Effect of SRA on the expansive behaviour of mortars based on sulphoaluminate agent

The effectiveness of an expansive admixture based on calcium sulphoaluminate, in the presence and in the absence of a shrinkage reducing admixture, is investigated in mortars manufactured with different water to binder ratios. The results show that the mortar free expansion increases with gypsum content, but the amount of mortar free contraction and the time to reach the mortar steady state remain unaffected. Moreover, the mortar expansion strongly depends on the adopted water to binder ratio; in particular the greatest expansion is reached when the highest water to binder ratio is used. A threshold water/binder value exists below which mortars do not expand. Addition of a shrinkage reducing admixture strongly amplifies the free expansion of mortars but not the mortar expansion loss that follows. On the other hand, the shrinkage reducing admixture does not affect restrained mortar expansion but only reduces restrained mortar shrinkage. The presence of the shrinkage reducing admixture slightly delays cement hydration, always decreases the mortar compressive strength and it changes dramatically the morphology of the ettringite fibers.