Our company is engaged in the development and manufacture of a mortar-epoxy-fiberglass material that is used in concrete forms, boats and other applications. It has been called to our attention that the reactivity of glass with concrete might significantly deteriorate some of the properties of our material. We have not yet noticed any appreciable deterioration that we can attribute to such a reaction. We are not certain whether the absence of the phenomenon is caused by certain additives we are using or to a lower degree of reactivity than usual.
The alkali aggregate reaction between glass and hydrated portland cement produces hydrous alkali silicates that may cause expansion and disruption when they develop in concrete. It is likely to occur most rapidly in moist concrete at temperatures of about 100 to 120 degrees F. Even at these temperatures the reaction is slow. The steady progressive buildup of expansive pressures in concrete may nevertheless destroy the structure. One method of control is to use a cement with an alkali content below 0.60 percent. Another is to include in the mix a suitable pozzolanic material such as fly ash or calcined diatomaceous earth so that the reaction is completed rapidly and harmlessly while the mix is still plastic. It is possible that you will have little or no trouble with this reaction. The epoxy in your material may reduce the amount of contact between mortar and glass. The material may also have such a consistently low moisture content that the reaction is further retarded. A good petrographer familiar with concrete could quickly tell you whether any damage has yet occurred in old pieces with long service records. If you are actually experiencing some deterioration you could adjust your mix with a pozzolan or use a low-alkali cement. A concrete testing laboratory could help in evaluating the adjustment of your mix by using an appropriate adaptation of the methods given in ASTM C 441 or C 227.