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Why does slag cement concrete sometimes temporarily turn green?

Slag cement concrete sometimes turns green for a short period of time due to the oxidation state of sulfide sulphur compounds during portland/slag cement hydration.  This is normally a temporary condition and the concrete, upon exposure to air and sunlight, will ultimately become lighter than 100% portland cement concrete.  The condition is prolonged with extended moist curing, or on formed surfaces, especially when the form is left on for extended periods.  Often, slag cement concrete that is many years old will manifest greening when the concrete is broken up (in the non-surface interior portions of the concrete).

Greening is not harmful, and is almost always temporary.  In virtually every case where concrete is exposed to air, the greening will disappear and the concrete will be lighter that concrete made with other cementitious materials (see related FAQ on concrete color and article on Canadian Embassy, where greening occured, then disappeared). However, applications that will not be exposed to air and will be constantly moist may have permanent greening, such as swimming pools; therefore SCA suggests that slag cement not be use in this type of application when aesthetic concerns are important (greening will have no effect on other concrete properties, such as strength or durability). 

SCA normally suggests that concrete that displays greening be left alone, and the greening will disappear.  However, if the greening period is extended and waiting is not an option, applying a solution of hydrogen peroxide can help oxidize the concrete.  Another effective solution is calcium hypochlorite (usually used for pool shock).  Make this into a paste, apply and leave for a few minutes, wash off thoroughly, and let it take effect.

More information can be found in SCA's SCIC #10 and in a "Problem Clinic" discussion in Concrete Producer magazine