Aluminate Hydrate Research Articles

Kinetics of pozzolanic reaction and carbonation in ceramic – lime system: Thermogravimetry and solid-state NMR spectroscopy study

Waste red-clay ceramic powder (RCC) is a pozzolanic material which is currently not commercially used, but high amount of “brick fraction” of construction and demolition waste may become a source for a cheap pozzolanic additive for Portland cement and lime-based binders. A sort of specific of RCC, compared to other pozzolans, is the higher content of non-reactive minerals. Pozzolanic reaction, taking place in lime-based system, is inevitably accompanied by carbonation of the lime; these two processes are concurrent with respect to the lime consumption. Products of hydration (C–S–H and calcium aluminate hydrate phases) and carbonation (CaCO3) are contributing to the strengthening of the system. The goal of this research has been to evaluate the rate and mechanism of hardening of the RCC – lime mixtures. The relative rate of pozzolanic reaction and carbonation was studied over one-year period in RCC – lime pastes by help of thermal analysis and 23Na, 27Al and 29Si Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS NMR), accompanied by porosimetry and compressive strength determination. The kinetics of lime conversion to hydration products and CaCO3 was determined by the help of thermogravimetry and described by Jander's equation. The highest rate of pozzolanic reaction, as well as the highest strength, reached mixture of 70% of RCC and 30% of lime; such high effective pozzolana content is caused by the phase composition of ceramic powder – it contains lower amount of reactive species, compared to metakaolin or other pozzolans. The formation of AlVI hydration products (AFm and AFt phases) took place within initial 3 days in all mixtures, while the content of C–S–H hydrates grown over the whole year of the experiment.

A density functional theory study on the structure formation of Al(III) carboxylate complexes in aqueous aluminum sols

AbstractIn‐depth studies of the structure of precursor sols is very important for understanding the subsequent sol to gel transition. Here, density functional theory was applied to study the reaction of Al powder with formic and acetic acid in aqueous solution to find the structure of Al(III) complex in precursor sols. The Al3+ ion obtained by dissolving Al powder in aqueous solution of formic and acetic acid would preferentially coordinate with water molecules to form aluminum hydrates. As the reaction energy barrier of aluminum hydrates with CH3COO− and HCOO− was comparable, both aluminum formoacetate and other aluminum carboxylate monomers could be obtained. Most Al(III) complexes formed by other carboxylate monomers were similar to that prepared from aluminum formoacetate. However, some Al(III) monomers and Al(III) dimer with different structural characteristics might be detrimental to the formation of linear polymers. The formation of other aluminum carboxylate monomers had great influence on the structure of precursor sols.

Impact of aging on the hydration of tricalcium aluminate (C3A)/gypsum blends and the effectiveness of retarding admixtures

Abstract In the production of concrete from cement powder and water, setting behavior of the slurry is determined by the formation of ettringite (Ca6Al2(OH)12 · (SO4)3 · 26 H2O) from tricalcium aluminate (Ca9Al6O18, abbreviated C3A) and gypsum (CaSO4 · 2 H2O). Due to the high reaction potential of cement and water, premature hydration can occur after unintentional exposure to moisture. Model binary mixtures of C3A and gypsum stored at 90% relative humidity and 35 °C produced ample amounts of ettringite, which subsequently reacted with atmospheric CO2 to CaCO3, Al(OH)3 and gypsum. Investigated were the two main polymorphs of tricalcium aluminate encountered in cement, pure, cubic C3A and orthorhombic C3A in which calcium is partially substituted by sodium or potassium. Alkali substituted C3A converted to ettringite faster and more completely than pure C3A. Ettringite from prehydration caused a seeding effect, which promotes crystal growth and accelerates bulk hydration of the C3A/gypsum mixtures. Set retarders commonly applied in cement were dissolved in the mixing water prior to hydration to investigate their ability to counteract this acceleration. Sodium gluconate merely delayed the crystal growth but does not prolong the hydration process overall. Potassium pyrophosphate retarded much more effectively by suppressing the seeding effect via removal of calcium ions from the hydration reaction.

