What was the earliest use of Portland cement?
You may not know it, but you made my answer more simple than it would have been if you had asked about the history of "concrete." That answer would have gone to the dawn of recorded history. However, my knowledge of Portland cement tells me the initial use was 1820. The Portland cement I was taught about was used in mortar as a replacement for lime. Masons became more experienced with Portland cement and began to make a concrete out of the mixture of stone, mason sand, cement and water.
Portland cement has increased in demand to become a construction industry material standard, most projects contain concrete or mortar made from Portland cement.