effects of cold weather on portland cement

My husband and I are planning to have one and a half inch thick bluestone layed on an existing 30 year old poured concrete patio that is over a garage. While we are not able to see any obvious cracks in the concrete slab, there is an area toward the middle where water leaks into the garage below. Our mason plans to set the bluestone in portland cement with a sealer mixed in. I have 2 questions: Should we also apply a liquid sealer such as drylock beforehand. Also, we are doing the job late in the fall in New England when temperatures are freezing at night. Will the cooler weather prevent the portland cement from properly curing? I have read about hydration problems with concrete. Does the same problem occur with portland cement and grouts and mortars?

Thanks for your help!

Susan