Old school was calcium chloride mixed with water. CaCl is a salt that has the same effect as the salt used on icy roads and rusts out the rims in about 12 years. The advantage is that it is the heaviest liquid medium used for ballast at about 11.3 lb per gallon.
Rim Guard is the current best option. Freeze protection down to -35°, non-rusting, and environmentally safe at 10.3 lb per gallon.
Windshield washer antifreeze and plumbing antifreeze weigh about 8 lb per gallon and freeze protection and weight dependong on the strength used. The consensus is that it does not promote severe rusting and is relatively environmentally safe.
Water is acceptable if there is no risk of freezing, but rusting will occur. Weight is 8.3 lb per gallon.
Fill level is recommended at 75% of tire volume and various methods have been used. The easiest is to break the bead on the valve side of the tire and pour in the fluid.
Metal valve stems are required for Rim Guard and recommended for the other liquids.
Please note that humid air will also rust rims from the inside, just not as rapidly as water.
My snow clearing tractors get loaded tires as a matter of course, and have for the past 37+ years. Since most of those tractors are also equipped with FELs, ballast is essential and Rim Guard is the desired liquid. Unfortunately, it is sometimes not available locally and another option is required.
The tire valves do not get plugged up, but CaCl will do a serious number on the gauge used to check the pressure. Once a loaded tire is installed on the tractor and the pressure is correct, I measure from the ground to the top of the rim edge and note the measurement. From then on no gauge is required to get the same pressure. Just match the measurement.
Different brands of a given size of tire with a similar tread pattern can be quite a bit different in diameter if the tires are mixed on the tractor. The measurement method used for checking pressure can be used to get the same axle height even though the pressures can be substantially different.
Do not over pressure loaded tires. Loaded tires have a much smaller air chamber and bumps in the road spike pressures rapidly.
The upside is greatly improved traction. The downside is more expensive repairs of punctures. The last set of CaCl loaded tires on my GT went 12 years with only adding a bit of air once a year or so. My SCUT has had 3 punctures in the past 9 years. The last one cost $117 to repair and replace the lost fluid. The next time, it will cost a tire due to weather checking.
The combination of weather checking, tubes, and liquid ballast is not good. If the checking opens up on the inside of the tire due to higher than normal load on the tire, the tube gets pinched and torn. (That was the cause of my last flat. I installed the 800 lb hoe on the back of my SCUT, and then used the hoe to lift something without aid of the stabilizers.)
Automotive engine antifreeze is a NO-NO! It is toxic to animals if the tire leaks.
Depending on the tire and the liquid chosen, the weight of liquid ballast will equal or greatly exceed the weight of a set of expensive cast iron wheel weights. The 26x12-12 turf tires on my GT carried 160 lb of CaCl . . . each. The same weight in cast iron wheel weights will cost between $200 and $900, depending on what brand and where you find them, and when mounted on the wheel, they will stick out far enough to bang your shins on.
