Table 2.

Overview of foaming methods for geopolymer concrete.

Foaming Method	Foaming Agent	Aluminosilicate Sources	Alkaline Activator	Curing Conditions			Results	Ref.
				T (°C)	T (h)	RH (%)
Chemical foaming	Al powder	Fly ash	Sodium hydroxide and sodium silicate	60	24	n.r.	The ratio of alkaline activators impacts the extent of foaming.	[35]
Chemical foaming	Al powder	Fly ash	Sodium hydroxide	60	24	n.r.	Al powder delays the strength development and influences the gel formation.	[80]
Chemical foaming	Al powder	Metakaolin	Sodium hydroxide and sodium silicate				Increasing Al content leads to an increase in porosity and a decrease in thermal conductivity.	[74]
Chemical foaming	Al powder and H2O2	Fly ash	Sodium hydroxide and sodium silicate	70	24	n.r.	H2O2 as foaming agent leads to small pores and a compressive strength of 3.7 MPa (2.0 wt. % H2O2) Al powder as foaming agent leads to larger pores and a compressive strength of 3.3 MPa (0.2 wt. % Al powder).	[81]
Chemical foaming	H2O2	Metakaolin and fly ash	Sodium hydroxide and sodium silicate	40	24	65	NaOH concentration has an effect on compressive strength and thermal conductivity.	[82]
Chemical foaming	H2O2	Metakaolin and fly ash	Sodium hydroxide and sodium silicate	40	24	65	H2O2 content affects the physical properties (porosity, mechanical resistance and thermal conductivity).	[42]
Chemical foaming	H2O2	Metakaolin and fly ash	Sodium hydroxide and sodium silicate				Increasing H2O2-content leads to a decrease in compressive strength and thermal conductivity, and an increase in porosity.	[73]
Chemical foaming	H2O2 and different surfactants	Metakaolin	Sodium hydroxide and sodium silicate	RT + 60	10 + 24	n.r.	The surfactant influences the morphology and topology of the network and therefore the mechanical properties.	[83]
Chemical and mechanical foaming	H2O2 and SDS	Granulated blast-furnace slag and fly ash	Sodium hydroxide and sodium silicate	60	24	n.r.	Foam stabilizer enhances the pore size distribution and the pre-made foams are more stable.	[34]
Mechanical foaming	SDS	Granulated blast-furnace slag	Sodium hydroxide and sodium silicate	60	24	n.r.	A lower Si/Al ratio leads to higher strength, and a higher Si/Al ratio leads to a lower amount of crystalline structures.	[84]
Mechanical foaming	SDS	Granulated blast-furnace slag and fly ash	Sodium hydroxide and sodium silicate				Increasing XG concentration leads to an increase in compressive strength and a decrease in thermal conductivity	[34]
Mechanical foaming	Super-plasticizer	Fly ash	Sodium hydroxide and sodium silicate	RT + 60	24	n.r.	Higher compressive strength with heat curing (60 °C).	[29]
Syntactic foams	Cenospheres	Fly ash and ground granulated blast-furnace slag	Sodium metasilicate	RT	n.r.	n.r.	Strong bonding between cenospheres and the geopolymer matrix results in a compressive strength of 17.5 MPa at a density of 978 kg/m3 and a thermal conductivity of 0.28 W/(m K).	[77]
Syntactic foams	Fly ash cenospheres	Metakaolin	Potassium silicate	80	144	n.r.	Increasing the amount of cenospheres leads to a decrease in compressive strength, thermal conductivity and density.	[84]
Syntactic foams	Hollow glass microspheres	Fly ash	Sodium hydroxide and sodium silicate	60	24	n.r.	Increasing the amount of hollow glass microspheres leads to a decrease in density and compressive strength.	[85]
Syntactic foams	Hollow phenolic microspheres and hollow glass microspheres	Metakaolin	Sodium hydroxide and sodium silicate	40 + 60 + RT	2 + 24 + 144	n.r.	Increasing the amount of hollow microspheres leads to a decrease in compressive strength.	[78]

SDS = sodium dodecyl sulphate, n.r. = not reported, RT = room temperature, XG = Xanthan Gum.