This is a measure of the amount of dissolved calcium in your pool water. Depending on the other balance factors, the Australian Standard recommends a range of 150-250ppm.
Calcium Hardness cannot usually be tested with the standard 4 in 1 test kit so you will need to take a water sample to your nearest pool shop or use a mobile service technician for testing.
Calcium is particularly important in concrete pools as unlike fibreglass or vinyl pools concrete is porous. If water does not have calcium it will look for it and the first place it is more than likely going to get it from is the grout on concrete wall.
Do you have calcium deposits visible on the pool? It is often confused with too much calcium...in actual fact in most cases this is calcium deposits as the water has pulled it from the surface. We recommend checking your levels and balancing the pool ensuring all levels are correct.
When adding calcium to the pool, use small amounts, run the filter and test the effect after several hours. Adding large amounts of any chemical to achieve large changes can result in large problems.
And always consult your local pool professional before buying chemicals or using them in your pool.