Research on effect of limestone and gypsum on C3A, C3S and PC clinker system

The effect of substitution of limestone for gypsum on hydration rate, hydration products, setting time and strength development of C3A, C3S and PC clinker systems were studied separately. Following results were obtained: The hydration of C3A was greatly stayed by gypsum, and that CaCO3 accelerated the hydration of C3A. There were ettringite and C3A·CaCO3·11H2O as products of C3A–CaCO3–CaSO4·2H2O paste but no C3AH6 was detected. That’s the reason why setting time of C3A–CaCO3–CaSO4·2H2O paste could be controlled by partial substitution of CaCO3 for CaSO4·2H2O in proportion of 20% ∼ 60% which was also favorable to strengths of C3A–CaCO3–CaSO4·2H2O paste. Both CaCO3 and CaSO4·2H2O could accelerate hydration of C3S and increase the strengths of C3S paste. CaSO4·2H2O enhanced the strength of C3S paste by going into C–S–H in some way and CaCO3 did it by reacting with Ca(OH)2. The enhancement of CaCO3 to C3S paste was less than that of CaSO4·2H2O. CaCO3 and CaSO4·2H2O coexisting were favorable to later strengths of C3S paste. The results of PC clinker with limestone and gypsum were in agreement with those of C3A–CaCO3–CaSO4 ·  2H2O system and C3S–CaCO3–CaSO4·2H2O system. The properties of PC clinker with gypsum and limestone as regulator for setting time were similar to those with gypsum alone when the substitution of limestone for gypsum was controlled below or equal 50%.