If the mortar has not had a chance to fully cure, the expanding crystals will destroy the bond from within the mortar. Although there may be no visible damage, the job is ruined and will never have a proper bond. If freezing occurs after the mortar has cured, there will be no damage. A second problem, caused by slow curing times, involves grouting. To prevent this, wait an extra 2-4 days, until the mortar has cured sufficiently.
Heaters do help, but can also be the cause of other problems.
The dry out the winter air, which is already low in humidity. This delays and may even prevent curing in the heater's immediate area. They also give off carbon monoxide, a deadly poison, which requires thorough ventilation on the site. Use damp curing techniques to ensure adequate moisture will be present for at least three days. Make sure there is adequate ventilation.