Abstract
The effect of magnesia on ternary systems composed of limestone, metakaolin and calcium hydroxide, alkali activated with sodium silicate, sodium hydroxide, and sodium sulphate was studied by determination of the compressive strength, X-ray powder diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM). Pastes activated with sodium silicate and sodium sulphate showed strength regression caused by a formation of an unstable prone to cracking geopolymer gel. The presence of magnesia in sodium hydroxide-activated system hindered this trend by promoting a formation of more stable crystalline phases intermixed with brucide. In general, magnesia densified the binder matrix by promoting a formation of amorphous phases while sodium hydroxide produced the most porous microstructure containing high amount of crystalline phases.
Highlights
It is estimated that the production of one ton of Portland cement clinker causes emission of approximately 800 kilograms of carbon dioxide into the atmosphere. e cement industry alone accounts globally for a staggering 8% share of a total annually manmade carbon dioxide [1]. e cement industry is continuously modernizing the production process and uses more secondary cementitious binders such as fly ash, limestone, or blast-furnace slag to improve the situation. ere are alternative cementitious binders including the so-called geopolymers; Provis et al [2] and Purdon [3]
E effect of magnesia on ternary systems composed of limestone, metakaolin and calcium hydroxide, alkali activated with sodium silicate, sodium hydroxide, and sodium sulphate was studied by determination of the compressive strength, X-ray powder diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM)
Addition of magnesia to systems containing limestone and metakaolin worsened the workability while addition of calcium hydroxide had no effect. e observed worsening of the workability can be directly related to increased surface area and presumably, as not determined, lowering of the particle packing density [30]
Summary
It is estimated that the production of one ton of Portland cement clinker causes emission of approximately 800 kilograms of carbon dioxide into the atmosphere. e cement industry alone accounts globally for a staggering 8% share of a total annually manmade carbon dioxide [1]. e cement industry is continuously modernizing the production process and uses more secondary cementitious binders such as fly ash, limestone, or blast-furnace slag to improve the situation. ere are alternative cementitious binders including the so-called geopolymers; Provis et al [2] and Purdon [3]. The incorporation of slag into the fly ash-based system shortens the setting time, increases the compressive strength, and densifies the microstructure by formation of phases composed from modified calcium silicate hydrate gel (C-S-H). It includes aluminum modified calcium silicate hydrate (C-AS-H) gel and sodium aluminosilicate hydrate (N-A-S-H) gel [19, 20]. Slow dissolution of fly ash can be compensated with application of higher curing temperature, enabling formation of cross-linked products, densification of the microstructure, and increasing the strength of the solidified binder matrix [21]. No additional conductive coating was applied to any of the specimens. e backscattered electron (BSE) detector was used to obtain all images
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