I thought it was calcium chloride. Might be remembering wrong but that's whats popped into my head when I read the question.
looks like both...from the pendot site:
PennDOT utilizes an anti-icing program by applying liquid magnesium chelorid, liquid calcium chloride or salt water to the road before snow starts falling. This is done to prevent precipitation from forming a strong, icy bond with the pavement. Anti-icing is done on I-95, 1-76, and I-676 in Philadelphia; on the US 202/PA 611 Bypass, PA 132, US 13, I-95, PA 413, PA 313, PA 309 in Bucks County; on US 1, US 202, US 30 and PA 100 in Chester County; on the US 1 Bypass, I-476, and I-95 in Delaware County; and on I-76, I-476, and US 422 in Montgomery County.
I did not correct their spelling... ;-)