Hybrid binders based on alkali sulfate-activated Portland clinker and metakaolin

摘要

The development of hybrid binders based on combinations of conventional Portland clinkers and alkali-activated aluminosilicates as a mean of improving the sustainability of the cement production and reducing CO2 emissions is currently attracting increasing interest. The normal approach in these binders, as with pure alumino-silicate systems, is to use alkali silicate or hydroxide activators; although the production of these chemicals is energy intensive compared to alkali sulfates. New alkali-activated binders in the system Na2SO4 (NS) – white Portland clinker – metakaolin (MK) have been synthesized by designing the precursor mixes based on the desired final phase assemblages. Compared to conventional blended Portland cements, these systems have the added advantage of allowing the use of much higher amounts of supplementary cementitious materials (SCMs) (up to 60% MK of total binder) without any loss in performance. The activation-hydration products of these binders formulated with different contents of NS and MK/(MK+limestone) ratios have been primarily assessed by 29Si and 27Al MAS NMR along with powder X-ray diffraction (XRD) at 28 days of curing. 29Si MAS-NMR shows the formation of silicate chain structures with Al substitution (Q2(1Al) sites) within the binder. The incorporation of tetrahedrally coordinated Al in the C-S-H phase is also observed by 27Al MAS NMR. These spectra also reveal that at lower levels of NS addition, AFm phases (e.g. monosulfate and/or monocarbonate) and AFt phases (ettringite) are the main aluminate phases formed in approximately equal proportions, whereas an increased NS content in the binder leads to the formation of additional ettringite at the expense of the AFm phases. Formation of an X-ray amorphous aluminosilicate gel is also identified in all samples, and the content of this phase increases as a function of reaction time and the content of NS in the binder, which is associated with the activation of the MK included in the system.