Hydration of alkali-activated slag: comparison with ordinary Portland cement

Abstract

A multi-method approach was used for the investigation and comparison of alkali-activated slag binders (AAS), pure slag and ordinary Portland cement (OPC). X-ray fluorescence, X-ray powder diffraction, granulometry, calorimetry, thermo-gravimetric analysis and environmental scanning electron microscope investigations of the microstructure with energy dispersive X-ray analyses were used to characterise the cements and their hydrate phases. In addition, the chemical composition of the pore solution, including the different sulphur-containing ions, was analysed. The precipitation mechanisms during binder hydration in the AAS and OPC systems exhibit significant differences: in AAS the formation of the ‘outer product’ C-S-H is much faster than in OPC. The high Si concentrations in the pore solution during the early hydration of AAS are related to the fast dissolution of Na-metasilicate. The fast reaction of Na is an important factor for the voluminous precipitation of C-S-H within the interstitial space already during the first 24 h. In addition to the Na-metasilicate component, the high fineness of the slag represents a further important factor for the fast hydration of AAS. The small slag particles (< 2 μm) are completely dissolved or hydrated within the first 24 h, whereas hydration of the larger particles is much slower. The fast formation of a gel-like matrix in AAS is the product of a fast ‘through solution’ precipitation, which contrasts with the slower dissolution-precipitation mechanism of a ‘topotactic’ growth of C-S-H in OPC. The chemical and mineralogical characterisation of solid and liquid phases and their changes with time are the basis for thermodynamic modelling of the corresponding hydration process, which is presented in a second paper.

Chemical acceleration of a neutral granulated blast-furnace slag activated by sodium carbonate

“…The concentration of Al is also high in all pore solutions, except the control paste, indicating that Al/Si ratios are considerably higher than that recorded by other researches [29][30][31]. The low Al concentration in the pore solution of the base paste additionally confirms a slow rate of dissolution of slag particles.…”

“…7 to Fig 9. Normally the microstructure of AAS paste is dominated by the relatively fast "through solution" precipitation of a gel-like C-S-H matrix in the pore space; and the internal space between the slag grains is readily completely filled with C-S-H gel after 24 hours when stronger activators (sodium silicate, sodium hydroxide) are used [29]. It can be seen that the control paste activated with sodium carbonate shows little hydration at early age (Fig.…”

“…The strength development of AAS concretes is governed by the interactions between solid and liquid components; that is dissolution rates, saturation/chemical equilibrium of the pore solution, precipitation or hydration rates and microstructural evolution [29]. The following sections are thus focused on these various aspects of AAS concrete hydration; and changes caused by the accelerating admixtures.…”

Influence of slag composition on the stability of steel in alkali-activated cementitious materials

“…Hummel et al [26] indicated by thermodynamic calculations that HS -is the dominant sulfur species under highly alkaline conditions, which will generate reducing conditions. However, Gruskovnjak et al [27] observed that SO 3 2-may dominate the sulfur speciation under moderately reducing conditions due to kinetic effects, whereas under more strongly reducing conditions, S 2 O 3 2-, HS -and a series of polysulfides will be dominant. The evolution of the speciation of sulfur during slag hydration has been determined by X-ray absorption near-edge structure (XANES) spectroscopy [13,28], where it was observed that the sulfur in an anhydrous granulated blast furnace slag is mainly present in reduced form, S 2-and S 0 , while in the activation of the slag, those sulfur species react with oxygen and alkaline solutions to form S 2 O 3 2-and SO 4 2-.…”

Hydration and properties of sodium sulfate activated slag

“…Other authors reported that strength of sodium silicate and sodium hydroxide activated slag increased with the increase in slag fineness [40] up to 6000 cm 2 /g [51]. Gruskovnjak et al [52] used a solid waterglass, solid Nametasilicate pentahydrate (Na 2 SiO 3 .5H 2 O) as a source of activator to activate slag. They reported that the high fineness of the slag represented a further important factor for fast reactivity, in which the small slag particles (< 2μm) were completely hydrated within the first 24 h after mixing, whereas hydration of the larger particles was much slower.…”