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XRD patterns recorded from the pastes hydrated in the absence of sulfur. Key: A, tricalcium aluminate; C 3 A; Ca 3 Al 2 O 5 ; 38-1429. #, calcium hydroaluminate; C 4 AH 19 ; Ca 4 Al 2 O 7 ?19H 2 O; 14-0628. O, hemicarboaluminate; Ca 8 Al 2 C ¯ H 23 ; Ca 4 Al 2 O 6 (CO 3 ) 0.5 (OH)?11.5H 2 O; 41-0221. M, monocarboaluminate; Ca 4 Al 2 C ¯ H 11 ; Ca 4 Al 2 O 6 (CO 3 )?11H 2 O; 41-0219. X, hydrogarnet; C 3 AH 6 ; Ca 3 Al 2 (OH) 12 ; 24-0217. Note that the main peak for C 3 A at 33.2u was throughout the study, but was very weak beyond 3 days.
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Raman spectroscopy has been used to follow the hydration of one of the principal components of Ordinary Portland Cement (OPC) clinkers, tricalcium aluminate, both in the absence and in the presence of calcium sulfate. Direct in-situ analysis of the hydrating paste surface was possible. Spectra were recorded regularly during the first 24 hours of hy...
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... Instead, a remarkable amount of C 3 AH 6 was obtained, indicating that the phase transitions from C 4 AH X to C 3 AH 6 might occur. Although C 3 AH 6 phase is stable in an environment of 20 °C, it is commonly reported that C 4 AH X phases are predominantly produced in short curing durations [6,47,48]. Therefore, these findings are not typical trends. Since it is challenging to determine this only with TGA, more detailed explanations will be provided with QXRD results. ...
In this study, the impact of triisopropanolamine (TIPA) on both the crystallographic and surface properties of cubic and orthorhombic tricalcium aluminate (C 3 A) were investigated together with their hydration behavior and strength development. When TIPA was added during the grinding process, the pre-hydration and carbonation of C 3 A were effectively prevented, and crystal structural changes of C 3 A were confirmed. It leads to altering the hydration mechanism of C 3 A phases: in the case of cubic C 3 A, it promotes the formation of Al-hydrogarnet phases instead of OH-AFm phases even on the first day of curing. Similar hydration behavior was observed with orthorhombic C 3 A, but the phase transition of OH-AFm phases to Al-hydrogarnet occurs during a specific time period in 1–3 days. The latter was revealed as a very interesting endothermic reaction which can be the only heat absorptive behavior in complex cement hydration as reported so far.
... The hydration kinetics of C 3 A can be substantially modulated by soluble sulfates to mitigate the impractical occurrence of "flash set" 18 . Cubic C 3 A rapidly reacts with CaSO 4 to form ettringite (AFt) within minutes 15,22 , succeeded by an induction period lasting several hours in isothermal calorimetric curves 13,14 . This induction period typically concludes with a renewed hydration heat release, converting AFt into sulfate-bearing Al 2 O 3 -Fe 2 O 3 -mono (AFm) phases [13][14][15] . ...
Portland cement (PC) is ubiquitously used in construction for centuries, yet the elucidation of its early-age hydration remains a challenge. Understanding the initial hydration progress of tricalcium aluminate (C3A) at molecular scale is thus crucial for tackling this challenge as it exhibits a proclivity for early-stage hydration and plays a pivotal role in structural build-up of cement colloids. Herein, we implement a series of ab-initio calculations to probe the intricate molecular interactions of C3A during its initial hydration process. The C3A surface exhibits remarkable chemical activity in promoting water dissociation, which in turn facilitates the gradual desorption of Ca ions through a metal-proton exchange reaction. The dissolution pathways and free energies of these Ca ions follow the ligand-exchange mechanism with multiple sequential reactions to form the ultimate products where Ca ions adopt fivefold or sixfold coordination. Finally, these Ca complexes reprecipitate on the remaining Al-rich layer through the interface-coupled dissolution-reprecipitation mechanism, demonstrating dynamically stable inner-sphere adsorption states. The above results are helpful in unmasking the early-age hydration of PC and advancing the rational design of cement-based materials through the bottom-up approach.
... A large body of research has been devoted to the analysis of the C 3 A → C 3 AH 6 hydration reaction involving numerous intermediate solid phases forming, via studying either mixed clinker phases or neat C 3 A. C 3 A is known to transform into various layered double hydroxide (LDH) phases, e.g. C 2 AH 8 , C 4 AH 13 , and/or C 4 AH 19 [7,[37][38][39][40][41][42], preceding the formation of katoite, C 3 AH 6 [37][38][39][42][43][44][45][46]. The dissolution equilibria of C 3 A as well as C 3 AH 6 have been studied in detail, and corresponding thermodynamic solubility products for both solids have been determined [9,15,22,47]. ...
