Drying process

The initial characterisation of the four original matrix compositions (M1 – M4) was carried out using pellet specimens. For the evaluation of the mechanical properties of these samples, a splitting-tensile strength test was used. The mechanical properties will be addressed in chapter 4.2. Here, only some general aspects of the drying behaviour of the pellet specimens shall be described.

As mentioned before, all matrix compositions investigated in this study produced viable solid geopolymers after the standard curing process. The developed hardness of all compositions after curing was good, and except for composition M3, which appeared somewhat softer than other mixtures, all compositions resisted a simple scratch test with a spatula. The demoulded pellets (as well as all other samples) were typically dried under ambient conditions on a lab bench. Some small variations (approximately 20 ± 2 %) were observed between different batches of composition M1, most likely due to temperature and humidity fluctuations of the lab environment. The weightloss of the different compositions also varied slightly. The typical drying behaviour of the pellet specimens of all four geopolymer compositions is shown in Figure 4.1. A level of constant weight was usually reached after around three days.

Figure 4.1: Typical weightloss of pellet specimens of compositions M1 – M4 during the drying process under ambient conditions

No noticeable shrinkage of the pellets could be measured over the course of the drying process. However, the formation of drying cracks was a major issue for pellet specimens. The other unreinforced sample types, i.e. cylinder and bar specimens, on the other hand, were not affected by drying cracks. The cracking of pellet specimens was only observed after a number of samples had already been successfully fabricated and mechanically tested (see chapter 4.2.2 for more detail). Although some cracking was observed among these initial samples, the majority of these samples appeared reasonably strong and crack free. In comparison between the four

different geopolymer compositions, M2 appeared to have the highest and M3 the lowest tendency to crack. These initial mixtures were mostly prepared by hand- mixing but some batches of supposedly crack free pellets could also be fabricated from mechanically mixed geopolymer binder. At some stage throughout the experiments, a whole batch of samples was found to be subjected to a number of macroscopically visible cracks. Surprisingly, however, most of these cracks seemed to have disappeared the following day. This phenomenon was subsequently repeatedly observed. It was noticed that the initial crack formation generally occurred within the first 24 – 48 hours of the drying process. The subsequent disappearance of some fine cracks, however, only occurred on a macroscopic level. Wiping the sample surfaces with isopropanol alcohol (IPA) was found to be a fast and simple method to reveal these cracks without the need of time-consuming microscopic analysis. In fact, wiping the sample surfaces with IPA revealed not only the disappeared cracks but for most pellets also a considerable number of previously unnoticed microcracks, see Figure 4.2. Therefore, it is arguable if the earlier samples were truly crack free or also included microcracks. The disappearance of cracks in the specimens is most likely caused by minimal shrinkage in the pellet specimens, inducing a closing force on the present cracks.

Figure 4.2: Visibility of micro- and macrocracks in M1 pellets before (a) and after (b) wiping the sample surfaces with IPA. The appearance of several microcracks in the seemingly crack free samples 1 and 4 are clearly visible

Subsequently, considerable efforts were made to control and prevent the cracking of pellets including the drying under different controlled humidities and the addition of small amounts of glycerol as reported by Barbosa et al. [89]. However, all attempts proved largely unsuccessful and crack free pellets were only obtained in occasional instances. Since sample cracking was only an issue for the pellet specimens, the fabrication of this sample type was eventually abandoned.

It was initially believed that the cracking problem may be a result of the particular chemical compositions of the geopolymers or the particular matrix system. However, since the formation of drying cracks was not observed in bar and cylinder specimens of the same compositions, the chemical composition is not considered the main cause for the cracking. Nevertheless, the fact that some compositions appeared more prone to cracking than others indicates that the chemical composition, in particular the alkali content, may have some effect. Another possible explanation may be related to the overall sample size. This is based on the fact that only the larger pellet specimens were subject to cracking. Also, the surface-to-volume ratio of the specimens might have an effect. Both pellet and bar specimens had similar volumes but the surface-to- volume ratio of the bar specimens was nearly double compared to the pellet specimens. This may unfavourably affect the water evaporation during the drying process, resulting in cracks. Although the development of drying cracks in larger specimens may be overcome technologically by a more carefully controlled drying process, the present results suggest that the fabrication of crack-free metakaolin geopolymers with a cross section of more than roughly 10 x 10 mm becomes increasingly problematic. However,