Dissolution and early hydration of tricalcium aluminate in aqueous sulfate solutions

The great reactivity of tricalcium aluminate (C3A) with water and subsequent rapid precipitation of insoluble products are serious challenges to learning how salts affect its interfacial chemistry and physics during dissolution. Those challenges are addressed here with rapid in situ surface nanotopography monitoring and characterization of hydration products as C3A dissolves in aqueous sulfate solutions (Na2SO4, MgSO4, and CaSO4) with varying pH. These measurements shed light on how pure dissolution and early hydration of C3A are influenced by sulfates. Bulk dissolution experiments confirm a cation-specific effect of sulfates on the inhibition of C3A dissolution and of etch pit evolution. Soon after the initial contact with sulfate solutions, precipitation of hydration products occurred and the morphology and composition of products were strongly affected by the types of sulfates.

Phase and Microstructural Evolution of Calcium Aluminum Phosphate Cement for Potential Biological Applications: Effect of Aluminum Hydroxide Sources

Calcium aluminum phosphate cement (CAPC) requires a practical composition for biological applications. This research aims to investigate the effect of aluminum hydroxide sources (aluminum hydroxide, boehmite, and hydratable alumina), where SECAR 71 cement and phosphoric acid are in the composition, regarding the mechanical, biological, and microstructural properties of CAPC. In vitro assessments were evaluated by immersing the samples in simulated body fluid (SBF) solution for 7 days, 14 days, and 28 days, and by MTT testing (cytotoxicity of MG-63 cells). The results revealed that the composition including hydratable alumina was superior concerning the lowest final setting time (50 min), in situ formation and growth of hydroxyapatite on its surface, as well as the compressive strength which reaches to 42 ± 10 MPa after 28 days in SBF solution. However, regarding the cytotoxicity, the cements consisting of boehmite have priority. A more detailed biological analysis is recommended to evaluate the clinical application of CAPC.

Multi-Technique Characterization of a Fine Fraction of CDW and Assessment of Reactivity in a CDW/Lime System

This study analysed the fine particle (<5 mm) waste generated during siliceous or calcareous (depending on the composition of the original aggregate) concrete waste crushing. In the absence of industrial applications, such waste is amassed in open-air stockpiles on construction and demolition wastes (CDW) management plant grounds. The aim pursued was to find an outlet for that material in the cement industry. The starting waste, sourced from six Spanish management facilities, was characterised for its chemical and mineralogical composition, physical properties and pozzolanicity. The mineralogical phases in the CDW/lime system and their variations during the pozzolanic reaction were likewise identified. The findings showed that the fine waste consisted primarily in quartz, calcite, micas and feldspars, with smaller fractions of kaolinite and cement anhydrous phases. No portland cement hydration phases were identified. All six types analysed exhibited medium to low pozzolanicity, with the highest values recorded for the siliceous waste. Ettringite, C–S–H gels and calcium aluminate hydrates (C4AH13, C4AcH12) were identified during the pozzolanic reaction in CDW/lime system. Therefore, this type of waste can be reused as supplementary cementitious material with low-medium pozzolanic activity.

Modelling of multi-minerals kinetic evolution in hyper-alkaline leachate for a 15-year experiment.

Cement has been widely used for low- to intermediate-level radioactive waste management; however, the long-term modelling of multiple mineral transfer between the cement leachate and the host rock of a geological disposal facility remains a challenge due to the strong physical-chemical interactions within the chemically disturbed zone. This paper presents a modelling study for a 15-year experiment simulating the reaction of crystalline basement rock with evolved near-field groundwater (pH = 10.8). A mixed kinetic equilibrium (MKE) modelling approach was employed to study the dolomite-rich fracture-filling assemblage reacting with intermediate cement leachate. The study found that the mineralogical and geochemical transformation of the system was driven by the kinetically controlled dissolution of the primary minerals (dolomite, calcite, quartz, k-feldspar and muscovite). The initial high concentration of calcium ions appeared to be the main driving force initiating the dedolomitization process, which played a significant role in the precipitation of secondary talc, brucite and Mg-aluminosilicate minerals. The modelling study also showed that most of the initially precipitated calcium silicon hydrate phases redissolved and formed more stable calcium silicon aluminium hydrate phases. The findings highlight the importance of a deep and insightful understanding of the geochemical transformations based on the type and characteristics of the host rock, where the system is under out of equilibrium conditions, and the rates of mineral reactions.

