Foaming Agents in Concrete

Foaming agents are chemical compounds with low specific gravity that increase the permeability of concrete. Generally, foaming agents are used in conjunction with aggregates to improve their workability. They can also help reduce the volume change of the concrete during curing by enhancing hydration and improving the pore structure of the foam. Foaming agents can be produced with a variety of materials such as waste gas, limestone, gypsum, fly ash and sand. However, the resulting concrete has a low compressive strength because the air voids are prone to collapsing [1].

The addition of foam stabilizer or PPF can increase the mechanical properties of foamed concrete. However, lightweight foamed concretes fabricated with the addition of foam stabilizer and PPF exhibit a decline trend in density, thermal conductivity and flexural and compressive strengths as their mass percentage increases with foam agent addition from 5wt% to 7wt%. This is because the large number of pores generated by decomposition of the foaming agent can lead to an adverse effect on the pore structure.

In addition to foam stabilizer and PPF, the use of fly ash has been shown to increase the compressive strength and thermal conductivity of foamed concretes. This is because fly ash is a low-density material that participates in the hydration of cement and enhances the pore structure of foamed concrete.

The SEM image given in Figure 4 shows the microstructure of foam concrete when WMP replaced silica sand and RHA was added to the mixture. The addition of RHA has improved the pore structure of the concrete by extending the size of CSH gels. The addition of marble waste has also enhanced the pore structure by increasing the number of finer grained particles in the mix.