I am trying to dehydroxylate glass powder. The powder is a mix of different types of glasses. Thus there are absolutely considerable amounts of hydroxyl units in the microstructure. I've checked the literature, but I've found different temperature and time of the diffusion for the removal of Si-OH in glasses. According to Prof. Davidovits's book, Geopolymer chemistry, the removal of OH groups in IV-fold coordination of Al happens at a temperature somewhere between 700-800 C in kaolinite clay. I know also the physically and chemically bonded waters evaporate at 100 and 300 C respectively regardless of the attached element. However, is this true for hydroxyl units as well? Should the dehydroxylation of Si-OH happens at 700-800 C similar to that of Al-OH sequences?
In my book, Chapter 8, page 164, I write:
"In the early stage of development of silicone chemistry, it was also thought that intra-dehydroxylation might occur within the -OH group of the same Si atom with formation of the -Si=O oxide (Kiping, 1927). Yet, this has not been proven, so far (McGregor, 1955; Noll, 1968). In silicones, dehydroxylation would exclusively occur between two neighboring Si(OH)2 silanols. What happens for Si chemistry may not necessarily be true for Al. The steric arrangement displayed in Figure 8.7 shows that the V-fold coordination of Al results from the existence of this particular covalent sequence Si-O-Al=O."
I, personally, would not try to de-hydroxylate -Si-OH silicas into the siloxo link -Si-O-Si-. It is not reactive to alkalis. Go to Chapter 11, Silica-based geopolymers, Section 11.6.1. The de-hydroxylation temperature is in the range 200-250°C.
Dear Sina,
Prof. Davidovits is already on Research Gate, so you might ask to him directly. But I think that you have to understand that an amorphous material isn’t necessarily reactive to be used as precursor for geopolymerization process. You told about mixing of different glass powders , without other mineralogical and chemical informations is really difficult to know how much could be reactive your glass powder premix. Usually glass , that is amorphous of course , has composition very specific for example ceramic Frits are boron silicates , green bottle are rich on Iron and heavy metals , crystal is very rich on lead (more than 40%) and brown glass (for belgique beer) has inside Manganese and other dark metals to protect beer fermentation process from sunlight. Inside all of these glasses you have big lack of alumina so in terms of reactivity seem not good . For Geopolymer precursor you must find good source of amorphous silicon aluminate for this reason maybe your materials aren’t the best ones. When in the book you find this temperature, discussion was referred to metakaolin production using flash kiln, that permits to obtain the most reactive metakaolin . If you try to fire a glass at that temperature you will understand my words better. Some glasses will be already fusing , other will start to sinterize and other can have other reactions , depending on composition.
By the way, ask to Joseph directly , you will have more details about.
Best Regards
Alex Reggiani
Thanks Alex Reggiani for your answer.
Very helpful indeed. I have already sent a direct message to Prof. Davidovits and I am still waiting.
I am not using glass powder as the main precursor. I know it contains very little alumina. But I am trying to use it along with metakaolin in hope of consuming lower amount of soluble silica. I totally agree with you about different glass microstructural properties. The point is, my glass is a mixed type. I did XRD and XRF and I know the major characteristics of the material. I did the same process for the glass, as Prof. Davidovits mentioned in his book for metakaolin, and actually I found some enhancement on the final product. Unfortunately, I don't have easy access to NMR to figure out what is going on exactly, and I think TGA/DTA doesn't help in this case too. However, I am very curious to know what is happening scientifically. The dehydroxylation of Silica species was my initial thought and target, but I have no clue about the enhancement and its relationship with the dehydroxylation. This is why, I want to know, how do we typically dehydroxylate Si species?
Once again, thanks for your answer.
Best wishes,
Sina Dadsetan
In my book, Chapter 8, page 164, I write:
"In the early stage of development of silicone chemistry, it was also thought that intra-dehydroxylation might occur within the -OH group of the same Si atom with formation of the -Si=O oxide (Kiping, 1927). Yet, this has not been proven, so far (McGregor, 1955; Noll, 1968). In silicones, dehydroxylation would exclusively occur between two neighboring Si(OH)2 silanols. What happens for Si chemistry may not necessarily be true for Al. The steric arrangement displayed in Figure 8.7 shows that the V-fold coordination of Al results from the existence of this particular covalent sequence Si-O-Al=O."
I, personally, would not try to de-hydroxylate -Si-OH silicas into the siloxo link -Si-O-Si-. It is not reactive to alkalis. Go to Chapter 11, Silica-based geopolymers, Section 11.6.1. The de-hydroxylation temperature is in the range 200-250°C.
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