Low carbon cementitious materials: Sodium sulfate activated ultra-fine slag/fly ash blends at ambient temperature

摘要

Na2SO4 (NS)-activated slag as a kind of alkali-activated slag (AAS) had great development potential in low carbon cementitious materials field, because NS could be obtained from natural resources with much lower CO2 emissions than NaOH and water glass. However, the main issue was its slower strength development at both early and later ages. With intention to mitigate this problem, ultra-fine slag was used, and the increased CO2 emissions caused by ultra-fine slag expected to be balanced by the use of fly ash (FA). In the present study, NS-activated ultra-fine slag/FA was designed. Effects of NS dosage and FA content on the compressive strength were researched, and the activated process was assessed by chemical shrinkage, XRD, TGA, pH value, ion concentration, NMR, and SEM. Finally, CO2 emissions were evaluated. Results showed that compressive strength of slag with D50 = 3.1 mu m activated by 4% Na2O-E of NS reached 36.2 MPa and 77.3 MPa at 28 d and 90 d age. The main hydrates of NS-activated slag were ettringite and C-(N)-S-H. Moreover, in the NS-activated ultra-fine slag/FA mortars, 20% FA significantly reduced the 3 d compressive strength, but slightly increased the 28 d compressive strength. CO2 emissions of this blend seemed much lower than that of NaOH or water glass-activated slag. These results suggested one alkali-activated binder with low carbon emissions. (C) 2020 Elsevier Ltd. All rights reserved.