 
capillary action. This supplemental water facilitates the slow curing of 
the block and provides adequate water to complete the hydration process. 
Water in the original block slurry tends to evaporate into air in the 
pores and into the ambient air, cannibalizing water from the cement and 
possibly preventing the cement from completely hydrating. The absorption 
into the block by capillary action of supplemental water helps to offset 
the loss of water by evaporation into air in block pores. 
The hydrated block 116 is stacked on a pallet, shrink wrapped or packaged 
117 with other hydrated blocks on the pallet, and is set out in the sun to 
cure 118. The blocks 116 can also be placed in an oven, autoclave, or 
other desired location (heated or not) to be cured. The autoclave includes 
water vapor in the air. Heat and water can be utilized to increase the 
speed at which concrete cures. 
The graph of FIG. 2 illustrates the relationship between the viscosity of 
the slurry 111 and the length of time the slurry is in the mold. The 
primary critical viscosity X is indicated in the graph of FIG. 2 by 
horizontal line 121. After slurry 111 is placed in mold 112 at time zero, 
the slurry 111 rapidly fuses due to the heat from the mold and due to the 
heat contained or being generated in slurry 111 by the components of the 
slurry 111. When slurry 111 fuses, the viscosity of the slurry 111 drops 
123 below the primary critical viscosity X for approximately thirty to 
forty-five seconds. While the viscosity of slurry 111 is below the primary 
critical viscosity, air bubbles in the slurry 111 coalesce to form bubbles 
substantially all of which have a width in the range of 1/128 of an inch 
to about one-half of an inch, preferably 1/64 to one-quarter of an inch. 
Once the viscosity of slurry 111 is no longer below line 121, the bubbles 
in the slurry 111 can no longer coalesce. The viscosity of slurry 111 can 
not be below line 121 for too long, otherwise unacceptably large bubbles 
form in the slurry 111. 
While it is presently preferred that slurry 111 be fairly stiff when poured 
into mold 112, it is possible that a rapidly setting slurry can be used 
which, when poured in mold 112, has a viscosity 122 which is less than the 
critical viscosity but is increasing in the manner shown in FIG. 2. In 
such a case, it is sometimes advisable to utilize a foaming agent which is 
more resistant to surfactants or other surface tension reduction means so 
that the bubbles in slurry 111 are more resistant to breakage and not all 
of the bubbles escape from the slurry 111 before the viscosity of slurry 
111 reaches the critical viscosity 121. One possible problem with this 
approach is that the heavier particles in the slurry tend to sink under 
gravity toward the bottom of the mold, producing a block which is stronger 
at the bottom than at the top. When the slurry 111 poured into a mold is 
stiff and prevents the migration of bubbles, it also tends to prevent sand 
and other components from being pulled toward the bottom of the mold under 
gravity. Similarly, if the slurry 111 produced by mixer 100 is fairly 
stiff and prevents the migration of bubbles, but begins to fuse prior to 
being poured into mold 112, then gravity can pull slurry components toward 
the bottom of the mold, again resulting in a block having a bottom which 
is heavier and stronger than the top of the block. 
As would be appreciated by those skilled in the art, the time that the 
viscosity of the slurry 111 is less than the primary critical viscosity 
can be adjusted as desired, as can the time required for a block to set 
after slurry 111 is poured in a mold. If, for example, it is desired to 
take eight minutes (or an hour, a day, etc.) instead of four minutes for 
the block to set, then the amount of heat added to slurry 111, mixer 100 
or mold 112 or block 114 can be decreased to slow the formation of bubbles 
or to slow the setting and hardening of the slurry 111. The quantities of 
air, water, cement, foaming agent, surfactant, heat, etc. and other 
components in slurry 111 can, as would be appreciated by those of skill in 
the art, be varied to control or alter the time it takes a block to set or 
to control or alter the proportion of air or bubbles in a cementitious 
block or other structural member which is manufactured in accordance with 
the principles of the invention. 
