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