Dark patches or efflorescence migrate through stone.
Use the site evidence to agree a project-specific method with the responsible engineer, concrete producer, manufacturer or specialist applicator.
Dark patches or efflorescence migrate through stone.
Check stone porosity, water source, adhesive and grout compatibility.
Moisture migration or incompatible fixing material can discolour porous stone.
Stop water source and select tested stone-compatible setting and sealing materials.
Make a sample panel with proposed materials and actual stone before full work.
Observe sample through wet and dry cycles and confirm appearance.
Avoid acid cleaning or sealing before identifying the stone and stain mechanism.
References inform the topic; they do not endorse a specific product for this project.