Q: I'm interested to know what do you use any chemicals, heat or steam prior to the demolding ? How long do you need to to wait before demolding ?

A: (Bruce Schundler) Release agents are used in most molded products, and I think the same release agents are used as in other cast or molded concrete products. The same would apply to the use of heat or steam--both of which are used simply to accelerate the curing time of concrete blocks and products.

Q: I live near Socorro, NM where there is a large perlite mine, but they don't expand the perlite. Can you say something about using un-expanded perlite in concrete mixes?

A: (Bruce) Both forms of puerilities are used in concrete mixes, but it does depend on what one wants to accomplish with the concrete mix.

Q: I am sending this message from New Delhi. Recently we made a mix for cellular light weight concrete using a small scale system. It was found that the unit weight is 1200 kg/cum, but when the same mix was made in the batching plant for large scale production the unit weight was found to be 2300kg/cum. Foam is directly mixed in batching plant mixing drum and in other trials, first we have produced the concrete and foam is mixed in transit mixer. Why it is so? Kindly advise the suitable procedure.

A: (Lee Campos) The assumption is that the general weights of the individual materials (and more specifically, their densities) in each situation does not change prior to mixing. For a constant weight (kg), the only explanation for the rise in density (“it was found that the unit weight is 1200 kg/cum. but when the same mix was made in the batching plant for large scale production the unit weight was found to be 2300kg/cum”) would be due to a decrease in volume (cum). This is quite possible in a large-scale batch mixing plant. If the materials are “over-mixed” the perlite or vermiculite materials might be more-or-less pulverized and therefore occupy less volume. Both perlite and vermiculite are susceptible to this phenomenon, but in particular perlite (especially large-grain, horticulture grade) is sensitive to over-mixing. It turns out that the solution is easier than one might imagine. Simply be sure not to over-mix the perlite/vermiculite materials. This can be accomplished by adding them last to the Portland cement and water and then just mix them long enough to thicken the slurry. Typically in a transit-mix truck, this takes maybe 45 seconds – two minutes is plenty.

Q: I have naturally occurring pumice from pea-size to two feet across! I'm looking into both pumice-crete and earthbag uses and am wondering what sizes are most appropriate. Is there any advice for using the very large pieces?

A: (Kelly) I used 3/4" and minus scoria to fill my earthbags in the home that I built, and I believe that is it roughly that size that is typically used in pumicecrete. The larger pieces can either be crushed further, or used for decorative purposes I suppose.