An untapped source of amorphous SiO2, industrially generated Indian biomass ash (SA)—90% amorphous, with composition of ~60% SiO2and ~20% unburnt carbon—can be used to produce cementitious and alkali‐activated binders. This study reports dissolution of amorphous Si from SA in 0.5 mol/L and 1 mol/L aqueous NaOH, with and without added Ca(OH)2, at SA:Ca(OH)2wt% ratios of 100:0, 87.5:12.5, and 82.5:17.5. Monitoring of elemental dissolution and subsequent/simultaneous product uptake by ICP‐OES offers an effective process for evaluating utility of industrial wastes in binder‐based systems. After 28 days in solution, up to 68% of total Si is dissolved from SA in 1 mol/L NaOH, with values as much as 38% lower in the presence of Ca(OH)2, due to the formation of tobermorite‐like C‐S‐H. FTIR,29Si MAS‐NMR, and XRD are used to characterize solid reaction products and observe reaction progress. Product chemistries calculated from ICP‐OES results and verified by selective dissolution in EDTA/NaOH—namely, Ca/Si of 0.6‐1 and Na adsorption of 1‐2 mmol/g—are found to be consistent with those indicated by aforementioned techniques. This indicates the efficacy of ICP‐OES in estimating product chemistry via such a methodology.
In situ quasi-elastic neutron scattering study on the water dynamics and reaction mechanisms in alkali-activated slags
In this study, in situ quasi-elastic neutron scattering (QENS) has been employed to probe the water dynamics and reaction mechanisms occurring during the formation of NaOH- and Na 2 SiO 3 -activated slags, an important class of low-CO 2 cements, in conjunction with isothermal conduction calorimetry (ICC), Fourier transform infrared spectroscopy (FTIR) analysis and N 2 sorption measurements. We show that the single ICC reaction peak in the NaOH-activated slag is accompanied with a transformation of free water to bound water (from QENS analysis), which directly signals formation of a sodium-containing aluminum-substituted calcium–silicate–hydrate (C–(N)–A–S–H) gel, as confirmed by FTIR. In contrast, the Na 2 SiO 3 -activated slag sample exhibits two distinct reaction peaks in the ICC data, where the first reaction peak is associated with conversion of constrained water to bound and free water, and the second peak is accompanied by conversion of free water to bound and constrained water (from QENS analysis). The second conversion is attributed to formation of the main reaction product ( i.e. , C–(N)–A–S–H gel) as confirmed by FTIR and N 2 sorption data. Analysis of the QENS, FTIR and N 2 sorption data together with thermodynamic information from the literature explicitly shows that the first reaction peak is associated with the formation of an initial gel (similar to C–(N)–A–S–H gel) that is governed by the Na + ions and silicate species in Na 2 SiO 3 solution and the dissolved Ca/Al species from slag. Hence, this study exemplifies the power of in situ QENS, when combined with laboratory-based characterization techniques, in elucidating the water dynamics and associated chemical mechanisms occurring in complex materials, and has provided important mechanistic insight on the early-age reactions occurring during formation of two alkali-activated slags.
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- Award ID(s):
- 1362039
- NSF-PAR ID:
- 10301637
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 21
- Issue:
- 20
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 10277 to 10292
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)Conventional drying of colloidal materials and gels (including cement) can lead to detrimental effects due to the buildup of internal stresses as water evaporates from the nano/microscopic pores. However, for these gel materials the underlying nanoscopic alterations that are, in part, responsible for macroscopically-measured strain values (especially at low relative humidity) remain a topic of open debate in the literature. In this study, sodium-based calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel, the major binding phase of silicate-activated blast furnace slag (one type of low-CO 2 cement), is investigated from a drying perspective, since it is known to suffer extensively from drying-induced microcracking. By employing in situ synchrotron X-ray total scattering measurements and pair distribution function (PDF) analysis we show that the significant contributing factor to the strain development in this material at extremely low relative humidity (0%) is the local atomic structural rearrangement of the C-(N)-A-S-H gel, including collapse of interlayer spacing and slight disintegration of the gel. Moreover, analysis of the medium range (1.0–2.2 nm) ordering in the PDF data reveals that the PDF-derived strain values are in much closer agreement (same order of magnitude) with the macroscopically measured strain data, compared to previous results based on reciprocal space X-ray diffraction data. From a mitigation standpoint, we show that small amounts of ZrO 2 nanoparticles are able to actively reinforce the structure of silicate-activated slag during drying, preventing atomic level strains from developing. Mechanistically, these nanoparticles induce growth of a silica-rich gel during drying, which, via density functional theory calculations, we show is attributed to the high surface reactivity of tetragonal ZrO 2 .more » « less
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null (Ed.)The long-term durability of cement-based materials is influenced by the pore structure and associated permeability at the sub-micrometre length scale. With the emergence of new types of sustainable cements in recent decades, there is a pressing need to be able to predict the durability of these new materials, and therefore nondestructive experimental techniques capable of characterizing the evolution of the pore structure are increasingly crucial for investigating cement durability. Here, small-angle neutron scattering is used to analyze the evolution of the pore structure in alkali-activated materials over the initial 24 h of reaction in order to assess the characteristic pore sizes that emerge during these short time scales. By using a unified fitting approach for data modeling, information on the pore size and surface roughness is obtained for a variety of precursor chemistries and morphologies (metakaolin- and slag-based pastes). Furthermore, the impact of activator chemistry is elucidated via the analysis of pastes synthesized using hydroxide- and silicate-based activators. It is found that the main aspect influencing the size of pores that are accessible using small-angle neutron scattering analysis (approximately 10–500 Å in diameter) is the availability of free silica in the activating solution, which leads to a more refined pore structure with smaller average pore size. Moreover, as the reaction progresses the gel pores visible using this scattering technique are seen to increase in size.more » « less
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Abstract Microcrystalline zeolites of the gismondine family are often reported in alkali‐activated and blended cement systems. However, little is known about gismondine's compatibility with other cementitious phases to determine stability in long‐term phase assemblage. Experimental studies were conducted to investigate the compositional field of gismondine stability in the lime‐alumina‐silica‐hydrate systems, with a particular focus on understanding the compatibility of gismondine with other cement phases such as C‐S‐H, ettringite, monosulfate, strätlingite, katoite, gypsum, calcite, portlandite, alkali, silica, and aluminosilicate phases. Results show that gismondine‐Ca forms readily at ~85°C in high aluminosilicate compositions; and persists in the presence of calcite, gypsum, ettringite, katoite solid solution, low Ca tobermorite‐like C‐S‐H, silica and aluminosilicate phases, at 20‐85°C. However, gismondine‐Ca reacts with: (a) monosulfate, producing ettringite‐thaumasite solid solution; (b) portlandite, forming tobermorite‐like C‐A‐S‐H gel and siliceous katoite at >55°C; (c) aqueous NaOH, generating gismondine‐(Na,Ca), a garronite‐like zeolite P solid solution; and (d) strätlingite leading to the conversion of strätlingite to gismondine indicating the metastability of strätlingite with respect to gismondine at 55°C. The outcomes are discussed to provide insights into the long‐term phase assemblage of relevant cement systems such as lime‐calcined clay, alkali‐activated materials, and potentially ancient Roman concrete.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9cp00889f
