An excess of which substance was probably used in mixing the material during installation of a castable burner if the front breaks up after very little use?

When mixing materials for a castable burner, the balance of water is crucial for ensuring proper setting and strength of the material. An excess of water can lead to structural weakness in the castable material. This is because too much water will create a mixture that is too loose, resulting in inadequate bonding between the components once set. If the front of the burner breaks up after little use, it indicates that the material did not have the strength necessary to withstand the operational conditions it was subjected to, which could stem from overly diluted material.

In contrast, while solvents, sand, and cement play important roles in the mixture, their excesses would lead to different types of issues. An excess of solvent might impact the curing process, while too much sand could result in a mixture that's too dry and difficult to work with. An excess of cement could cause the mixture to become overly rigid but not necessarily weak. Therefore, it is the excess of water that is primarily responsible for the breakdown of the burner front after minimal usage.