EFFECT OF BORAX ON CALCIUM SULFOALUMINATE CEMENT PROPERTIES

This work reports the effect of borax on the performance of calcium sulfoaluminate (CSA) cements which contain different amounts of gypsum. The heat evolution, setting time, compressive strength, dimensional stability and hydration products were investigated. The results indicated that borax significantly retards the early hydration and setting time of CSA cement. The setting time is prolonged with an increased amount of additional borax. The addition of gypsum influences the early hydration of CSA cements with borax. The early strength of the mortars with borax suffers a decrease for mortars containing 5 % gypsum. The addition of borax leads to an increase in the mechanical strength of the mortars with 15 % and 20 % gypsum. An appropriate amount of borax as a retarder is beneficial to the strength development of CSA cements. The shrinkage and expansion rates of the cements with borax are limited. The presence of borax inhibits the hydration of ye'elimite and the depletion of gypsum at 1 day. The effect of borax on the ettringite formation is related to the gypsum content of the CSA cements.