In-situ XRD study of the temperature-dependent early hydration of calcium aluminate cement in a mix with calcite

In the present work the hydration of white CAC, inert alumina-filler and CaCO3 was investigated at different temperatures with in-situ XRD. Experiments were performed at 10 °C, 23 °C, 40 °C and 60 °C. Overcoming or mitigating the phenomena of calcium aluminate hydrate conversion by adding reactive additives to calcium aluminate cement is often described in literature. When a sufficient amount of calcite is added, carbonate-AFm (monocarbonate and hemicarbonate) is expected to be the dominant hydrate phase. Calcite however, acts as an inert filler and barely influences the calcium aluminate hydration products at 10 °C. With rising temperature monocarbonate contents increase as well as the reaction rate of cement phases. At 60 °C minor amounts of C3AH6 precipitate during the early hydration. Special attention is given to the role of CAH10 and C2AHx during hydration. While CAH10 is dominant at 10 °C, C2AHx precipitates in a temperature range between 23 and 60 °C as a precursor phase for carbonate-AFm phases. These are not found at 10 °C, which is probably due to the lack of the precursor phase (C2AHx). The hydration of the mix at 10 °C with an initial addition of C2AHx shows that carbonate-AFm-phases can be precipitated in addition to CAH10 during the first 96 h.

Chemical–physical–mechanical stability of MKG mortars under sulfate attacks

Geopolymer-based materials have attracted increasing interest in construction and infrastructure engineering due to their high mechanical properties but low carbon dioxide emissions. However, it remains unclear whether this type of material can work in a stable way in harsh environments. This study investigated the chemical–physical–mechanical stability of metakaolin-based geopolymer (MKG) mortars under the immersing actions of sodium sulfate and magnesium sulfate solutions up to 180 days by testing their visual appearance, mass, dimensions, strength, microstructure and chemical components. Results show that the immersion actions do not cause obvious changes in mass, dimensions or compressive and flexural strengths of the MKG mortars. A white deposit, consisting of magnesium hydroxide, magnesium sulfate, magnesium carbonate and magnesium silicate aluminate hydrate, appeared on the surfaces of the high-alkali MKG specimens immersed in magnesium sulfate solution for 30 days, and disappeared at later ages due to the continual consumption of magnesium hydroxide. Mg–Na exchanges occurred for the MKG mortars immersed in magnesium sulfate solution. Overall, the chemical–physical–mechanical stability of geopolymer-based materials depends on the material components and interactions with the environment. Caution should be used before any engineering applications, and relevant issues deserve further rigorous investigation in the future.

Effects of a Portland cement additive rich in SiO2 and Al2O3 in microstructure densification of RAP incorporated RCCP mixes

Past studies shows that the introduction of extra calcium silicate hydrate (C-S-H) gels, by means of various supplementary cementitious materials, did not show much significant effect on the strength of the roller compacted concrete pavement (RCCP) mixes made with reclaimed asphalt pavement (RAP) aggregates. Therefore, this paper is an attempt to refine the pores of RAP-RCCP mixes by inducing filler effect with the help of SiO2 and Al2O3 particles, derived by means of a Portland cement additive. The incorporation levels of the additive were 0, 2, 4, 6, 8, and 10%, respectively, which were introduced into the RCCP mixture containing 50% RAP aggregates (R-50 mix). Based on the scanning electron microscope images, it was observed that the asphalt cohesion failure was prevalent although the additional calcium silicate hydrate gels were able to refine the microstructure of the mortar paste. However, improvement in the pore structure of the RAP-RCCP mixes was observed as the SiO2 content increases. Moreover, the presence of Al2O3 particles could also react with the calcium carbonate present in the concrete (as shown by x-ray diffractometer patterns) to produce hemicarboaluminate and monocarboaluminate which could act as a filler and densify the interfacial transition zone of the RAP concrete. This promising effect was observed when the additive was utilized in a proportion of 10% wherein a marginal enhancement of 2–7% was observed in the compressive, flexural, and split tensile strength of the R-50 mix. In fact, the calcium aluminate hydrate phases formed on reaction between the Al2O3 particles and the calcium hydroxide could also induce higher progression reaction within initial days itself. Meanwhile, the relationships established in the present study also indicated a strong relation co-exists between the compressive and flexural strength, and compressive strength and the abrasion resistance as well.

