Incorporation of Al in C-A-S-H gels with various Ca/Si and Al/Si ratio: Microstructural and structural characteristics with DTA/TG, XRD, FTIR and TEM analysis
30 Nov 2017-Construction and Building Materials (Elsevier)-Vol. 155, pp 643-653
TL;DR: In this article, the system of different Ca/Si and Al/Si molar ratios were investigated and it was shown that incorporation of Al increases main basal spacing, amount of bounded water and decreases crystallinity of C-(A)-S-H (calcium (aluminium) silicate hydrate).
Abstract: Systems of different Ca/Si and Al/Si molar ratios were investigated. Incorporation of Al increases main basal spacing, amount of bounded water and decreases crystallinity of C-(A)-S-H (calcium (aluminium) silicate hydrate). Transmission electron microscope observations showed that aluminium results in formation of more compacted, foil-like microstructure. FTIR revealed the presence of rings within the structure of C-(A)-S-H. Low Ca/Si ratio promotes Al incorporation into C-(A)-S-H, while in case of high Ca/Si ratio aluminium is also incorporated into AFm. The results show, that Ca/Si ratio is of key significance deciding on Al incorporation into C-(A)-S-H in hydrating SCMs bearing blended systems.
TL;DR: The development of low-carbon binders has been recognized as a means of reducing the carbon footprint of the Portland cement industry, in response to growing global concerns over CO2 emissions from the construction sector as mentioned in this paper.
Abstract: The development of low-carbon binders has been recognized as a means of reducing the carbon footprint of the Portland cement industry, in response to growing global concerns over CO2 emissions from the construction sector. This paper reviews recent progress in the three most attractive low-carbon binders: alkali-activated, carbonate, and belite-ye'elimite-based binders. Alkali-activated binders/materials were reviewed at the past two ICCC congresses, so this paper focuses on some key developments of alkali-activated binders/materials since the last keynote paper was published in 2015. Recent progress on carbonate and belite-ye'elimite-based binders are also reviewed and discussed, as they are attracting more and more attention as essential alternative low-carbon cementitious materials. These classes of binders have a clear role to play in providing a sustainable future for global construction, as part of the available toolkit of cements.
TL;DR: This study balances the positive and negative effects of adding MSWI fly ash to the backfill by controlling its quantity in the binders, thus establishing an optimal concentration of 49 wt.% steel slag, resulting in very stable growth in strength and control of leaching risks in subsequent periods.
Abstract: As a binder to completely replace Portland cement for mine backfilling, the use of clinker-free cementitious materials combined with municipal solid waste incineration (MSWI) fly ash is proposed to achieve the targets of low-cost green backfilling, safe disposal and resource utilisation of bulk urban hazardous waste and metallurgical solid waste. This study balances the positive and negative effects of adding MSWI fly ash to the backfill by controlling its quantity in the binders, thus establishing an optimal concentration of 49 wt.% steel slag (SS), 21 wt.% blast furnace slag (BFS), 10 wt.% MSWI fly ash and 20 wt.% flue gas desulfurisation (FGD) gypsum. It is also reported that the filling performance of slurry (A2) satisfied strength requirements and is very suitable for long-distance transportation according to filling parameters. The leaching levels of the target elements (Cr, Ni, Zn, As, Cd, Sb, Pb, Hg and dioxins) for A2 matrix are lower than the required maximum concentration limits for the underground class Ⅲ water standard. Furthermore, the risk of leaching harmful constituents is mainly controlled by the pH of the environmental and the excellent buffering capacity of the matrix can reduce the potential leaching risk. The encapsulation, precipitation and adsorption of low-solubility double salts, such as hydrate calcium chloroaluminate (HCC) and ettringite, are the solidification/stabilisation (S/S) mechanism of series A on harmful substances. In addition, the high degree of polymerization(Ca/Si = 1.18 < 1.2, at 90d), the formation of long-chain C-S-H gels in binder A2-2, the dense pore structure lead to very stable growth in strength and control of leaching risks in subsequent periods.
TL;DR: In this article, the carbonation of portlandite, ettringite, and calcite was investigated at 57% RH and 91% RH using X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, and the phenolphthalein spray test.
