French to English Tech/Engineering Materials (Plastics, Ceramics, etc.) Refractory

This is from a document regarding waste incinerators and a refractory process. I've come across "debris flows" as the only possibly viable translation of "laves" in this context, but I'm not convinced this is correct. The full sentence is as follows:

Plus les déchets sont chargés en fondants (Na, K, Ca….) plus les laves sont agressives pour les matériaux réfractaires ; de plus la présence de ces fondants fait baisser la température de solidification des laves.

Thank you!