Controlled morphologies and hydration process of hydratable alumina by using citric acid

Hydration mechanisms of hydratable alumina (HA) in citric acid at different pH are investigated in this study, aiming to improve rapid hydration process and massive hydration heat. By adding citric acid to adjust solution pHs to 2, 3, and 4, the crystal phase, morphologies, and microstructure of the HA hydrates are characterized by X-ray powder diffraction (XRD), differential scanning calorimetry (TG-DSC), and transmission electron microscopy (TEM), respectively. The results show that HA exhibits different hydration behaviors in different concentrations of acidic solutions. With decreasing pH of citric acid, the crystallinity of HA decreases whereas the dehydration process slows down. Excessive citric acid could displace hydroxyl which generates from pseudoboehmite gel. Meanwhile, the excess citrate ion can be absorbed on the surface of HA by forming numerous micelles, then compound to inhibit further hydration. Proper addition of citric acid helps to delay the hydration of HA and achieve better balance between bonding performance and hydration speed.

Effects of Aluminum Incorporation in Tobermorite Structure on Chloride Diffusion: A Molecular Dynamics Simulation Study

In this paper, the effects of different aluminum to silicon ratios in silicate chains of calcium silicate hydrates (C-S-H) are evaluated on the diffusion coefficient of chloride ions by molecular dynamics method. Tobermorite is a crystalline phase that is used for studying C-S-H properties in nano scale, because of its analogous chemical composition to C-S-H. Aluminum incorporation in C-S-H and the formation of calcium aluminosilicate hydrates (C-A-S-H) is due to both of hydration of tricalcium aluminate (C3A) in portland cement and aluminum oxides in pozzolans. There exist different Al/Si ratios in the tetrahedral chains of C-A-S-H depending on available aluminum oxides in cementitious raw materials. In order to compare the simulation results with previously-published experimental researches on cement pastes, a novel method is introduced here to calculate Al/Si ratio in tetrahedral chains of C-A-S-H using pozzolan replacement ratio in cementitious paste. MK (metakaolin) and FC3R (Fluid Catalytic Cracking Catalyst Residue) are the pozzolans that are used to validate the obtained results in this paper. Results showed that diffusion coefficients of chloride ions in C-A-S-H decrease by Al/Si ratio increasing in the tetrahedral chains as it was observed experimentally in previous researches.

Effect of phosphogypsum on physiochemical and mechanical behaviour of cement stabilized dredged soil from Fuzhou, China

Chemical treatment is one of the popular methods dealing with geoenviromental problems related to soil, such as waste dredged materials, soil with heavy metal pollutions, among others. This paper performed a series of laboratory tests to investigate the effect of phosphogypsum on water content, density, pH, unconfined compressive strength and deformation modulus of cement stabilized soil. It is found that the water content and pH value decreased whereas the density increased, as the phophogysum content increased. These changes were attributable to the formation of ettringite related to the chemical reaction between phosphogypsum and cementitious materials (calcium aluminate hydrates). The unconfined compression strengths of the cement stabilized soil with phosphogypsum content of 1.4% to 8.6% were approximately 1.7 to 9.4 times of those without phosphogypsum. Particularly, the earlier strength of the stabilized soil with phosphogypsum cured at 3 and 7 days increased by 1.3 and 2.1 times in comparison with those lacking phosphogypsum. The mineralogical changes revealed that when the cementation bonding was sufficiently formed at 28 days of curing, the increase in ettringite with phosphogypsum content behaved positive influence on strength development of cement stabilized soil. When cementation bonding was relatively low before 7 days of curing, adequate phosphogypsum content might produce moderate level of ettringite, tended to swell and make denser fabric, resulting in the strength increase. It should be also noted that over-dosage of phosphogypsum might cause volume increase and structure disruption, leading to the strength loss.

