You can test if it is lime by dropping it in acetic acid / vinegar / other acid - lime fizzes, cement does nothing. It also makes a different sound that cement when you hit it with a trowel.
Current hypothesis- water drawn up between layers of render as it’s touching ground. Can’t escape through cement render, goes inside instead. We have v.mild damp at base of this wall.
You can test if it is lime by dropping it in acetic acid / vinegar / other acid - lime fizzes, cement does nothing. It also makes a different sound that cement when you hit it with a trowel.
Can you post more photos of the lower walls?