Q: I'm wanting to use fly ash as a cement admixture in a ferrocement water tank, but rumors exist that it may not be drinking-water safe because of heavy metal contaminations. Do you have info or can you refer me somewhere else?

A (Paul Sarnstrom): I have heard that any heavy metals present in concrete, where some of the Portland cement has been replaced by fly ash, are locked up in the concrete and that it is not a problem but as I remember this reference was not specifically to water tanks. I think that the heavy metals in fly ash will differ significantly both in types and quantities in different samples depending on the type of coal burned and the completeness and cleanliness of the combustion process for a particular sample.

A (Kelly): I did a little search on this topic and came up with the following summary from this study: http://greenwood.cr.usgs.gov/energy/factshts/163-97/FS-163-97.html Radioactive elements in coal and fly ash should not be sources of alarm. The vast majority of coal and the majority of fly ash are not significantly enriched in radioactive elements, or in associated radioactivity, compared to common soils or rocks. This observation provides a useful geologic perspective for addressing societal concerns regarding possible radiation and radon hazard.
      The location and form of radioactive elements in fly ash determine the availability of elements for leaching during ash utilization or disposal. Existing measurements of uranium distribution in fly ash particles indicate a uniform distribution of uranium throughout the glassy particles. The apparent absence of abundant, surface-bound, relatively available uranium suggests that the rate of release of uranium is dominantly controlled by the relatively slow dissolution of host ash particles.
      Previous studies of dissolved radio elements in the environment, and existing knowledge of the chemical properties of uranium and radium can be used to predict the most important chemical controls, such as pH, on solubility of uranium and radium when fly ash interacts with water. Limited measurements of dissolved uranium and radium in water leachates of fly ash and in natural water from some ash disposal sites indicate that dissolved concentrations of these radioactive elements are below levels of human health concern.

Another website, http://www.hps.org/publicinformation/ate/q3121.html , quotes L. Max Scott, CHP, " Anything extracted from the earth's crust usually contains trace amounts of naturally occurring radioactive material—namely uranium, thorium, radium, their radioactive decay products, and potassium. The amount of radioactive material in fly ash may vary by a factor of more than 10, depending on the source of the coal. Generally coal mined in the western states is lower in radioactivity than eastern state coal. The radioactivity tends to stay with the fly ash and because of weight lost during burning, on a weight basis, the amount of radioactivity increases in fly ash. Since brick, concrete blocks, and concrete are made from material extracted from the earth, they also contain naturally occurring radioactive material. The concentrations are generally comparable to that of coal.

The scientific community has not reached a definitive conclusion regarding the health effects of very low radiation exposure. Some feel there is an effect, others feel there is not an effect. Generally one can state that the health effects of living in a house made of fly ash block would be similar to that resulting from living in a house made of brick or concrete block. I don't think that you can find fly ash or much of anything else that is absolutely free of radioactive materials. Even wood contains trace amounts of radioactive carbon."

Several other sites seem to confirm these findings, so based on this, I would not be overly concerned with potential radiation from the use of fly ash in building products.

C: I believe that in the US you have products that change the structure of cement products ie, concrete blocks,concrete slabs in fact any product that contains cement, so that they become very water proofed up to 4". One product I know of is used by NASA on the concrete launching pads. No fabric to tear or hope that it was installed properly or damaged by poor back filling methods.

R (Paul Sarnstrom): I have no idea exactly what he is talking about. Sodium silicate [waterglass] can be applied to the surface of concrete to waterproof it, I am not sure it would penetrate 4". There are of course other topical treatments that claim to waterproof concrete but again I don't think any of them would penetrate more than 1/2"=1" let alone 4".

Concrete block is a different creature altogether. It uses a less Portland cement in the mix. Just enough Portland and sand to stick the aggregate together. Blocks are usually more porous than poured or placed concrete.

There are also additive that go into concrete as it's being mixed that can aid in waterproofing. Another possibility is to add 20% - 30% fly ash or other pozzolan as a partial replacement for the Portland cement. The particle size of fly ash is much smaller than Portland so you get a better gradation of particle size leading to fewer and smaller voids.

One of the best ways to insure waterproof characteristics in concrete or mortar is to keep the water/cement ratio low. If the water is kept to .35-.4 parts to 1 part cement by weight, it is generally considered waterproof. Such a mix is also very stiff and may require the use of additives and or vibration to properly place it.

C: I know about the mixture used in the concrete mix; the Trojan masonry sealer works in a different way. It is a Polyester Polymer which penetrates the concrete to seal, it doesn't sit on the surface of the concrete product. One of our construction companies uses this product to seal the inside of very large planter boxes, it stops water seeping out onto the carpet /tiles etc, it saves this company millions of dollars in damages.               

Out here, we excavate perhaps differently than you do when building basements. Here we excavate about 3' larger than the basement and the abutments are sloped at about 45-55 degrees, after the slab/footings are poured and the walls are built a trench is dug around and about 18'' out and 18" deep from the slab/footings, a 4" drainage pipe is placed into the trench and covered with 1/2" screenings, the walls and footings are then sealed , by what ever means the contractor uses,  the back fill is with screenings and then earth which slopes away from the wall, at the top of the excavated slope a spoon drain is put in. Any water will now follow the excavated slope to drainage trench below the footings/slab, and will with all things being equal keep water away from the wall, and it's protective coating! A web address www.enviropacific.com will give you lots of information on the Trojan products. I feel that it is better than bitumenous membranes, and other such products which can be damaged.

C: I designed a new way to make ferrocement water jars (pot-shaped cisterns common in Thailand).  My method is lighter, faster, uses less material and is lower cost.  The secret?  It uses a completed jar as a form.  So instead of spending lots of time making a form and using lots of materials to make it strong, you just wrap burlap bags and thin wire around a jar.  And it should produce perfectly symmetrical pots each time.  I plan to work with a local company to test it out.