Effect of CaO on the Autogenous Shrinkage of Alkali‐Activated Slag Mortar

Abstract

The autogenous shrinkage of alkali-activated slag (AAS) is significantly higher than that of ordinary Portland cement (OPC). The higher risk of concrete cracking due to autogenous shrinkage is a critical drawback to wider use of this promising alternative binder. The effect of CaO content on the autogenous shrinkage of AAS mortar was investigated. The autogenous shrinkage of AAS mortars was determined by comparator. The pore structure of the pastes was determined by mercury intrusion porosimetry. The hydration products of the pastes were determined by Fourier transform-infrared, thermogravimetric analysis, X-ray diffraction, and 29Si solid-state magic-angle spinning nuclear magnetic resonance. The results show that the amount of portlandite increases as CaO content increases. CaO in the paste causes the partial replacement of C-S-H(I) (low stiffness) by C-S-H(II) (high stiffness). The hydration reaction of AAS is inhibited by the addition of CaO. The increase of polymerization degree of C-(A-)S-H and rearrangement of C-S-H(I) during hydration are inhibited by the addition of CaO, and micropores closure is also inhibited. Therefore, the autogenous shrinkage of AAS mortar decreases with the increase of CaO content.

Supercritical CO2-Induced Evolution of Alkali-Activated Slag Cements

“…The area contributed by the unreacted slag in the uncarbonated Na 2 SO 4 -activated sample is about 75%, which indicates the reaction degree of the slag in Na 2 SO 4 -activated samples is very low. The CaO- and MgO-activated samples consist of ~65% unreacted slag whereas the area of reacted slag has reached about 50% in NaOH- and Na 2 SiO 3 -activated slags [ 63 ]. The area contribution from slag has reduced drastically upon carbonation.…”