This revolutionary idea has me thinking. Plastic has been used for thousands of years--it's just that the modern definition and composition of "plastic" did not start until 1907. A good example of "plastic" is rubber used from a rubber tree. It was used by the Mesoamerican tribes as far away as the third millennium BCE.
The problem is that natural rubber trees are indigenous to the jungles of Latin America, and the rubber tree plantations used in India and China were cultivated from seeds taken by British scientists and financiers.
Let us pretend, then, that in an alternate Earth, a species of wild rubber is common in the Mediterranean coastlines of southern Europe and had been used by the Romans for cultivation for their sap. This would be the sort of natural plastic to be used for concrete construction, to make the structure last longer against the elements than subsequent "Portland reinforced" concrete. In OTL, volcanic ash already added resistance to Roman concrete, but in this alternate Earth, could using rubber, one of the most natural of plastics, have the same effect?