Head gasket replacement options

[Rus]

Update on the situation:

As I mentioned previously, I ordered the coolant testing kit and received it this week. Today I finally had a chance to perform the test as per the kit instructions. The test fluid showed no presence of combustion gases in the coolant. Now I am quite puzzled since it seems that the head gasket is now out of the question. Could the coolant be causing this issue? When I recently changed the hoses and thermostat I only drained the radiator. I did not drain the block, so only about 1/2 of the old coolant was replaced. After that procedure, the pressure accumulation didn't stop, but it was no longer as severe as before I performed the work. So could it be the coolant that has degraded? Before I replaced hoses and thermostat I tried to bleed the system and found that the coolant in the expansion tank actually boiled a few minutes after the car warmed up to operating temp. Today this did not happen (1/2 new coolant?). What exactly happens when coolant starts to break down?

Edit: Looked up some MSDS info on ehtylene glycol and found that it can produce CO and CO2 at its decomposition temperature of 413 degrees C. This obviously is not relevant in a car engine. What is of interest is that ethylene glycol is not very compatible with acids or bases. Thus, the pH of the coolant mixture may have an adverse effect on ethylene glycol. I guess I'm looking at a full drain, flush, and refill of the system as the next step.