Found this on Castrol USA site:
Many engine failures are the result of improper antifreeze usage. To protect equipment from coolant–related damage, it is critical to know the different antifreeze types, required mix ratios and problems that could result from improper use. A 100 percent antifreeze solution will turn to a solid at 9°F (–13°C). For best all–around protection, use a 50:50 mixture of antifreeze and water. Concentrated antifreeze needs water for chemical balance and optimum performance.
Antifreeze to Water Mix
Coolant should not be mixed with hard tap water. Hard tap water has excessive calcium and magnesium deposits that can cause scaling, which will result in inadequate heat transfer. To avoid scaling, use only soft or de–ionized water that is not treated with salts or chlorides, when mixing water and antifreeze. (OEMs publish limits in parts per million (ppm) for hardness, chlorides, sulfates and total dissolved solids for the water used to dilute antifreeze.)