The primary difference between MTA and Portland cement is that, MTA: (A) Lacks potassium, (B) lacks Al, (C) lacks Mg, (D) lacks Fe
Answer is A. The primary difference between both types of MTA and Portland cement are a lack of potassium and presence of bismuth oxide. White MTA contains lower amounts of Fe, Al and Mg than grey MTA.
ref. MTA review, JOE 2010, pg 17