As shown in Fig. 1a, the 1h settlement distances of the foam prepared by yeast and CTMAB are significantly improved compared with foam prepared by using 0.50 wt% CTMAB alone with water or yeast solution. This is mainly due to the increase of surface active molecules in the solution system, which reduces the surface tension of the solution and increases the stability of the foam. However, when the CTMAB content is relatively low in the CTMAB/yeast solution, the number of surface active molecules and entrained solutions attached to the liquid film are little, the self-healing ability of the liquid film is poor, resulting that the foam is easily destroyed. When the content of CTMAB is high, a large number of surface active molecules attached to the liquid film make the foam gravity larger, leading to foam rupture. At the same time, the volume expansion of CTMAB/yeast solution is significantly improved. When the amount of CTMAB is 0.50 wt%, the average foaming multiple of the foam prepared by the yeast and CTAB composite system is the largest, reaching 36.7 times (Fig. 1b). In addition, Fig. 1c shows the bleeding ratio of different foam, which is an important index to measure the stability of foam. When the content of CTMAB is 0.25 wt%, the pore size of the foam are not uniform, and the wall thickness is thin (~ 29 µm), resulting in a bleeding ratio of 83.6%, which makes them easy to defoam (Fig. 1d). When the content of CTMAB is 0.50 wt%, the foam with uniform size, wall thickness of about 38.24 µm and suitable liquid volume between bubbles makes them stability better (Fig. 1e). As shown in Fig. 1f, when the content of CTMAB is 0.75 wt%, the pore size of foam increases, the wall thickness of the foam is as high as 55.42 µm, and the liquid between the bubbles accumulates, which easily causes the foam to burst, thereby increasing its bleeding ratio. As results, we selected the content of 0.5 wt% CTMAB with yeast solution to prepare bio-based geopolymer in subsequent experiments.
As shown in Fig. 2a, the dry density of silica fume-based geopolymer without adding foam is 1050.7 kg/m3. with the increase of foam content, the dry density gradually decreases. But when the foam content increases to 30g, the decrease of dry density is not obvious, because the geopolymer slurry has high viscosity and large surface tension, which makes it difficult for foams to exist in the stable, thus resulting in the defoaming phenomenon. As the dry density decreases, the porosity increases, resulting in an increase in the ability of water absorption (Fig. 2b). When the foam content is 30g, the water absorption of lightweight silica fume-based geopolymer is as high as 64.6%. It is shown in Fig. 2c that the thermal conductivity is positively correlated with dry density of geopolymer. Compared to the high thermal conductivity of the silica fume-based geopolymer without adding foam [0.275 W/(m·K)], the thermal conductivity of the silica fume-based geopolymer obtained with the foam content of 20g, 25g and 30g were 0.123 W/(m·K), 0.094 W/(m·K) and 0.088 W/(m·K), respectively. Meanwhile, it can be seen from Fig. 2d that with the continuous hydration process, the compressive strength increases with the increase of age, whether foam is added or not. The compressive strength of the block gradually decreases with the increase of the foam content at the same curing age. In addition, the change of foam content has almost no effect on the microstructure of silica fume geopolymer (Fig. 2e, 2f, 2g, 2h). Compared with the silica fume geopolymer without adding foams, the N-A-S-H gel in the silica fume-based geopolymer with adding foams is mostly flocculated, because the incorporation of foam can play a barrier effect so that the N-A-S-H gel tends to disperse. The presence of a large number of flocculated products leads to the increase of pores in the sample, which leads to the reduction of compressive strength of the sample, the increase of water absorption, and has a beneficial effect on heat insulation performance.
In order to fully prove the universality of bio-based foaming agents for the preparation of lightweight geopolymer, fly ash was used instead of silica fume in the experiment. The dry density reached 1107.3 kg/m3 without adding foam, and the dry density decreased gradually 1023.8 kg/m3, 730.0 kg/m3, 678.4 kg/m3, respectively, when the content of foam are 15g, 20g and 25g (Fig. 3a). The water absorption of the fly ash-based geopolymer increases, and the thermal conductivity decreases with the increase of foam content (Fig. 3b, 3c). As shown in Fig. 3d, the compressive strength of the samples gradually decrease with the increase of foam contents, and increase with the growth of curing age. A large number of fibrous cementitious materials accumulate into a near-spherical structure as shown in Fig. 3e, 3f, 3g, 3h. Because the Si-O-Al covalent bonds of the highly-activated metakaolin were firstly destroyed during the reaction, and combined with the alkaline substances to form a fibrous cementitious materials, which was attached to the surface of the spherical fly ash particles. As the degree of cementification deepens and the degree of polymerization increases, the nearly spherical particles gradually become larger and the compactness of the structure gradually increases. Finally, the unique three-dimensional network structure is formed, which constitutes the skeleton of the fly ash-based geopolymer.