Why does slaked lime water lose alkalinity over time? When I throw 3 handfuls of white lime $\ce{Ca(OH)_2}$ into 60 gallons of tap water, the initial pH value is 11.4, but unknowingly it changes to pH 7. 

What is the chemistry behind it? 

How long does it take to completely change to neutral pH at open space? 

I put lime in the water to pasteurize straw for mushrooms, and I need to know how long and how to make neutral pH faster so that I can pour the neutral water in the soil, so it won't ruin the soil.
If you dissolve slaked lime in water it will absorb carbon dioxide from the atmosphere over time.
Carbon dioxide will react with hydroxide ions from the slaked lime forming mainly hydrocarbonate ions. Hydrocarbonate ions will equilibrate with water and at about pH &lt; 8 carbon dioxide will slowly be liberated from the solution. After some time, no significant amounts of hydroxide ions will be left from the slaked lime and the pH of the solution will close into neutral pH.

Well, what will happen to the calcium ions from the slaked lime? You will probably find them as an insoluble deposit at the end. This is because carbonate ions will also be formed during the process, forming calcium carbonate of very low water solubility.