Study on high-efficiency CO2 absorption by fresh cement paste

Abstract

In order to achieve high efficiency absorption of CO2 by fresh cement paste, a special CO2 absorption device was designed. Then the influences of stirring rate (SR), water-to-cement ratio(W/C), superplasticizer type and its addition order on the CO2 absorption rate (CO2 AR) and CO2 ultimate absorption amount(CO2 UAA)were investigated. Moreover, the fluidity and mechanical properties of cement paste with different CO2 absorption amount were determined. According to the experimental results, with increasing absorption amount of CO2, the cement paste gradually loses fluidity and becomes hardened, indicating CO2 absorption of cement paste has reached to CO2 UAA. Meanwhile, increasing SR can effectively increase CO2 AR, while the CO2 UAA cannot be increased. Increasing W/C results in an increase in both CO2 AR and CO2 UAA. On the other hand, superplasticizer added before CO2 absorption neither can help to increase CO2 UAA nor improve fluidity of the carbonated cement paste. Superplasticizer added after CO2 absorption can improve fluidity of the carbonated cement paste but is barely helpful for increasing the CO2 UAA. The influence of CO2 absorption on early mechanical strength of the carbonated cement mortar is not obvious while the later mechanical strength of the carbonated cement paste can be improved. SEM and EDS analysis indicate that with an increase in CO2 absorption amount (CO2 AA), water in the cement paste is gradually consumed, forming columnar crystals of ettringite and calcium carbonate and increasing the cement particles and the flocculation structure per unit volume of cement paste. As a result, cement paste gradually loses its fluidity.