I was reading about Roman concrete and couldn't find any information on the but how did ancient people identify materials such as quicklime or different metals when many of these chemicals were found bonded to other stones. How did they discover or identify aluminum?
Well, Aluminium wasn't fully extracted and identified until 1825 by Danish scientist Hans Christian Oersted.
Other elements and compounds, though, can be occasionally found in their "pure" states, or at least an easily-worked oxidized state, in nature (like sulfur, salt, copper, iron, and occasionally lead), or can be generated by comparatively technologically simple means (like burning limestone to get quicklime, or getting phosphorus from urine). Some of these, we actually know when, where, and by whom they were discovered (e.g., phosphorus was discovered by Hennig Brand in 1669).
As for how substances were identified? Same way geologists and such still do it today: by its physical properties. Sulfur is yellow and smelly; quicklime will burn your skin unless it is slaked; mercury is liquid at room temperature; salt is crumbly and tastes salty; lead is silvery-grey and very malleable with a low melting point; etc.
Identification of metal ores I believe is not terribly difficult to do visually. Cinnabar (mercury ore) is red, malachite (copper ore) is green, etc. Additionally, the same sorts of rocks tend to show up in the same places. If you got real gold out of an ore vein once, chances are the rocks 100m down the valley are going to be pretty similar in composition. Different ores can be reduced, melted, or smelted at different temperatures, which also helps with separating metals.