Abstract: The carbonation of portlandite, calcium silicate hydrate (C-S-H), and ettringite was investigated at 57% RH and 91% RH using X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, and the phenolphthalein spray test. The experiments show that the carbonation of portlandite, ettringite, and C-S-H with Ca/Si = 0.7 is significantly faster at 91% RH than at 57% RH. Little effect of RH is observed for C-S-H with higher Ca/Si. Portlandite and C-S-H with Ca/Si = 0.7 carbonate only partially at 57% RH; complete carbonation is observed if the relative humidity is increased to 91% RH. In contrast, the carbonation of C-S-H with Ca/Si = 1.2 and 1.5 is complete at both relative humidities. The carbonation rate of C-S-H decreases with decreasing Ca/Si ratio, both at 57% and 91%RH. Carbonation at 57% RH promotes the formation of vaterite and aragonite over calcite; the precipitation of amorphous calcium carbonate is observed for C-S-H with Ca/Si = 0.7.
TL;DR: In this paper, gypsum, steel slag, and water were mixed, compaction-shaped, and carbonation-cured as a means of improving the strength of the steel slags.
Abstract: The carbonation of steel slag to produce building material is a useful way to increase the utilization of steel slag and absorb carbon dioxide. In this study, gypsum, steel slag, and water were mixed, compaction-shaped, and carbonation-cured as a means of improving the strength of the steel slag. It was observed that gypsum promoted an increase in both the compressive strength and the CO2 uptake of steel slag. CO2 uptake was positively correlated with strength. Microanalysis indicated that the main hydration product were C-S-H phases and ettringite, while the main carbonation products were calcite and monocarbonate (C3A. CaCO3.11H2O). Gypsum is speculated to promote the rapid hydration of steel slag to form ettringite (C3A.3CaSO4.32H2O), which then reacts with CO2 to produce monocarbonate; thus, gypsum plays a catalytic role in this system. The results of this study therefore provide theoretical guidance for the preparation of steel slag–gypsum carbide building materials.
TL;DR: In this article, a combination of selective chemical extraction and Fourier transform infrared (FTIR) spectral subtraction and deconvolution is used to discover Si-O peaks hidden in the single broad FTIR peak.
Abstract: It has been reported that municipal solid waste incineration bottom ash (IBA) can be a potential precursor for alkali-activated materials (AAM). This study investigates chemical composition and structure of calcium-containing phases in an alkali-activated IBA (AA-IBA) binder by a novel combination of selective chemical extraction and Fourier transform infrared (FTIR) spectral subtraction and deconvolution. Salicylic acid/methanol extraction is used to isolate the calcium-containing phases from the AA-IBA binder. X-ray powder diffraction and FTIR spectroscopy are used for sample characterization. Spectral subtraction is carried out to assign FTIR peaks of calcium-containing phases and deconvolution is used to discover various individual Si-O peaks hidden in the single broad FTIR peak. Results show that the AA-IBA consists of about 20 wt.% calcium silicate hydrate (C-S-H) and pirssonite (Na2Ca(CO3)2·2H2O). Chemical structure of the C-S-H in AA-IBA is found to be broadly similar to that in aged Portland cement paste, with possibly a higher degree of polymerization of the silicate chains. The methodology established in this study is significant and can greatly benefit the development of sustainable construction materials because many industry by-products and solid wastes are Si and/or Al rich, which could be potential AAM precursor.
TL;DR: In this paper, the mid-, near-, and far-infrared (IR) spectra of synthetic, single-phase calcium silicate hydrates (C-S-H) with Ca/Si ratios (C/S) of 0.41-1.2 were analyzed.
