Mulch is, as you know, a protective covering of rotting vegetable matter spread to reduce evaporation and soil erosion. Primarily, it should comprise of vegetable matter. Since it seems that you have access to a lot of sawdust, you may use it but only after understanding the full implications of using it.

First of all, you must ensure that the sawdust is derived from wood that has not been treated for termite prevention and any other toxin for durability. Assuming that it is not, you should know the following two important elements of using sawdust as mulch.

Soil microorganisms need nitrogen as well as carbon for completion of the decomposition process. Wood chips, straw and sawdust have more carbon content, which means that the soil microorganisms will draw nitrogen from the soil causing nitrogen deficiency. This can have a negative effect on plant growth.

However, the issue of nitrogen deficiency can be addressed by using a fertilizer with high nitrogen content. An organic solution is to either let the sawdust decompose first or mix it with grass clippings, which are rich in nitrogen.

The second point to keep in mind is that if you decide to decompose sawdust first before using it, then you must stir it periodically. The reason is that if large heaps of any organic much is left to become soggy and compressed, it tends to become anaerobic and extremely acidic. However, you will get adequate warning if that happens because the heap will start giving a sour, ammonia smell.