What causes hardening in cement or concrete?
This is very good question. The Concrete Primer by the American Concrete Institute answers the question this way. "When Portland cement is mixed with enough water to form a paste, the compounds of the Portland cement react and combine with the water to form a slowly developing cementious crystalline structure which adheres to and binds together the intermixed sand and stone particles into a hardened mass - commonly called concrete. This crystalline structure binds the mass together with growing strength and becomes very hard over time. As long as moisture is present this maturing crystalline structure of hydration products continues to add strength to the concrete mixture and may do so for many years but at a decreasing rate.
The institute says "decreasing rate," they mean an increase in strength but at a much slower pace than the initial strength gain. A concrete mixture can change from 500 pounds per square inch in compressive strength resistance at one day of age to 4000 pounds per square inch in 28 days of age. Remember, concrete needs to gain strength in environments of F 40 and above.