Effect of the composition of well-hydrated slag cement pastes on their reactivity as SCMs after enforced carbonation

Recent studies have shown that enforced wet carbonation of recycled cement paste (RCP) is able to provide these materials with a pozzolanic reactivity, enabling their application as SCMs in new cement or concrete. This pozzolanic behavior results from a Si-Al-gel created during the carbonation reaction of hydrated cement paste. The original composition of the hydrated cement paste has a major effect on the composition of these gels and their structure, which also affects the pozzolanic reactivity of such carbonated materials. This study investigates the effect of the composition and structure of the Si-Al-gel formed during enforced carbonation on its pozzolanic reactivity. For this, various cement pastes that contained ground granulated blast furnace slag in various amounts were prepared. The samples were hydrated, sealed for at least 120 days, and consequently carbonated immersed in a 0.1 mol NaOH solution through which gaseous CO2 was bubbled. After carbonation, the samples were dried in an aerated oven at 40 °C. The resulting carbonated recycled cement pastes (cRCP) were then investigated for their pozzolanic reactivity according to the R3 test. The results indicate apparent differences between the reactivity of the various cRCP samples. The cRCP obtained from the CEM III showed the highest heat of hydration in the R3 test, while the cRCP from the CEM I showed the lowest results of all cRCPs. This correlates with the fact that the carbonated CEM III showed the highest amorphous content and lowest bulk Ca/Si ratio of all cRCPs, while the carbonated CEM I showed opposite results. To elucidate these observed differences in reactivity in more detail, the gel structure of the cRCPs is currently investigated by Fourier-transform infrared spectroscopy (FTIR) and 29Si as well as 27Al NMR. This study helps to identify promising candidates of RCP to be used as SCM after carbonation treatment.     «

Recent studies have shown that enforced wet carbonation of recycled cement paste (RCP) is able to provide these materials with a pozzolanic reactivity, enabling their application as SCMs in new cement or concrete. This pozzolanic behavior results from a Si-Al-gel created during the carbonation reaction of hydrated cement paste. The original composition of the hydrated cement paste has a major effect on the composition of these gels and their structure, which also affects the pozzolanic reactivit...     »