Effect of Na2CO3 on the tensile creep of slag-fly ash systems activated with Na2SiO3

Abstract

The use of Na2CO3 in alkali-activated system (AAS) has attracted an increased interest since it provides a suitable setting time, suitable mechanical properties and lower cost. However, the tensile creep behavior of Na2CO3-activated AAS remains unknown, which is crucial for controlling its early cracking. Therefore, this study aims to investigate the effect of Na2CO3 on the tensile creep of slag-fly ash systems activated with Na2SiO3 (ASF). The total Na2O equivalent (Na2O of hybrid activators/binder, by mass) was maintained at 6%. Various Na2CO3 contents (i.e. the ratio of Na2O equivalent of Na2CO3 to total Na2O equivalent) were used in this paper, namely, 0, 1/3, 1/2 and 2/3. The compressive strength, flexural strength, elastic modulus, internal relative humidity, autogenous shrinkage, tensile creep and microstructures (X-ray diffraction, thermogravimetry, mercury intrusion porosimetry, scanning electronic microscopy-energy dispersive spectroscopy and nanoindentation) were investigated for the ASF system. Results indicate that the tensile creep of ASF mortar (ASFm) firstly increases and then decreases as the Na2CO3 contents increases. ASFm with Na2CO3 content of 1/2 has the highest tensile creep, which is 236.3% higher than that of ASFm without Na2CO3. When the Na2CO3 content increases from 0 to 1/2, Na2CO3 promots C(N)-A-S-H gel formation, increases the autogenous shrinkage and decreases the contact creep modulus of gels, leading to the increases of the tensile creep of ASFm. However, excessive Na2CO3 (content＞1/2) reacts preferentially with Ca ion dissolved from slag to generate CaCO3, inhibiting the gel formation. Moreover, CaCO3 plays a role in limiting the gel deformation, which remarkably reduces autogenous shrinkage and increases the contact creep modulus of gels, and therefore the tensile creep of ASFm is reduced.