Abstract
Delayed strength development and long setting times are the main disadvantageous properties of Na2CO3-activated slag cements. In this work, combined auxiliary activators of Ca(OH)2 and Mg(OH)2 were incorporated in one-part Na2CO3-activated slag binders to accelerate the kinetics of alkali activation. The properties and microstructure evolution were investigated to clarify the reaction mechanism. The results showed that the additions of auxiliary activators promoted the hardening of the pastes within 2 h. The 28 days compressive strengths were in the range of 39.5–45.5 MPa, rendering the binders practical cementitious materials in general construction applications. Ca(OH)2 was more effective than Mg(OH)2 in accelerating the kinetics of alkali activation. The dissolution of Ca(OH)2 released more OH− and Ca2+ ions in the aqueous phase to increase alkalinity in the aqueous phase and promote the formation of the main binding gel phase of calcium-aluminosilicate hydrate (C-A-S-H). An increase in the Ca(OH)2/Mg(OH)2 ratios increased autogenous shrinkage and decreased drying shrinkage of the binders. The formation of a compact pore structure restricted the water evaporation from the binders during the drying procedure.
Highlights
Alkali-activated slag (AAS) cements are of growing interest as a clinker-free alternative to ordinary Portland cements (OPC), and the application has the potential to lower the CO2 footprint in the construction industry [1]
It is important to measure the volume stability of Na2 CO3 -activated slag cements before widespread field application. To cope with these challenges, we investigated the compressive strength and autogenous and drying shrinkage of one-part Na2 CO3 -activated slag cements modified with both Ca(OH)2 and Mg(OH)2
The setting times of Na2 CO3 -activated slag binders modified with Ca(OH)2 and
Summary
Alkali-activated slag (AAS) cements are of growing interest as a clinker-free alternative to ordinary Portland cements (OPC), and the application has the potential to lower the CO2 footprint in the construction industry [1]. The concentrated aqueous alkali (i.e., Na and K) hydroxide or silicate solutions are widely used as the activators for preparing AAS binders, facilitating the dissolution of the glassy phases in slag, polymerization of the dissolved ionic species, and precipitation of the C-A-S-H gel phase [11]. These strong bases of activators are still carbon-intensive and expansive, contributing to the majority of energy consumption and carbon emission in the production of AAS cements [12,13]. The dry mixtures of cements are prepared by premixing the solid alkali sources with slag, and can be used to OPC via the “just add water”
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