The comparison of the chemical compositions of the original fly ash and the foamed geopolymer concrete is presented in Table 1. Calcium oxide (CaO) made up 21.6% of the content of the original fly ash, so it must be classified as a class C fly ash (containing more than 20% of CaO) and the ratio of Si:Al was about 3. This fly ash composition is representative of fly ash from the combustion of the sub-bituminous coal that is used in Malaysian power plants [27]. In addition, the content of iron oxide (Fe₂O₃) was high, which accounted for the darker color of the fly ash [27]. The powder sample of geopolymers LW1 and LW2 showed increases in the content of SiO₂. This was due to the reaction between fly ash and the alkaline activator (mixture of sodium silicate and NaOH), which is known as geopolymerization. This process occurs through a mechanism involving the dissolution of the aluminum and silicon species from the surfaces of waste material (fly ash) as well as the surface hydration of undissolved waste particles, followed by the polymerization of active surface groups and soluble species to form a gel and, subsequently, a hardened geopolymer structure [28]. Also, the mass percentages of SiO₂ and Al₂O₃ in LW2 were greater than they were in LW1 due to the heat-induced, rapid geopolymerization process.

Figure 1 shows the compressive strengths at days 1, 7, and 28 for the foamed geopolymer concretes that were cured at room temperature and at 60 °C for the average of 3 samples. For each of the test days, the maximum compressive strength was observed in the samples that had been cured in the oven (LW2). The maximum compressive strength values for the LW2 samples for days 1, 7, and 28 were 11.0 MPa, 17.6 MPa, and 18.2 MPa, respectively. Thus, we concluded that the curing temperature influenced the strength of the geopolymers [29]. The increase in strength of the LW2 samples was nearly complete after seven days, as evidenced by the fact that the strength had increased only slightly on day 28. However, for LW1, the results showed significant differences in strength for day 1, day 7, and day 28. This proved that heat treatment is required to expedite the rate of development of the strength of the geopolymers.

The average density of LW1 is 1650 kg/m³ and for LW2 are 1667 kg/m³ as stated in Table 2. The porosity of the foamed geopolymer concrete is the sum of the entrained air voids and the voids within the paste. The higher compressive strength of LW2 samples was due to their lower porosity and water absorption. The LW1 samples had 15.29% porosity and 2.35% water absorption, whereas the LW2 samples had substantially lower corresponding values of 6.78% and 1.22%, respectively, as shown in Table 2. According to BS 1881: Part 122: 1983, low water absorption is deemed to be anything less than 3%, and both types of samples had water absorption values that were less than 3%. Since LW2 is more dense (higher density than LW1), it produced lower porosity and water absorption as mentioned above.

The XRD pattern of fly ash was obtained as shown in Figure 2. The main components of the fly ash were quartz, mullite, anhydrite and f-CaO [24]. Figure 2 also shows the foamed geopolymer concrete, which consisted mostly of amorphous content. When comparing the XRD pattern of the original fly ash with the hardened geopolymer, it can be seen that the crystalline phases that existed in the fly ash originally (quartz and mullite) apparently have not been altered by the activation reactions. The fly ash also was made up of an amorphous phase, as indicated by the broad hump registered between 2θ = 20 °C and 30 °C [30].

Additionally, the broad hump between 2θ = 20 °C and 40 °C indicated the characteristic of amorphous gels, including geopolymeric gels and calcium silicate hydrate (C-S-H) gels. This shows that the geopolymeric reaction and the hydrate reaction occurred at the same time [24].

Figure 3 shows the IR bands of the fly ash and the foamed geopolymer concrete, and Table 3 summarizes the IR bands obtained from the FTIR analyses. The IR spectrum of fly ash shows main absorption bands at 1004, 1428, 2358, and 3715 cm⁻¹. The broad component at 1004 cm⁻¹ is due to the Si-O-Si and Al-O-Si asymmetric stretching vibration [31–36] and it becomes sharper and shifts towards lower frequencies (LW1 = 976 cm⁻¹ and LW2 = 969 cm⁻¹) in lightweight geopolymer. This indicates the formation of a new product (the amorphous aluminosilicate gel phase) due to dissolution of fly in alkaline activator [31–36]. In addition, the band at 1428 cm⁻¹ was due to the stretching vibrations of the O-C-O bond indicating the presence of sodium bicarbonate that is suggested to occur due to the atmospheric carbonation of a high alkaline NaOH aqueous phase, which is diffused on the geopolymeric materials surface [31,33,35]. Meanwhile, the broad IR bands at 3715 cm⁻¹ and 2358 cm⁻¹ represent the stretching and deformation vibration of OH and H-O-H groups, respectively, from the weakly-bound water molecules that were adsorbed on the surface or trapped in the large cavities between the rings of the geopolymeric products [24,37].

