PURPOSES : Experimental findings pertaining to the mechanical properties and microstructures of calcium sulfo-aluminate (CSA) cement and amorphous calcium aluminate (ACA) cement based-repair mortars incorporated with anhydrite gypsum (AG) are described herein. METHODS : To prepare the mortars, the CSA or ACA as binders were adopted and the ratio of water–binder was fixed at 0.57. For comparison, mortar made of Type I ordinary Portland cement (OPC) was prepared. The fluidity, setting time, compressive and bond strengths and absorption of the mortars were measured at predetermined periods. In addition, the microstructures of paste samples using OPC, CSA or ACA were visually examined through SEM observation. RESULTS : The ACA-based mortars showed the increases in the fluidity, and the acceleration of the setting time. Furthermore, the ACAbased binder effectively enhanced the compressive and bond strengths of the mortars owing to amount of formation of C2AH8 hydrates. Meanwhile, the mortar with ACA showed an excellence absorption. CONCLUSIONS : Comparing with those of CSA-based mortars, the mechanical properties of ACA based-mortars were more remarkable. However, further studies regarding the durability of repair mortars using aluminate-based binders must be conducted to obtain the optimal mixture.

Concrete strength is not only an important factor in design and quality control, but it also represents the overall qualityof concrete. The use of admixture has been increasingly prevalent in the recent cases of concrete production as a meansto improve the functionality of concrete. Of particular note, fly ash is added in either the cement or the ready-mixedconcrete production stage with the general mixing ratio being about 15%; however, using fly ash slows down the initialhydration of the binding material, which can in turn cause a delay in acquisition of strength. In this study, calcium sulfoaluminate (C4A3S; CSA) was added to improve the initial strength of cement after the use of fly ash, and its effect instrength improvement was analyzed. The substitution ratios of fly ash were 0, 10, 20 and 30%, and the amount of CSAadded to improve the initial strength was 8% of the fly ash weight. The results of the experiment showed that adding CSA resulted in high calorific values at peaks 1 and 2 of hydration heat, and an X-ray diffraction analysis showed thatthe amount of unhydrated materials was higher with increasing substitution ratio of fly ash. An increase in CSA wasalso shown to lead to a higher amount of ettringite being generated in the early ages. In conclusion, addition of 30% flyash and 8% CSA led to an ettringite production that was 3 times higher than the mixing ratio of fly ash, which effectivelyimproved the initial strength. The same phenomenon was observed in the electron microscope analysis. Based on theseresults, it was determined that adding CSA in an amount that equaled to 8% of fly ash weight can promote the productionof ettringite, thereby improving the initial strength, which gets reduced by the use of fly ash.