Tricalcium aluminate hexahydrate, C 3 AH 6 , forms during the hydration of the oxide form, C 3 A. Modelling the resulting phase composition over time has been the subject of intensive research; yet, understanding the effect of pH, a crucial parameter of cementitious mixtures, has remained elusive. To this end, we studied the stability of C 3 AH 6 in a wide pH range via the addition of HCl. We compared the behavior of conventional 'cement' C 3 AH 6 and 'Bayer' C 3 AH 6 , synthesized via alkaline digestion. Upon addition of HCl to mixtures in H 2 O or NaCl, both phases slowly transform in an equilibrium reaction into Friedel's salt, C 4 ACl 2 H 10 , a layered double hydroxide (LDH) with chloride ions being intercalated between the layers. Consequently, C 3 AH 6 , C 4 ACl 2 H 10 , and Al(OH) 3 are likely to coexist in the pH range of 11-12. Furthermore, we find the reverse process, i.e. C 4 ACl 2 H 10 → C 3 AH 6 to be kinetically hindered, consistent with the high stability of C 4 ACl 2 H 10 compared to hydroxide-intercalated LDHs.
... XRD curves are similar to other types of G-cements [62], showing similar crystals. The lack of ettringite and monosulfoaluminates is due to the low content of C 3 A and gypsum in cement [63,64]. Katoite and silicious hydrogarnet can be formed from C 2 S and C 4 AF and they have a similar chemical formula depending on fluid condition, sulfate activity, and temperature. ...
The effect of wet supercritical CO2 (90 °C and 20 MPa) on the performance of cement paste (PC) modified with bacterial nanocellulose (BNC) was investigated. The pore structure of carbonated cement shows clogging over the outer rim of the samples. In contrast, near the sample core, the characteristic peak of pore size distribution shifted towards smaller pores analyzed by mercury intrusion porosimetry. The effect of the carbonation over time on mechanical properties shows increasing alteration. XRD results show more crystalline phases of hydrated cement in the BNC samples before carbonation. Cement reinforced with BNC shows lower density, a reduction in its porosity, and experiences fewer porosity changes at the cement core. Furthermore, its mechanical performance was less affected by the carbonation process.
... Upon contact with water in the absence of sulfate, the reaction of C 3 A results in the formation of calcium aluminate hydrates (C-A-H phases) of different stoichiometric composition within a very short time. With continued hydration, the thermodynamically stable phase C 3 AH 6 forms from the C-A-H phases with the release of water [41][42][43]. This reaction is significantly accelerated at elevated temperatures (>30 • C) [38]. ...
The carbonation of C3A and pre-hydrated C3A was studied at different initial pH values, temperatures, pre-hydration and carbonation times. Based on these investigations, the reaction kinetics of C3A under humid conditions were considered in more detail. The reaction products were characterized by quantitative X-ray diffraction (QXRD) and thermogravimetric analysis (TGA). C3A shows no significant reaction when carbonated directly in suspension, however, it has been observed that pre-hydrated C3A can be carbonated. A higher reaction degree was obtained with increasing hydration time. Consistent with reaching a stable pH, no further reaction of the hydrate phases was detected after ~20 min, indicating fast kinetics of the carbonation reaction. At temperatures ≤21 °C, the CaCO3 polymorph vaterite was found to be the main phase. At a pre-hydration temperature of 40 °C, C3AH6 appeared as a stable hydrate phase, but showed no reaction with CO2 under our experimental conditions.
... Compared with other test methods, Raman spectroscopy has advantages such as [5,6]: 1) simple sample preparation; 2) rapid testing; 3) superb ability in recognizing both crystalline and poorly-crystallized phases; and 4) optional quick mapping and capability to reveal the global structure evolution of materials when combined with confocal microscopy. Taking advantage of these merits, Raman spectroscopy has been proven to be a valuable tool for characterizing clinker phases and hydration products, including tricalcium silicate (C 3 S) [7], dicalcium silicate (C 2 S) [8], tricalcium aluminate (C 3 A) [9][10][11], tetracalcium aluminoferrite (C 4 AF) [9,10], gypsum (CaSO 4 ⋅2H 2 O) [12], calcium hydroxide (CH) [13], calcium silicate hydrate (C-S-H) gel [14,15], ettringite (AFt) [16], and monosulfate (AFm) phases [4,9,17]. ...
