Complex sentence: At the time, the boilers were sealed with a cement-like substance which required days-if not weeks-to set properly, time would not allow this and the seal quickly failed once the trial runs were restarted.
Split into simpler sentences: At the time, the boilers were sealed with a cement-like substance which required days-if not weeks-to set properly. Time would not allow this and the seal quickly failed once the trial runs were restarted.