Optimization of process parameters affecting final foam properties

There are different parameters that can affect the foaming process and, consequently, the final features of the ceramic foams produced. For this reason, in order to optimize the properties of the final products obtained, the influence of some of these process parameters on the final porosity of the foams has been investigated.

In particular, the attention has been focused on three main factors: a) Alkalinity of the activating solution

b) Amount of pore forming agent used c) Curing temperature

All the ceramic foams produced are based on a starting mixture with the following composition: 70 % wt. SS solution, 8.7 % wt. Na2SiF6, 21.3 % wt. metakaolin (MK). All

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the powder components were previously homogeneously dry mixed and then the SS solution was added [35].

The parameter that gives important information about the alkalinity of the activating solution is the ratio R = SiO2/Na2O, that is a characteristic ratio for sodium silicate

solutions; it is quite clear that the increasing of this ratio leads to a reduction of the alkalinity of the solution considered. Two different SS solutions, one with R=3 and another one with R=2, have been tested in producing two different geopolymeric foams. The foams were produced adding each activating solution to the raw powder materials (MK, Na2SiF6 and metal silicon) previously homogenized and dry mixed. Then, the pastes

obtained were cast in cylindrical open molds and cured at 40°C for 24 hours. In Figure 3.15 are reported respectively the sample obtained using SS solution with R=3 (3.14a) and the one obtained using SS solution with R=2 (3.14b). As it is possible to see from Figure 3.15, the sample produced starting from a more alkaline SS solution looks like a more “expanded” foam, with a higher and irregular open porosity, while the foam obtained with the SS solution with R=3, appears much more dense and characterized by a more regular and smaller porosity.

(a) (b)

Figure 3.15 Geopolymeric foam samples produced using SS solution with R=3 (a) and R=2 (b)

Regarding the amount of pore forming agent used, two different foams have been produced with two different percentages of metal silicon, one with 10% wt of silicon (Figure 3.16a) and the other one reducing the amount of silicon to the 8% wt (Figure 3.16b).

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(a) (b)

Figure 3.16Geopolymeric foam samples produced using 10% (a) and 8% (b) of metal silicon

It is evident from Figure 3.16 that the reduction of the percentage of metal silicon used to create porosity inside the ceramic matrix, did not lead to significant changes or differences in the final structure of the two foams produced. This means that the difference between the two percentages is maybe too slow to influence the effective capacity of the silicon to produce bubbles inside the matrix. So, it is clearly preferable using, considering the same final properties, a lower amount of metal silicon, especially because the presence of eventual unreacted silicon could represent a source of defect and can contribute to increase weight and density of the samples while, for these kinds of materials, low density and lightness are critical points.

It is well known that curing temperature represents an important parameter in the geopolymer production. In particular, it has been proved that prolonging the curing time and increasing the curing temperature (up to 60°C) can speed up the harden process and improve the physical properties of a geopolymer sample. However, curing at too high temperatures (80° and 100°C) or for longer time (more than 72 hours) do not provide any significant improvement in chemical and mechanical properties [36]. For this reason, the two different curing temperatures selected to study their influence on the final properties of the produced foams are 40°C and 70°C. Figure 3.17 shows how the foam samples cured respectively at 40°C (Figure 3.17a) and 70°C (Figure 3.17b) looked like; the two samples are extremely similar in appearance, size and type of porosity. This underlines that the curing temperature does not influence in a significant way the finished products obtained, for what concerns the porosity of samples. This could be also a consequence of the addition of the sodium hexafluorosilicate, which, acting as catalyst, chemically promotes the gelification of the sodium silicate and of the entire geopolymeric matrix, affecting in this way the consolidation of the system especially during the first moments of the process. In particular, the catalyst, accelerates the reactions that lead to the

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hardening of geopolymeric matrix and, for this reason, in this specific case, the effect of curing temperature is not so much significant as it would have been expected from literature [36].

So, it is possible to conclude that the best curing temperature to use for the production of geopolymeric foams is the lower one, 40°C.

(a) (b)

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