Natural Pozzolans

Natural Pozzolans are an ancient form of cementious material and have been used for thousands of years. They have been occurring naturally since, well...practically forever, at least since there was life on earth. Called Pozzolan for the village of Pozzuoli where great volcanic eruption from Mount Vesuvius had deposited a large volume of volcanic ash in 79 AD. Pozzuoli, located near Naples, made the most of this misfortune and beginning mining this material which was an already proven material with cementious properties. Pozzolan was likely the first cementious material being used in the ancient world. Timelines show uses of such materials dating back to 3000 BC when both the Eqyptians and Chinese were incorporating them inmajor infrastructure projects and beyond. I have heard, but can not yet verify, that when the very first civilizations began taking shape, around 6500 BC, in the area of what is now Syria and Jordan, it is likely that they were crafting their buildings, which can still be found today, with mortars consisting of Natural Pozzolans.

 

Today Natural Pozzolans have a use in concrete that is very similar to that of fly-ash.

They can be used as a partial replacement for the cement often in the range of 15% to 35% (per PCA Concrete Manual). They can improve sulfate resistance an can help manage the reactivity to alkali-silica. The physical nature of  Pozzolans helps to reduce permeability.

Natural Pozzolans that you are likely to come across are usually processed materials. They are heat treated in a kiln and are then pulverised into a fine powder.  These Pozzolans include Calcined Clay, Calcined Shale, and Metakaolin

Natural Pozzolans are classified by ASTM C 618

I have worked with a few companies that were both developing and using Natural Pozzolans. They seemed to have a Pozz-itive effect.

For more information on Natural Pozzolans and other Cementious Materials, Look Here:
http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/

Fly ash in Concrete

Fly ash has become a big part of the precast concrete industry. It is a useful addition to the cementious portion of a concrete mix. Because of its unique molecular shape it makes stripping molds off of dry cast products smoother, aids in longer product lives in certain applications, and beacause it is a by product of burning coal utilizing it within concrete is a beneficial way to capture and store an otherwise harmful material. But what is fly ash?

As I mentioned fly ash is usually a by product of burning coal. However fly ash is technically any ash, typically from a furnace that is airborne. Fly ash is usually used to replace 15-25% of the cementious material in a concrete mix. There are several benefits to using fly ash. A key one is that fly ash has excellent resistance to acids and its use in a concrete mix helps the product to resist the premature degradation caused by exposure to excrement and other acidic organic materials that can be found in the soil.fly ash is also resistant to sulphate and salt.

The round shape of the fly ash molecule adds an elegant elasticity and smoothness to the concrete mix. This is a great asset when pumping concrete and when stripping dry cast products in their green state.

Unfortunately, or fortunately, based upon your view point, Fly ash has become very difficult to get due the combination of an increased demand by concrete producers and, more significantly, greater environmental restrictions on the production and consumption of American coal.

Understanding Mixer Output

One of the most confusing parts of buying (or selling) a concrete mixer is understanding what the output of concrete will be. this not only because different manufactures have different names for the same sized mixers, some metric others imperial. The same mixer will have different output capacities based on the mix design. it is also different whether it is weight or volume  of output that is being analyzed. Generally speaking the weight of the material that you put into the mixer will (almost) always be the same as the weight of concrete that comes. But volume is a different story.

When you have a pile of stone, a good portion of that pile is actually air. There are voids (air space)between the individual stones. Even grains of sand have air voids where they are not touch adjacent grains. in the process of mixing concrete the voids between grains of sand are fill with hydrated cement dust, and the voids in the rock are fill by the combination of sand cement and water. Therefore, if you have cubic foot of stone, and 25% of that volume is air voids, you can expect that you will see that volume of reduction in your finished concrete (as the sand will fill these open spaces). Additionally, there will be further reduction as the cement particles fill the voids left in the sand. Generally, this reduction is around 1/3 to 1/4, though much greater or lesser reductions are not uncommon.

Admixtures can also play a notable role in volume changes to concrete, I think. But a lot of stuff with Admix is beyond me, and I am in know way an expert in that regard.

Really, the important this about mix volume reduction comes down to, how big of a mixer do you need to fill this form, or this Tucker, or this bucket, or machine hopper, etc, etc, etc. with X amount of concrete of X type. the kicker is if you don't get this answer right you will be living with it for years.

Here is my recommendation for getting the right size concrete mixer for your plant... Plan on shrinkage and understand mixer volume capacities. Get the technical information on the mixers you are considering. Compare the volume of the mixers to be sure that you are getting a mixer that can produce the volume of concrete you need. If it is a pan or a planetary mixer make sure that you don't have to go far above the paddles on your mix, as that will not mix as well, and it can be hard to make a mix homogeneous from top to bottom. With Twin shaft and Horizontal shaft mixers the concrete is blended more easily at greater depths, but over filling can lead to a mess.

There is of course much more to evaluating mixers. This is but a small part, but I hope that you find it helpful.