When bubbles in slurry 111 collapse and coalesce during the time the 
viscosity 123 is below the critical viscosity, substantially all of the 
resulting larger bubbles have sizes in the range of 1/128 of an inch up to 
about one half of an inch, preferably 1/64 of an inch up to one-quarter of 
an inch. The resulting larger bubbles includes many different bubble 
sizes, much like concrete aggregate often includes many different sizes of 
stones. As used herein with respect to bubbles or pores in a cementitious 
structural member, the term "substantially all" designates at least 75% of 
the total bubble or pore volume in a block or other cementitious 
structural member or in the slurry in a mold. The larger bubbles formed in 
the slurry are often asymmetrical. Presently, it appears that the larger 
the bubble, the more likely it will be asymmetrical. The network of 
different sized asymmetrical and symmetrical or nearly symmetrical bubbles 
which forms in the cementitious material permits the cement in the slurry 
to form an interconnected rigid network extending between and adjacent 
bubbles and through the block and which has, when the density of the block 
is about twenty-five pounds per cubic foot, a compressive strength of 
about 250 psi. The density of the block of the invention is typically in 
the range of about five pounds per cubic foot to seventy-five pounds per 
cubic foot, preferably ten pounds per cubic foot to fifty pounds per cubic 
foot. As would be appreciated by those of skill in the art, the 
compressive strength of the block can be varied by varying the amount of 
cement or sand, the volume of water, the volume of air, etc. in the block. 
The proportions of binder, sand, water, etc. which are intermixed to form a 
slurry in accordance with the invention can vary as desired. Presently, 
however, in general, the weight percentage of cementitious binder 
(concrete or other cementitious material) is in the range of 10% to 75%, 
of filler (sand or other granular mineral materials) is in the range of 
zero to 55%, of gypsum or another "accelerator" which reduces setup time 
is from zero to 10%, of lime or another component which increases the 
viscosity of the slurry is from zero to 15%, of an anionic surfactant 
(which reduces the amount of water which must be mixed with cement) is 0% 
to 10%, of a non-ionic surfactant is from 0% to 10%, of a foaming agent 
(which may include water) is 0% to 50%, of a hydrotrope (usually used with 
a non-ionic surfactant) is from 0% to 20%, of water is from 5% to 50%, and 
of fibers is from 0% to 20%. The fibers are added to strengthen and reduce 
the brittleness of the resulting block or other cementitious structural 
member. The fibers can comprise polypropylene strands, carbon fibers, or 
fiber comprised of any other desired material. Other strengthening 
materials can be added. The presently preferred foaming agent is a 
hydrolyzed protein or blood. Any desired foaming agent can be utilized. An 
example of an anionic surfactant is melamine formaldehyde polycondensate 
(hereafter called "melamine"). Melamine functions to decrease the amount 
of water needed to produce a cementitious slurry. When the amount of water 
is decreased, the slurry sets up faster. Sodium 2 ethylhexylsulfate 
(NIAPRUF 08 surfactant) manufactured by Niacet Corporation of Niagara 
Falls, N.Y. is a anionic surfactant which can be utilized. TRITON X100, a 
non-ionic surfactant manufactured by Union Carbide, can also be utilized. 
Anionic and non-ionic surfactants reduce surface tension and facilitate 
the coalescing of air bubbles into larger size bubbles. 
A hydrotrope, for example a phosphate ester, can be utilized to keep a 
non-ionic surfactant in solution in alkaline conditions. Many anionic and 
non-ionic surfactants are known in the art 
Sodium benzoate, salt, or any desired preservative can be utilized to 
extend the life of organic foaming agents by preventing the oxidation of 
such agents. 
By way of example, and not limitation, in one particular embodiment of the 
invention, a cementitious composition is added to and admixed in mixer 100 
for up to ten seconds (or more if desired), after which a foam composition 
is added and mixed with the cementitious composition for about thirty more 
seconds before the resulting slurry is poured into one or more molds. The 
cementitious composition includes cement (15 to 80% by weight), sand (0.1 
to 68% by weight), gypsum (0.1 to 5.0% by weight), lime (0.1 to 16% by 
weight), an anionic surfactant (0.1 to 2.0% by weight), fibers (0.1 to 15% 
by weight), and water (15 to 50% by weight). The foam composition includes 
cow's blood (0.5 to 10% by weight), a non-ionic surfactant (0.1 to 2% by 
weight), an anionic surfactant (0.1 to 2.0% by weight), a phosphate ester 
(0.1 to 2.0% by weight), sodium benzoate (0.1 to 2.0% by weight), and 
water (85 to 99% by weight). 
