Black grout turning white on installation

We have had our floor re done 3 times now and the grout keeps turning white. What is happening the company is baffled and doesn't know why it is happening/?

Could be many things but the first three that come to mind are:

1. Mineral Deposits. Mopping and allowing water to sit on the grout and dry there. This could deposit minerals from the water onto the grout.

2. Efflorescence. Basically this is natural salts that occur in concrete and cement products such as grout that can at times (when moisture is present) rise to the surface in an effort to evaporate.

3. Sealer. Grouts that have been sealed with too much sealer can develope white areas. Sealers that have been applied over grout were moisture is present down below can turn white as the moisture attempts to rise.

Anything sound familiar?

What consistency is the grout when mixed? Thick like creamy peanut butter or runny like potatoe soup? Did the same contractor do it all three times? Is the grout powder black?