When the work does not match what was contracted: what people did, and the one thing that ends your leverage

A different material was installed than the one specified. Having been through it and asked others, the sequence matters more than the argument.

Do these first, before saying anything

• Photograph everything, including labels, packaging, stamps on the material and anything left in the skip. Once it is covered or removed, the evidence is gone.
• Find the written specification. The quote, the contract, the email confirming the choice. The whole thing turns on what was written down.
• Write down dates. When you noticed, what you were told, by whom.

Then

• Raise it in writing, factually, without a demand attached. "The contract specifies X and Y appears to have been installed, can you confirm" gets a much more useful answer than an accusation. Sometimes it is a genuine supply substitution nobody told you about.
• Ask for the remedy you actually want, and know which one that is: replacement, a price reduction, or completion of the missing parts. These are very different conversations.
• Keep it to the specification. Arguments about quality are subjective; the difference between what was contracted and what was installed is not.

The one thing that ends your leverage
Paying the final instalment. Almost everyone said the same: the outstanding balance is the only real leverage, and it disappears the moment it is paid. Withhold a proportionate amount, in writing, with the reason stated, and keep paying for the parts that were done correctly.

Worth knowing: manufacturer warranties on materials often depend on correct installation, so a substitution can void cover you thought you had. That is frequently a bigger cost than the material difference.

Rules and remedies vary by country, and for anything large it is worth actual professional advice rather than an internet consensus.