By way of further example, and not by way of limitation, in another 
particular embodiment of the invention, a foaming agent mixture is 
produced by mixing together one-half gallon of hydrolyzed blood powder, 
five gallons of water heated to ninety degrees F, ninety grams of 
non-ionic Triton X100 surfactant (Union Carbide), ninety grams of 
phosphate ester, and twenty grams of caustic soda (to put the pH of the 
mixture in the range of 7 to 12 so the blood will foam). Air is blown into 
the foaming agent mixture to produce a foam composition having many small 
(less than 1/32 of an inch wide) symmetrical bubbles. The foaming agent is 
heated to ninety degrees F. A slurry is prepared by mixing together for 
about five to ten seconds one hundred pounds of cement, one hundred pounds 
of sand, thirty ounces of Melment anionic surfactant (containing melamine) 
manufactured by W. R. Grace & Co., forty grams of polypropylene fiber, and 
twelve and a half gallons of water heated to ninety degrees F. The entire 
foam composition is mixed together with the slurry for about thirty 
seconds and is poured into ten block molds heated to one hundred and 
twenty degrees F. The molds are presently preferably, but not necessarily, 
heated to a temperature of from eight degrees F to one-hundred and eighty 
degrees F. After about four minutes, the slurry has set up and formed 
green blocks 10 which are removed from the molds, sprayed with water, and 
cured. 
An organic foaming agent like blood or hydrolyzed protein preferably has a 
pH of from 7 to 14 because it helps the agent form foam when intermixed or 
contacted with air. 
The pores in the cured block are often asymmetric, although some of the 
pores are symmetrical or nearly symmetrical. Although the width of 
substantially all of the pores varies widely from 1/128 of an inch to 
about 1/2 of an inch (preferably 1/64 to 1/4 of an inch), the volume of 
pores tends to be uniformly distributed throughout the block, i.e., each 
cubic inch of the block has about the same total volume of pore space, 
except near the outer surface of the block, which has a much lower volume 
of pore space because of the destruction of air bubbles by the heated mold 
surface. Although many of the pores in the blocks presently produced in 
accordance with the invention are interconnected, in another embodiment of 
the invention it is desirable to produce blocks in which a large majority 
of the pores are not interconnected. When the pores are not 
interconnected, the rate of evaporation of water from the block is 
reduced. 
As noted, a surfactant and heat are presently preferably utilized to reduce 
the surface tension of air bubbles in the slurry to facilitate the 
coalescing of the bubbles to form larger bubbles. Ultrasound, vibration, a 
surfactant(s) only, heat only, and any other desired means can be utilized 
in combination with or in place of the surfactant and heat to reduce 
surface tension or collapse bubbles to facilitate the formation of larger 
bubbles. 
As used herein, the width of a bubble is determined by calculating the 
diameter of a sphere having the volume of the bubble. The diameter of the 
sphere is the width of the bubble. Most bubbles in the finished 
cementitious structural members produced in accordance with the invention 
have a generally globular or elliptical shape. 
As earlier noted, air is presently preferably supplied in the slurry by 
mixing foam in the slurry. If desired, air can be supplied in the slurry 
by blowing air directly into the slurry to form bubbles; by mixing or 
beating the slurry to "fold" air into the slurry; by an in situ chemical 
reaction or another gas; or, by any other desired process. As utilized 
herein, the term air embodies any gas in the slurry which forms bubbles in 
the slurry. 
Blocks produced in accordance with the invention can have any shape and 
dimension, but the blocks presently preferably have hollows extending 
through the blocks such that when the blocks are stacked in rows one on 
top of the other, vertically oriented rebar can be extended through the 
hollows to footers and the hollows filled with concrete to form a 
vertically oriented concrete pillar extending through the hollows. The 
rebar and concrete pillar improve the compressive and shear strength of 
the stacked blocks. Further, the blocks preferably have a tongue and 
groove type configuration such that at least one tongue at one end of a 
block can be fit and seated in a groove in the end of an adjoining block. 
Further still, the blocks preferably have a tongue and groove type 
configuration on the top and bottom of the blocks such that tongues on the 
top of a first block can be fit and seated in grooves on the bottom of a 
second block stacked on the first block. Such a tongue and groove block 
configuration significantly strengthens a structure made with the blocks. 
One tongue and groove block design is illustrated in FIGS. 3 and 4 where 
block 10 includes tongue 14 and groove 17 which conform to and interfit 
with the groove 16 and tongue 15 on the end of a second identical block 
placed end-toend with block 10. Similarly, block 10 includes a lip 18 
which is generally in the shape of the number eight, which upstands from 
top 22, and which interfits with the recess 19 formed in the bottom 23 of 
a second indentical block placed on top of block 10. Block 10 includes 
parallel opposed sides 11, 24; parallel opposed ends 12, 13; and, openings 
or hollows 20 and 21 extending through block 10 from top to bottom. Each 
hollow 20 and 21 has a rectangular cross sectional area.