Intermolecular Attraction Cement
Country: South Africa
Date: Winter 2012-2013
I am in the concrete brick industry. We actually manufacture brick-making machinery. Now having left academia for this real-world operation, I had worked in some experimental data testing over my years here. I have become fairly familiar with cement in its resultant state in brick technology; but I wonder how much more room for improvement there can be.
To cut to the question, I find that much cement is wasted on the void space when compressing bricks. With more compression, more particles are densely packed in less volume so more cement "bridges" can be formed, but still there will be wastage of cement coating the aggregate particles where not needed.
Is there a way to increase the intermolecular attraction of a cement mixture so that it gathers more at the points of contact?
My simplified example is to take 3 ping pong balls and (x) amount of glue. It would surely be stronger to apply the glue at the points of contact rather than to dip the whole ping pong ball in glue and then stick. With the same amount of glue, it would be thinly coated. But if there were a way to have the glue coalesce a little bit to bunch up at the points of contact - that would be excellent! Certainly at ping pong ball size this is too much to ask, but with a less viscous material like cement particles around 4-6mm (0.2") stone - maybe something can be done.
Thanks for the question. I am writing this from my general knowledge--I have not researched this area thoroughly. Yes, there are adhesives that are added to increase the "sticking" power of concrete to surfaces. One of the areas that has been investigated has been additives to make concrete adhere better to steel and reinforcement bars (rebar).
The main concern I have with not filling the void volumes (as mentioned in the ping-pong example below) is that weathering of the concrete will be accelerated. The void volumes provide an ample space for water to fill and then to undergo freeze/thaw cycles.
From informal conversations with people who have taken construction classes, concrete is over-ordered 10% to make up for waste. So, if there is a way to cut this over-ordering, then that would be a good place to start.
I hope this helps. Please let me know if you have any more questions.
I know of no practical way to achieve what you are suggesting, but
there will be some serious disadvantages of doing this, even if it is
In a traditional brick block, filling all voids between aggregate particles
helps to spread the compressive load and reduce overall compressive
stress. If the cement were only present where aggregate particles
touch, compressive forces would be localized to just where the
aggregate particles touch, thus increasing local stress.
Further, making a brick block as you suggest will result in a porous
brick that would not block water entry into (for example) a basement
area. Another more serious problem would be that any water that was
absorbed into the voids between particles, will expand when frozen
and possibly break the bonds between aggregate particles, thus
seriously weakening the brick.
Well, I hope one of my responses fell into the category of “quite eloquent” but I have seldom heard them described as such. I know very little about bricks and cement so you have to excuse my ignorance. There are possibly some similarities between cement and paper coatings. In both cases, the objective is to maximize the concentration of particulate matter, and minimize the amount of water consistent with the requirement that the mixture flow. In paper coatings the solid component is often a suspension of “sand” SiO2 and calcium carbonate (CaCO3) and depending upon the application with other solid “pigments” (that is the term used in the paper industry, although they are almost always “white”).
The transition between a solid mass and a fluid is frequently very “sharp”. Less than 0.1% water can be the difference. In paper coatings this transition can be reduced by the addition of a small amount of low molecular weight surface active substances. Again, in the paper industry , called “dispersants”. The amount of dispersant can also be very small, of the order of a few tenths of a percent, depending upon specific composition of the solid component and the composition of the “dispersant”. This lowering of the viscosity of the suspension also allows entrapped air to escape. This increases the density of the paper coating (which in general is desirable, even if only a fraction of a percent. In the paper coatings industry, this is viewed as reducing the “bridging” between solid particles. The actual mechanism for all of this activity is complicated and largely empirical. Sometimes the “dispersant” is a low molecular weight water soluble electrolyte, and other times, higher molecular weight poly-electrolytes are more effective. It is a bit of trial and error.
It seems that in both cases, bricks and paper coatings, the objective is to maximize the concentration of solid material and minimize the amount of included air. All this may be well known cement chemistry, but that is out of my range of expertise.
