My first guess was the Temple of Protunus and the mysterious round temple next to it that is usually attributed to Vesta (but probably isn't for her). Both no longer have cement between the columns (and I'm not sure if they ever did) but are ancient for Roman standards in that they date to the Republican period.
There are three victory temples that have been incorporated into San Nicola in Carcere along the same stretch of the Tiber, all dating from the Republican period but with evidence of rebuilding in the Imperial period. There is brick between the columns, perhaps this is what you recall?
There is an archaic temple in the area (pre-Republican, maybe that qualifies as very ancient?) but it is buried underground, so I don't think you saw it. If you did, I'm very, very jealous!
If you can't find your answer here, you may want to check out the Oxford Archeological Guide to Rome, if there is an ancient block in Rome, they document it!