Concrete is gravel, sand, cement, lyme, and water, mixed in various ratios.
There's a lot of variations and additives that can change how quickly it cures, often to speed it up or slow it down to account for weather (temperature and humidity play a huge part in how it cures), or to modify the pace of how it cures so you can keep building on it if you're building vertically.
It's a simple concept that gets incredibly complicated very quickly.
Big rocks, little rocks, cement, water.
This is false. They have to PROVE that the repair that you made caused the keypad to fail in other to void your warranty, at least in the USA. Most people are misinformed as you are however, and they'll TELL you that's the case to make you go away, despite it being illegal.
https://en.wikipedia.org/wiki/Magnuson-Moss_Warranty_Act