I used portland cement as a binder because it is already used in dental medicine to prepare MTA cement. It works based on a series of chemical reactions that in the end make a mixture of chemical substances which slowly crystalize. Process of crystalization can take up to few months. The strength of the final material depends on how long has it been left to cure. This process can also be sped up with other chemicals or soaking the object in warm water.

Firs I have started with making a mixture of hydroxyapatite with 10%, 20%, 30%, 40% and 50% of portland cement in it. All of the mixtures worked. The best results were observed with 30% portland cement and 70% hydroxyapatite mixture. This material also had one bonus as I didn't have to sinter it as it was already hard enough. There is still one problem though: I don't know the effects of portland cement on human body. If there aren't any it could be used without problems as it is the easiest to use and make.

Printing with cement is a bit of a nuisance, as the wetting area is usualy at least twice the size of the drop. This can be prevented by using alcochols with big concentrations. There is also one more problem as this material cures even in plain air. Because of this I had to get big lumps out of the feed cylinder every day.

This material could be used just by itself as it is very reliable and easy to use. It could also be mixed with some other powders to give it color or some specific texture, but do not use it in conjunction with metals as non cured cement is alkaline and corrosive. I would say that this materials is mostly for artists, because it does not have any particular use in technology or engineering