Overall, the replacement of sand with unprocessed runof-station
fly ash had a significant beneficial effect on fresh
foamed concrete properties. Indeed, the FAcoarse mixes
exhibited enhanced consistence (greater spreadability and
flowability out of a modified Marsh cone) and rheology
(reduced apparent yield) compared with the sand concretes,
due to differences in the fine aggregate particle shape and
size. However, the FAcoarse concretes required up to three
times more foam than the calculated quantity to achieve the
design plastic density. This was due to foam instability,
possibly due to the highly fluid consistency of the base mix
and the adverse effects of the high residual carbon in the ash.
However, the slightly lower mix stability of the FAcoarse
concretes could potentially be overcome by using lower w/c
ratios than the sand mixes.
The use of fly ash in foamed concrete also significantly
benefited compressive strength development, particularly
after 28 days. At a given age, the FAcoarse concretes were up
to 6 times stronger than those of equivalent sand concretes.
From 28 days to 180 days, the fly ash mixes increased in
sealed-cured compressive strength by up to 2.5 times.
Although the FAcoarse concretes exhibited slightly higher
sorptivities than those of the corresponding sand mixes (due
to the higher volumes of sorbing paste in the former).
However, this did not have an adverse effect on its
performance in aggressive chemical solutions, and the
early indications are that this type of concrete could be
almost immune from attack.