Abstract: The mid-, near-, and far-infrared (IR) spectra of synthetic, single-phase calcium silicate hydrates (C-S-H) with Ca/Si ratios (C/S) of 0.41–1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H2O and OH environments. The main mid-IR bands occur at 950–1100, 810–830, 660–670, and 440–450 cm−1, consistent with single silicate chain structures. For the C-S-H samples, the mid-IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite-like structural environments of C/S ratios >1.2. The 950–1100 cm−1 group of bands due to Si-O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite-like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite-like domains cannot be determined from these data. A shoulder at ∼1200 cm−1 due to Si-O stretching vibrations in Q3 sites occurs only at C/S lessthan equal to 0.7. The 660–670 cm−1 band due to Si-O-Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near-IR region, the combination band at 4567 cm−1 due to Si-OH stretching plus O-H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si-OH linkages at C/S ratios greater than this. The primary Si-OH band at 3740 cm-1 decreases in a similar way. In the far-IR region, C-S-H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm−1, indicating the presence of CaOH environments, even though portlandite cannot be detected by X-ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C-S-H, which is described.
TL;DR: In this paper, a model for the structure of calcium silicate hydrate (C-S-H) as it is formed during the hydration of Portland cement is proposed, which is a simplified representation of the microstructure within the size range of about 1 to 100 nm.
Abstract: A model is proposed for the structure of calcium silicate hydrate (C-S-H) as it is formed during the hydration of Portland cement. One purpose of the model is to move toward an ability to evaluate the microstructure quantitatively, so that it can be related to properties on the one hand and processing on the other hand. It is a hypothesis intended to promote discussion and motivate experiments. Furthermore, the model is an attempt to rationalize disparate measurements of specific surface area reported in the literature by describing an underlying structure, which, when observed by different instruments, gives different results. It is a simplified representation of the microstructure within the size range of about 1 to 100 nm. The basic building block is a unit of C-S-H that is roughly spherical and approximately 2 nm across with a specific surface area of about 1,000 m2/g. These building blocks flocculate to form larger units. This paper describes the structure of the basic units and how they pack to form larger structures and microstructures. The model also explains a number of variant observations for such measured attributes as specific surface area, pore size, and density as determined by different techniques, as well as water content at different relative humidities.
TL;DR: In this article, a second generation model for the nanostructure of C-S-H based on the interpretation of water sorption isotherms is presented, which can help to establish quantitative relationships between the nano-structural and bulk properties.
Abstract: This paper describes a second generation model for the nanostructure of C-S-H based on the interpretation of water sorption isotherms. The cornerstone of the model is a description of the globules (used here to mean small brick like particles), which consist of solid C-S-H and internal water, and the distribution of water in the small pores between them. Microstructural changes that occur during drying and account for both reversible and irreversible shrinkage are described. Since globules are particles, the properties of C-S-H gel are best understood through application of the emerging granular mechanics. This new model should help to establish quantitative relationships between the nanostructure and bulk properties.
TL;DR: In this article, some aspects of the interpretation of IR spectra of inorganic solids are discussed, with emphasis on the factors influencing the vibrational frequencies of cation-oxygen co-ordinated groups.
Abstract: Some aspects of the interpretation of IR spectra of inorganic solids are discussed, with emphasis on the factors influencing the vibrational frequencies of cation-oxygen co-ordinated groups, namely the value of the co-ordination number, “isolated” or “condensed” state of the co-ordinated groups and vibrational interactions with neighbouring groups. These considerations are applied to the study of AlO stretching frequencies in a series of aluminates. Characteristic frequency ranges are as follows: “Condensed” AlO 4 tetrahedra 900-700 cm −1 , “Isolated” AlO 4 tetrahedra 800-650 cm −1 , “Condensed” AlO 6 octahedra 680-500 cm −1 , “Isolated” AlO 6 octahedra 530-400 cm −1 . Several cases of mixed vibrations (AlO + LiO) are found in the particular case of lithium aluminates from abnormal or erratic 6 Li 7 Li isotopic shifts.
TL;DR: In this article, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials.
Abstract: For first time, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials. The study involved a comparison with the C-S-H gel forming in a Portland cement paste. The structure of the C-A-S-H gels in AAS pastes depends on the nature of the alkali activator. When the activator is a NaOH, the structure of the C-S-H gel falls in between tobermorite 1.4 nm with a mean chain length of five, and tobermorite 1.1 nm with a mean length of 14. When waterglass is the activator the structure of the C-A-S-H gel is indicative of the co-existence of tobermorite 1.4 nm with a chain length of 11 and tobermorite 1.1 nm with a chain length of 14. This very densely packed structure gives rise to excellent mechanical properties.