The formation of CaCO3-based binder by carbonating high-dosage Ca(OH)2 + slag + NaHCO3 (HCHSN) cement paste

Normally, carbonation can cause the decalcification of C-(A)-S-H gel of alkali-activated cement and the degradation of mechanical properties. While, we propose another possibility to form CaCO 3 -based binder by carbonating the alkali-activated slag cement of high-dosage Ca(OH) 2 (30 %) + slag (70 %) + NaHCO 3 (5.11 % by weight of Ca(OH) 2 and slag) (HCHSN) in this study. The results supported that the compressive strength and volume stability of HCHSN cement paste can be improved by the carbonation curing and the carbonated HCHSN cement paste could reach a maximum compressive strength of 32.4 MPa. According to the micro-analysis of XRD, TGA, FTIR, 1 H NMR, BSE and SEM, the carbonated HCHSN cement paste was developed into a CaCO 3 -based binder with a CaCO 3 content approaching 60 % (based on the mass of the samples heated to 800℃). NaHCO 3 played a key role in the formation of the CaCO 3 -based binder, which not only accelerated the carbonation rate, but also promoted the carbonation of slag. The eco-efficiency assessment showed that compared to the production of 12.5 kg CO 2 /MPa/t from carbonated ordinary Portland cement, the carbonated HCHSN cement paste only produced 3.5 kg CO 2 /MPa/t, making it a very green CaCO 3 -based cementitious materials. • The alkali-activated slag was transformed into a carbonate binder, where composite phases binding CaCO 3 and unreacted slag. • Carbonation curing provided HCHSN cement paste with good compressive strength, volume stability, and refined pore structure. • Carbonation promoted the conversion of CaO to CaCO 3 in the slag and the maximum CaCO 3 content was close to 60 %. • The carbonated HCHSN cement paste was a very green CaCO 3 -based cementitious materials.