“The Romans never wrote the formula down.”
5 sources: berkeley.edu, edu.au, imperiumromani.com, uh.edu, unl.edu. If this refers to Roman concrete, Vitruvius did write down recipes and proportions for lime mortars and concrete ingredients. The University of California, Berkeley and a University of Nebraska–Lincoln study both describe concrete recipes recorded by Vitruvius. Those written recipes may not capture every building practice or the exact mix used in every Roman structure.
checked on the web by Perplexity
What Was Roman Concrete? The Secret of Rome's BuildingsUniversity of Nebraska - Lincoln
To improve today's concrete, do as the Romans did