It is now time to talk about the most interesting part of concrete – the paste. Paste is the weakest, least durable and most expensive part of concrete, but concrete wouldn’t work without it. Unfortunately, paste is also the most complicated part of concrete to discuss. If you really want to understand it, you have to talk to a cement geek, and I am not a cement geek. In fact, I only know the bare basics about cement and paste. If you want to know a little more, check out the Integrated Materials and Construction Practices document from FHWA at http://www.cptechcenter.org/publications/imcp/. If you really want to go down the rabbit hole, talk to the good folks at the Portland Cement Association, http://www.cement.org. If you just want to know enough to get by in the concrete world, keep on reading.
Paste is the material that holds together all the stronger, more durable and less expensive materials, namely rock and sand. You can divide paste design into two categories – 1) How much paste do we need to provide workability, and 2) What is the composition of the paste to produce the desired strength and durability. The two factors are interrelated in that a paste with a higher percentage of cementitious materials will usually be more viscous, requiring that the concrete mix will require more paste to produce a workable concrete. We usually want to minimize paste content subject to the fact that we need to achieve the specified strength, workability and desired durability in the face of environmental factors.
Paste for workability: How much paste is required for a workable concrete mix? That depends on the aggregate size, shape and the percentage of voids in the combined rock and sand. The paste acts to provide a lubricating layer between the combined aggregate particles. It must first fill the voids between the particles, then separate the particles so they can start to flow. However, the “flow” of cement paste isn’t exactly like the “flow” of water. If you want to take a dip into the cement geek end of the gene pool, look up “non-Newtonian fluid Bingham plastic” in Google. You will find a brief article in Wikipedia that will serve as a jumping off spot to a lot interesting articles. Think of things like Silly Putty and ketchup. There is a great YouTube video at http://www.youtube.com/watch?v=S5SGiwS5L6I.
How much paste does it take to create a workable concrete mix? Typical values quoted are 20-40%. For a great article on the paste content of self-consolidating concrete, check out http://www.icar.utexas.edu/publications/108/ICAR%20108-1%20(Proportioning).pdf. In that article authors Eric Koehler (now Dr. Eric Koehler, of Verifi fame) and Dr. David Fowler provide an interesting look into an alternate means of proportioning concrete mixtures. ICAR (The International Center for Aggregates Research) also published a bunch of great articles on the use of high-fines aggregates in concrete. The trouble is, you can’t just decide you only want 22% paste in your concrete mix and design it that way. You probably won’t be able to get the concrete out of the mixer. You must first add enough paste to fill the voids between the aggregate particles, then add more paste to separate the particles to provide workability. How much the particles need to be separated depends on particle shape and texture, and the viscosity of the paste.
Paste for strength and durability: Once you know how much paste you need, you must determine the composition of the paste. For my purposes, I’m going to divide paste into 5 components:
- Soluble cementing materials
- Insoluble fines (passing the #200 or 75 µm sieve)
- Chemical admixtures
As I have stated in previous blog posts, each component has a purpose and if any single component proportioned properly, the concrete won’t work well. Rather than provide a “mix design procedure” that tells you how much of each material is needed for a concrete mix, I want everyone to understand what each material does for the paste and for the concrete.
Water – Water lubricates the paste and fills the voids between solid cement particles and other fines. If you have too little water, the paste will be too thick on won’t flow. Too much water will weaken the paste and provide decreased durability. The amount of water necessary is dependent on the voids between the cement-sized particles and the ultimate workability you want to achieve. Entrained air bubbles can also fill spaces and act as a lubricant, so air entrained concrete usually requires less water for a given slump than non-air entrained concrete. Of course, as soon as the water comes in contact with the cement particles, some of the cement starts to dissolve and form calcium-silicate-hydrate gel (CSH). This gel is the material that later crystalizes and starts to produce strength in the concrete.
Air – Even in non-air entrained concrete, there will still be entrapped air. Entrapped air bubbles are much larger than entrained air bubble, so provide no protection from freeze-thaw damage. Entrained air bubbles are microscopic bubbles that act as “shock absorbers” and provide spaces for freezing water to expand into, minimizing damage to concrete during freezing. How much entrained air should be added to concrete? While today we normally talk about air as a percent of the total concrete mix, the original studies on air entrainment considered air as a percent of paste. Back in the late 1930’s the recommendation was that air be 9% or mortar or 16% of paste. (I’m still trying to find the reference on this but have temporarily misplaced it. I think it was work by TC Powers.)
Over the normal range of entrained air, increasing air content will decrease strength, typically by 5% for every 1% increase in air. However, adding limited air entrainment (on the order of 2-4%) to low cement factor mixes can actually increase strength by taking up space in the paste and reducing water demand to produce a specific fluidity in the paste.
Air bubble size has an impact on the efficiency of air entrainment to protect against freeze-thaw damage. The Air Void Analyzer, http://www.youtube.com/watch?v=lokHsHn99DU, makes it possible to determine air void sizing in plastic concrete.
Cementitious and supplementary cementitious materials – These materials dissolve in water, then react and recrystallize to form calcium silicate hydrate, the materials that hold concrete together. The more of these materials with respect to water, the stronger or more durable the concrete – up to a point. See http://www.commandalkonconnect.com/2012/08/13/the-0-26-wc-myth-conception/. I could spend another whole post talking about cementitious materials (and probably will) so for now let’s just leave it at that.
Insoluble fines – Some of the fines don’t dissolve, but they are still valuable to the paste. Depending on particle size, very small fines can fill in the spaces between larger particles and reduce the demand for water. Lafarge has been a front-runner in studying the impact of particle size distribution of cement-sized fines, but the concept remains the same as for aggregates. Silica fume and finely divided limestone powder are the two most common sources of insoluble fines.
Also, insoluble fines can act as “nucleation sites” for the formation of the CSH crystals. When the CSH gel is ready to crystalize, having a particle for the crystal to latch on to can help in the formation of the crystal. The more nucleation sites that exist and the closer they are to each other, the more crystals will start to form and the easier it will be for them to connect.
Admixtures – Admixtures used to be simple chemicals that were easy to talk about. They were soaps, sugars and surfactants (water softeners like lignins). Until the 1980’s admixtures were so easy to produce that some companies made their own. With the emergence of superplasticizers that has changed. Today optimizing admixtures can require molecular manipulations beyond any kitchen labs. Nanotechnology is becoming the watch-word for admixture production.
The thing to know about admixtures is that it is best to come up with the best concrete mix possible without the admixtures, then use admixtures to enhance the properties of an already good concrete mix. Using an admixture in a bad concrete mix forces the admixture to overcome the mix problems first, then enhance the concrete. This is not an efficient use of the admixture.
This concludes my introduction to paste. I will probably be adding on to the above sections later, depending on the responses I get back from this post. However, the next section to this series will probably be about the different techniques for optimizing aggregates. After that I will talk more about paste and its component materials.
I am NOT a cement guru. If anyone disagrees with anything I say, please speak up. What you read here is just my understanding of a complex material and I would love to learn more about it.
Until next time.