This study utilized a novel in situ fiber-optic Raman probe to continuously monitor the hydration progress of tricalcium silicate (C3S) and dicalcium silicate (C2S) without the need for sampling, from early hydration stage to later stages, and from fresh to hardened states of paste samples. By virtue of the remarkable ability of this technique in characterizing either dry or wet and crystalline or amorphous samples, the hydration processes of C3S and C2S pastes with different water-to-solid (w/s) ratios could be monitored from the start of the hydration reaction. The main hydration products, calcium silicate hydrate (C–S–H) and portlandite/calcium hydroxide (CH), have been successfully identified and continuously monitored for variations in their respective amounts in situ. The effect of w/s ratio on the hydration processes of C3S and C2S pastes was also considered. Meanwhile, the x-ray diffraction (XRD) and thermogravimetric analysis (TGA) results showed a great correlation with the in situ Raman test results about hydration products, which demonstrated the reliability of this technology. Moreover, the signal-to-noise ratio (SNR) of this Raman probe is significantly superior to existing technologies for in situ fiber-optic Raman spectroscopy. This remote fiber-optic Raman probe enables the use of Raman spectroscopy in future construction projects for on-site monitoring and evaluation of health conditions and performance of concrete structures.
... In the decelerating stage, the heat release rates of all five samples dropped to low values and changed slowly. At this stage, AFm was continuously generated with the consumption of AFt due to the high concentration of sulphate ions in the pore solution [48]. For the samples blended with chloride ions lower than 2%, the cumulative heat increased with the increase of chloride blending, while for the 5% Clsample, the cumulative heat was quite low in the first 19 h but increased to the highest value after that. ...
... The last commonly reported reaction product is hydrogarnet (Ca 3 Al 2 (OH) 12 ) [51,[57][58][59][62][63][64][65][70][71][72]. When Ca 3 Al 2 O 6 reacts with a stoichiometric amount of water, hydrogarnet is considered the only reaction product in equilibrium; OH-AFm and C 2 AH 8 are only intermediates [35] (Fig. 4). ...
... In the case of Schwiete et al. [98], this early AFm was OH-AFm, which disappeared in the further course of the reaction. The disappearance of early formed AFm was also reported by others [70,97]. ...
... In the sulfate-buffered stage, ettringite is the dominating [62,64,85,98,100] or only [23,29,65,70,77,95,97,101] precipitate. As minor products, OH-AFm/C 2 AH 8 [62], SO 4 -AFm [98,100] or mixtures of them [85] have been reported. ...
Experiments, simulations and derived theories for the hydration of tricalcium aluminate in Portland cement-related systems are summarized and further conclusions are drawn. Highly reactive calcium sulfates lead to an earlier sulfate depletion, possibly due to their faster dissolution and consequently enhanced sulfate consumption by forming more ettringite instead of AFm. Sodium in solid solution decreases the crystal symmetry of tricalcium aluminate and is released over-proportionally during dissolution. When reacting only with water, sodium in solid solution and in the pore solution decreases the dissolution. At lower humidity, hydration starts with a delay, and the minimum humidity that allows a reaction depends on the overall composition of the system. Organic admixtures may interfere with the balance between aluminate phases and calcium sulfates thus disturbing the silicate reaction. The retardations of aluminate phases by sulfate and silicate phases by aluminate are likely due to surface sorption of ions suppressing the dissolution.
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... It depicts that the main phases revealed by the XRD in Mix 0 (0% CKD) and Mix 3 (15% CKD) are Ettringite (hydrous calcium aluminum sulfate, Ca6Al2(SO4)3(OH)12·26H2O), calcium silicate hydrate (C-S-H) 5Ca2 SiO4·6H2O, and portlandite (Ca(OH)2). It is known in the phases of the clinker that the cement tricalcium aluminate C3A has the phenomenon of flash-set as soon as water is added [52]. To prevent this phenomenon, gypsum CaSO4. ...
... 2H2O is added to the clinker, and it combines with C3A to form a substance that surrounds C3A and delays the time of water reaching C3A; thus, delaying the setting time of the cement. Therefore, the forming of Ettringite is normal in Mix 0 and Mix 3 because the added OPC contains gypsum [9,52]. ...
... All the mixtures in all the sample preparation stages, starting from mixing, passing through casting and ending with curing, were conducted in exactly the same conditions. The only difference between It is known in the phases of the clinker that the cement tricalcium aluminate C3A has the phenomenon of flash-set as soon as water is added [52]. To prevent this phenomenon, gypsum CaSO 4 . ...
