Just had our wood burning fireplace chimney relined by a licensed chimney company. They used stainless steel liner down to the smoke chamber. They lined (parged) the smoke chamber itself with regular type-S cement. I understood they should have used a heat resistant mortar like Chambertech, but they insist that it's not necessary. Are they right? If not, will it likely still last 50 years or fall down when we first set a fire? The fire will only be used occasionally (say once a week in the winter).