SEM images in Fig. 4 displays the morphology of foamed samples.
The macrostructure shows that in all the samples there exist a lot
of pores with different diameters from few tens of microns up to
1–2 mm. SEM analysis also evidence that the pore concentration decreases
and also the pore size decreases with the content of bottom
ash. When the bottom ash is suspended in a water glass solution and
is subjected to microwave treatment, thanks to the microwave heating
the silicate groups will start to react and link to each other increasing
the viscosity of the slurry and forming an impervious skin.With further
rising of the temperature above 100 °C steam will be produced which
inflates the impervious skin originating the intumescence and foaming,
which creates a porous scaffold. Expansion will continue until the gelling
of silicate group and dehydration will form a rigid brittle network.
Excess pressure evolving inside the pores will eventually locally break
to let the steam be removed from the foam. In our study, the 60wt.% sodium
silicate sample BA60 shows interconnected pores with pore sizes
around 500 μm to 2 mm range (Fig. 4a). The same characteristics also
appeared in the sample with 50 wt.% sodium silicate BA50. When the
sodium silicate content became less than 50 wt.%, the foamability by
microwave irradiation was reduced and appeared with less pores and
decreased pore size as for BA60 and BA70 with sodiumsilicate contents
of 40 and 30 wt.% respectively