Abstract
In order to compare the properties of alkali-activated carbon steel slag (CSS) and stainless steel slag (SSS), the effects of sodium hydroxide/sodium silicate solution mass ratio (NH/NS), liquid/solid ratio and blast furnace slag (BFS) dosage on the compressive strength, hydration products and hydration degree of CSS and SSS were studied. Furthermore, a combination of X-ray diffraction (XRD), thermo-gravimetric analysis coupled with differential thermal analysis (TGA-DTA), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS) were used to characterize the morphology and structure of alkali-activated CSS-BFS and SSS-BFS cementitious materials. As the results revealed, the primary hydrate of alkali-activated CSS and SSS is C-(A)-S-H with Q2 [SiO4] units, which has a low Ca/Si ratio and includes inert phases like a CaO-FeO-MnO-MgO solid solution (RO) in CSS while cuspidine, magnesiochromite etc. in SSS. More active C3S and β-C2S promote the alkali activation of CSS, whereas the less active γ-C2S hinders the depolymerization of SSS. The incorporation of BFS does not change the hydrate, whose seed effect is helpful for accelerating the depolymerization and polycondensation of CSS and SSS, especially for SSS, and makes the hydrate increase significantly. Owing to the high SiO2 and Al2O3 contents of SSS, the C-(A)-S-H chain length is increased, thus facilitating the polycondensation effect. In this study, the optimal NH/NS of CSS and SSS is NH/NS= 1:2, and the optimal liquid/solid ratio is 0.29. Compared to CSS–BFS, the C-(A)-S-H gel produced by SSS–BFS has lower Ca/Si and Al/Si ratios. Unlike CSS, pure SSS is inappropriate as an alkali-activated precursor and needs to be co-activated with BFS.
Highlights
With the continuous development of global steel industry in recent years, bulk solid wastes like steel slag and blast furnace slag (BFS) have tended to grow
In view of the wide application of BFS in geopolymers, and the precedent of combining carbon steel slag (CSS) with BFS as an alkali-activation precursor [9,33], this paper comparatively investigates the effects of sodium hydroxide/sodium silicate (NH:NS) solution mass ratio, liquid/solid ratio and BFS dosage on the polymer strength, non-evaporable water content, hydrates and the microstructure
C2-29-50 and S2-29-50 can attain higher early compressive strengths, and their 90 d compressive strength are higher than the P.O 42.5 cement
Summary
With the continuous development of global steel industry in recent years, bulk solid wastes like steel slag and blast furnace slag (BFS) have tended to grow. Assuming that the CSS accounted for 15–20% of crude steel production [6] and that 1 t of SSS was produced for every 3 t of stainless-steel production [7], the global productions of CSS and SSS exceeded 250 and 16 million t, respectively, in 2018. BFS is widely used in the production of cement and concrete, while steel slag receives little utilization because of its low activity [8,9,10]. Steel slag is used mostly for low value-added applications, including asphalt concrete aggregates, fillers for foundation engineering, supplementary cementitious materials, etc. Researchers in various countries are committed to seeking effective ways to enhance steel slag and increase the utilization rate of steel slag
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