Air Carrier Captain reported two fume events on subsequent days on the same aircraft. The fumes were described as fumes accompanied with visible misting. On both events the aircraft returned to the gate where passengers and flight crew were deplaned. Aircraft was assigned to maintenance.

Narrative

Flight Crew experienced two fume events on subsequent days while operating the same aircraft. The first event occurred during taxi out when the aircraft and crew experienced a lengthy ground delay for hazardous weather in the terminal area and subsequent gate return to add additional fuel to operate the flight on an alternate route to the destination.The ground delay before the aircraft was able to return to the gate took over 2.5 hours from original pushback from the gate. The aircraft operated on one engine and the APU for more than half of the time period. Once the aircraft was assigned a return gate to was safely able to taxi to it; the Captain directed the #2 engine to be restarted; in order to more safely taxi the aircraft in the confined apron area adjacent to the gate (Gate XXX). It was during this restart of the engine that the aircraft cabin experienced visible misting and fumes; which was reported to the cabin by the Lead Flight Attendant. Shortly following this call; the flightdeck also experienced fume odors as well (no misting). Flight crew performed fume isolation procedures ans use of supplemental oxygen to mitigate the impact of the fumes in both the cabin and flightdeck. APU was shutdown as part of the isolation procedure.After returning to the gate; the Flight Crew was released from duty and went into crew rest. The Flight Crew was subsequently reassigned to operate the same flight on delayed departure status the next afternoon; using the same aircraft from the night prior.The aircraft was inspected by Tech Operations and the APU was not available for the later flight under an MEL status. The Flight Crew performed an alternate engine start of the #2 engine at the gate; using an external HP air cart. However; shortly after the engine was started and the aircraft was being pushed back; the Flight Crew and in-flight crew noticed fume odors in both the flightdeck and cabin.The Captain determined that the flight needed to immediately return to the gate on one engine; in order to shut down the fume source and remove the crew and passengers from the aircraft as soon as possible. The effects of the fumes in the flightdeck nearly incapacitated the First Officer from being able to talk due to profuse reactionary coughing from the fumes.