The really short answer is I do not think focusing only on cement
chemistry will be the most effective approach. Although once you start
changing additives and fillers to reduce cost, you may need to alter
cement chemistry to optimize performance. But altering chemistry of
cement alone I think will not yield the results you want. As you
undoubtedly know, concrete is enormously complex, and is the subject
of active research. Improving concrete, and building the theoretical
underpinnings of how it works, is very much ongoing! And, I am rarely
satisfied with really short answers, so here goes a long (and no where
near long enough, to be fair) explanation...and I will apologize in
advance, I did not go back and edit thoroughly, so it is a little
Your question involves two distinct elements, first, of concrete
strength, and second, of cost. Although you do not explicitly ask about
cost, you are asking about using less cement, an expensive part of
concrete, and your wording "wasted on void space" certainly recognizes
the importance of cost. So the question is, can you save money by
using less cement by means of changing its chemistry? I believe the
answer to that question is most appropriately 'no' -- if you want to
improve cost and strength, you are better off changing other aspects of
I will discuss the intermolecular attraction idea, but let me first
say that there are practical considerations that outweigh this
concept. I will discuss those as well. And, I am far from being a
concrete expert, so please take these thoughts with a grain of salt!
A basic rule of surface energy (a core phenomenon behind
intermolecular attraction) is 'like attracts like'. Hypothetically, if
you change the chemistry of your cement so that it is more attracted to
itself than to the aggregate, it is going to be less attracted to the
aggregate. It is going to wet (coat) the aggregate less effectively and
have a lower bonding strength to the aggregate. In effect, this will
exacerbate the problem -- the cement will tend to agglomerate more,
rather than spread over the aggregate. That reduces the performance of
the concrete. As an example, you could mix some hydrophobic plastic
balls, like polypropylene or polyethylene (representing aggregate),
with some clear oil, like soybean oil (representing void space), and
add some water-based food dye (representing the cement). The dye will
tend to form droplets because it is neither attracted to the oil (void
space) nor to the balls (aggregate).
In contrast, consider if you have a cement that is highly attracted to
the aggregate -- it will tend to wet the aggregate and not
agglomerate. It will not concentrate on just points of contact,
though, it will fully wet the aggregate. The way to illustrate this
would be with glass spheres, water-soluble dye, and some clear oil. In
this example, the dye will coat the surface of the glass balls
(because they are hydrophilic), but will not necessarily aggregate at the
points of contact.
However, this 'theoretical' discussion neglects some important
practical considerations. When mixing concrete (I mention this for
other readers -- I am sure you know this better than I do), it compacts
as entrapped air voids are released, and the desired, uniform
workability is achieved. The mixture is thick -- meaning that bulk
mixing action requires a lot of effort, and also that at small scales,
diffusion is much more important than convection. These conditions
make it difficult for cement to reconfigure itself driven solely by
intermolecular attraction. Mixing forces dominate
It turns out, in high-strength concrete, that several parameters
matter beyond wasted cement. One parameter closely related to the
chemistry of the cement is the bonding of cement to the aggregate (for
example, limestone aggregate has been shown to bond better than river
rock, resulting in stronger concrete). You can use a mix of larger and
smaller aggregate as well (the 'small' pieces fill the voids between
the 'large' ones). Much research has been performed on micro- and
nano-scale additives as well (and much more research has yet to be
And, *something* has to fill the spaces between the aggregate --
whether that is smaller aggregate particles, or fly ash or filler or
fiber or whatever -- once you mix the concrete, and compress the
concrete into molds, the entrapped air voids are going to be largely
pushed out, so it is not air in the voids. (Some admixtures are used to
keep micro-scale bubbles in the concrete, but these stabilize the
bubbles as spheres -- effectively acting as another type of aggregate,
not a continuous phase to fill voids.)
So, another approach to minimize the volume that cement can fill would
be with additives. Fly ash, for example, generally reduces the
strength of concrete, but is cheaper than cement. In contrast,
microsilica can increase strength, but can also introduce workability
challenges with higher loading, and increases cost. Specifically,
microsilica improves the bond between the aggregate and the cement. In
more technical concretes, a variety of fibers have also been used for
many years, and very low-cost fibers (such as those derived from
agricultural residues) can improve freeze-thaw performance, toughness,
impact resistance, an other brittle-failure issues.
So, in conclusion, I would argue that changing the composition of the
concrete can be *more* effective than only changing the chemistry of
the concrete. But changing the chemistry of the cement is also very
important in certain applications (improving dimensional stability,
increasing cure time, increasing hardening time, etc.), and especially
when considering the concrete system as a whole.
Hope this helps,
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