This study investigates the effect of the partial replacement of ordinary Portland cement (OPC) with cement kiln dust (CKD) on the engineering properties of pozzolanic concrete for use in block manufacturing. Ultimately, this could potentially reduce cement consumption and CO2 emissions. The study was carried out on cylindrical concrete samples prepared from five mixtures comprising 71.13% pozzolan and 14.16% water, with 0, 5, 10, 15, or 20% of the OPC fraction (14.71 wt.%) replaced by CKD. The samples were tested for density and compressive strength (UCS) at curing ages of 7, 14, 28, 56, and 91 days; and for voids and water absorption after 28 curing days. Results show that increasing CKD content lowered the voids and increased the water absorption. The 15% CKD sample had the highest UCS, regardless of the curing age. X-ray diffraction analysis confirmed that the phase responsible for hydration, calcium silicate hydrate, was higher in the 15% CKD sample than the control.
... Tricalcium aluminate was not detected, since its (weak) marker band at about 750 cm −1 [28] is overlapped with the stronger spectral feature of zirconia at 736 cm −1 . The Raman spectrum of just extruded Ceraseal ( Figure 3) confirms the presence of PEG [29], calcium silicates [30,31] and monoclinic zirconia [27,32]; tricalcium aluminate [33,34] was detected as well. ...
... The micro-Raman spectrum of the just extruded sealer ( Figure 6) confirmed the nature of the organic component [29], as well as the presence of tantalite [44], calcium silicates and aluminates; among aluminates, calcium monoaluminate and tricalcium aluminate were detected [30,31,33,34,42,45]. For this sealer, setting induced the formation of a calcium carbonate component; the bands of the organic phase weakened in both IR and micro-Raman spectra (Figures 5 and 6). ...
The aim of the study was to analyze the chemical–physical properties and bioactivity (apatite-forming ability) of three recently introduced premixed bioceramic root canal sealers containing varied amounts of different calcium silicates (CaSi): a dicalcium and tricalcium silicate (1–10% and 20–30%)-containing sealer with zirconium dioxide and tricalcium aluminate (CERASEAL); a tricalcium silicate (5–15%)-containing sealer with zirconium dioxide, dimethyl sulfoxide and lithium carbonate (AH PLUS BIOCERAMIC) and a dicalcium and tricalcium silicate (10% and 25%)-containing sealer with calcium aluminate, tricalcium aluminate and tantalite (NEOSEALER FLO). An epoxy resin-based sealer (AH PLUS) was used as control. The initial and final setting times, radiopacity, flowability, film thickness, open pore volume, water absorption, solubility, calcium release and alkalizing activity were tested. The nucleation of calcium phosphates and/or apatite after 28 days aging in Hanks balanced salt solution (HBSS) was evaluated by ESEM-EDX, vibrational IR and micro-Raman spectroscopy. The analyses showed for NeoSealer Flo and AH Plus the longest final setting times (1344 ± 60 and 1300 ± 60 min, respectively), while shorter times for AH Plus Bioceramic and Ceraseal (660 ± 60 and 720 ± 60 min, respectively). Radiopacity, flowability and film thickness complied with ISO 6876/12 for all tested materials. A significantly higher open pore volume was observed for NeoSealer Flo, AH Plus Bioceramic and Ceraseal when compared to AH Plus (p < 0.05), significantly higher values were observed for NeoSealer Flo and AH Plus Bioceramic (p < 0.05). Ceraseal and AH Plus revealed the lowest solubility. All CaSi-containing sealers released calcium and alkalized the soaking water. After 28 days immersion in HBSS, ESEM-EDX analyses revealed the formation of a mineral layer that covered the surface of all bioceramic sealers, with a lower detection of radiopacifiers (Zirconium for Ceraseal and AH Plus Bioceramic, Tantalum for NeoSealer Flo) and an increase in calcium, phosphorous and carbon. The calcium phosphate (CaP) layer was more evident on NeoSealer Flo and AH Plus Bioceramic. IR and micro-Raman revealed the formation of calcium carbonate on the surface of all set materials. A thin layer of a CaP phase was detected only on AH Plus Bioceramic and NeoSealer Flo. Ceraseal did not show CaP deposit despite its highest calcium release among all the tested CaSi-containing sealers. In conclusion, CaSi-containing sealers met the required chemical and physical standards and released biologically relevant ions. Slight/limited apatite nucleation was observed in relation to the high carbonation processes.
