Can you use mortar under an in ground pool liner?

It is generally not recommended to use mortar under an in-ground pool liner. Mortar is a cementitious material that can eventually crack and deteriorate when exposed to the constant moisture and chemicals present in a swimming pool environment. This can lead to leaks and damage to the pool liner. Instead, it's advisable to use a specialized pool underlayment or cushion that is designed to provide a smooth, even base for the pool liner and help protect it from punctures and tears. These underlayments are typically made from materials like geotextile fabric or foam and are specifically engineered for use in swimming pool applications.