MTA (Mineral Trioxide Aggregate) requires moisture to set because that is part of the chemistry of its setting reaction. MTA is derived originally from Portland cement.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5573456/
However, this does not mean that the root canal should be soaking wet
before MTA is placed.
MTA is used in situations where there is inevitably moisture (blood or tissue fluid) within a part of the tooth or root canal.
For example:
1. Sealing perforations
2. Sealing open root apices
3. Apexification of immature primary teeth
4. Dressing pulp stump wounds after a pulpotomy
In these situations, moisture contamination means that conventional root canal sealers will eventually wash out. MTA is able to set in the presence of moisture and is very biocompatible. This means that cementum and bone can grow onto the exposed MTA surface. This is especially important for healing of teeth with perforations or open root apices.
Since the root canal system itself still has to be thoroughly dried before MTA is placed, there is not enough moisture left behind for bacteria to proliferate. As the MTA sets, it crystallizes and water is needed for this process to occur.
In any case, root canal treatment does not completely sterilize (completely *remove* ALL viable microbes) the root canal system because bacteria are found within the dentinal walls and within complex anatomy (isthmuses and anastomoses) that cannot be directly instrumented. The aim of root canal treatment is to disinfect (*reduce* the amount of viable microbes) within the root canal system to a level where there are no symptoms and healing can occur.
