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Name: Dane
Status: other
Grade: other
Country: South Africa
Date: Winter 2012-2013


Question:
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.



Replies:
Hi Dane,

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. Thanks Jeff Grell


Hi Dane,

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 possible.

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.

Regards, Bob Wilson


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.

Vince Calder


Hi Dane,

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 choppy.

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 the concrete.

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! (pun intended!)

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 thermodynamics/diffusion.

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 done!).

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, Burr Zimmerman


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