Foamed geopolymer samples (LW1 and LW2) showed broad components at 3301 cm⁻¹, 2333 cm⁻¹, 3304 cm⁻¹ and 2343 cm⁻¹ which indicated the stretching vibration of OH and H-O-H, respectively [33]. Moreover, bands at 1652 cm⁻¹ and 1653 cm⁻¹ represent the bending vibration of H-O-H [30].

The microstructure of the original fly ash based on the SEM observation is shown in Figure 4. The fly ash consists of spherical, vitreous particles of different sizes. These particles are usually hollow, and some spheres may contain other, smaller particles in their interior [38]. The surface texture of fly ash particles appears to be smooth [39] and also some vitreous, unshaped fragments or quartz particles can be seen [32].

Figure 5a–d shows the foamed geopolymer concrete of LW1 and LW2 at different magnifications. The size of the pores in the foamed geopolymer concrete ranged from 4 μm to 37 μm, and the distributions of the pores in both samples were uniform, as shown in Figure 5a,b. As expected, the existence of these pores in the foamed concrete result in its being classified as lightweight concrete. However, at magnifications of 2000× and 5000×, microcracks were observed in the LW1 samples (Figure 5c,e), which contributed to their lower strength by increasing their water absorption and porosity.

The LW2 samples, which had a denser matrix (Figure 5d,f) than the LW1 samples, produced foamed geopolymer concrete that had greater strength. These stronger samples were heat cured, which facilitated the complete reaction between the fly ash and the alkaline activator to form aluminosilicate gel. Soon after the mixing process, the gel covered the fly ash particles and produced a dense matrix (complete reaction). Nevertheless, there were still some instances of incomplete reaction, as evidenced by the fact that the surface of the fly ash was covered with aluminosilicate gel rather a dense matrix having been formed. This situation was observed on both samples.

Unreacted fly ash was present in both samples. Fly ash with its original spherical shape (Figure 4) was located on the nearby dense matrix. From the SEM analysis, it was determined that the existence of microcracks and the incomplete formation of the dense matrix had caused water absorption and porosity of the LW1 samples to increase, thereby impairing their strength.

Fly ash, sodium silicate, sodium hydroxide (NaOH) and foaming agent (superplasticizer) were used to produce the foam geopolymer concrete. The fly ash was obtained from Manjung Power Station in Lumut, Perak, Malaysia. The chemical composition of the fly ash was determined by X-ray Fluorescence (XRF) as shown in Table 1. The microstructure of the fly ash is shown in Figure 4.

Sodium silicate and the NaOH solution were mixed together to act as the activator. NaOH pellets with 99% purity, made in Taiwan with the brand name of Formosoda-P were used to produce 12 M NaOH solution by adding the NaOH pellets to distilled water. This concentration was based on previous research [40] that indicated that the maximum strength of geopolymer was obtained when 12 M NaOH was used. Meanwhile, a technical grade of sodium silicate was obtained from South Pacific Chemical Industries Sdn. Bhd. (SPCI), Malaysia, with a chemical composition of SiO₂ = 30.1%, Na₂O = 9.4%, and H₂O = 60.5% (SiO₂/Na₂O = 3.2). The other characteristics were: specific gravity at 20 °C = 1.4 kg/cm³ and viscosity = 0.4 Pa s.

In order to produce a desirable strength in lightweight concrete, a trial and error process was commonly used [41]. Since no methods have been proposed for producing foamed geopolymer concrete, we decided to use a geopolymer paste-to-foam ratio of 1:2 (by volume). The foam geopolymer concrete was produced by using the pre-foaming method, in which the foam is produced separately and then mixed with the geopolymer paste. The foam was produced by diluting the foaming agent with water based on a foaming agent-to-water ratio of 1:20 by volume. Then, the foam was generated by using a custom-made foam-generating machine: LCM Model 02. The geopolymer paste was produced according to the ratios shown in Table 4.

The sodium silicate and NaOH were mixed together for three minutes and then mixed with fly ash for another five minutes. After the geopolymer paste was homogeneous, the foam was added and mixed for another five minutes before it was placed in the 50-mm mold. The samples were cured at room temperature (LW1) and 60 °C (LW2). The LW2 samples were cured in the oven for 24 hours and then left at room temperature (open air) until compressive strength testing were conducted. Meanwhile, LW1 was cured at room temperature (open air) until testing day.