The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate.

Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate (C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H2O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO2(OH)4]5- complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced 27Al and 29Si solid-state NMR experiments. We notably assign a narrow 27Al NMR signal at 5 ppm to the silicate-bridging [AlO2(OH)4]5- sites and show that this signal correlates to 29Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by 27Al NMR in C-A-S-H samples.

Prevention of potential strength degradation due to conversion of C2AH8 formed in CaO-Ca(HCOO)2-activated GGBFS binder using CaSO4

Recently, the combined use of calcium oxide (CaO) and calcium formate (Ca(HCOO)2) has been proposed as an alternative activator for ground granulated blast furnace slag (GGBFS) (referred to as CaO-Ca(HCOO)2-activated GGBFS system). However, this clinker-free binder system produces calcium aluminate hydrate (C2AH8), generally leading to the degradation of cementitious binder systems by its conversion to other phases. Thus, in this study, attempts to remove C2AH8 from this system without losing the strength are suggested by the use of two additives (i.e., calcium sulfate (CaSO4) and calcium nitrate (Ca(NO3)2), respectively). Results revealed that the above-mentioned additives completely remove C2AH8 in the initial curing stage via its conversion to ettringite or NO3- and NO2-AFm, respectively. In addition, the additives reduced the extent of the total shrinkage and the standard deviation of strength results of triplicate samples. However, CaSO4 exerted a beneficial effect on the strength, while Ca(NO3)2 exerted a marginal or detrimental effect on the strength. Therefore, this study suggested a new mixture form of an activator, comprising CaO, Ca(HCOO)2, and CaSO4, to activate GGBFS for maintaining strength without the conversion issue of C2AH8.

Evolution of aluminate hydrate phases in fly ash-cement system under the sulfate conditions

Abstract The aluminate hydrate phases play an important role in the sulfate attack of concrete. In this paper, the aluminum hydrate phases evolution of fly ash blended cement paste powders immersed in different sodium sulfate solution is studied. In order to accelerate the reaction, the fly ash blended cement paste powders are immersed in different sulfate solution at ambient temperature up to 28 d. The results show that the hydration of fly ash and the distribution of aluminum hydrates phases in the hydration products of fly ash-cement blends are significantly influenced by the sulfate. The addition of fly ash into the cements introduces the reactive Al3+ into the systems, leading to an increase of Al(IV)/Si and mean chain length of C-A-S-H. Furthermore, although the pozzolanic reaction of fly ash consumes the calcium hydroxide, where the calcium hydroxide is required for the formation of ettringite, the addition of fly ash is favorable for the formation of ettringite.

Ultra-fast fabrication of porous alumina film with excellent wear and corrosion resistance via hard anodizing in etidronic acid

This paper describes a fast-fabrication strategy for hard-anodized film prepared on 6063T5 aluminum alloy in environmentally friendly etidronic acid (HEDP). The mild HEDP anodizing was first investigated at a temperature range of 15–45 °C, just under the burning current densities. The voltage curves and microstructures of the anodized film prepared in HEDP (Eti-film) indicate that 35 °C is the most appropriate temperature for the fast fabrication of Eti-film, in contrast to the micro-arc oxidation (MAO) coating and the seriously surface-corroded Eti-film prepared at 15 and 45 °C, respectively. Then, the HEDP hard anodizing without burning was successfully conducted via a gradient-increase-current approach, resulting in a high growth rate (~ 2.1 μm/min), low porosity (< 10%), thick barrier layer (~510 nm) and branched nanoholes structure of the hard anodized Eti-film. The Eti-films were systematically investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and a nanoindentation tester. In addition, the wear and corrosion resistance of the Eti-film and the hard sulfuric acid anodized film (Sul-film) were compared. The results indicate that the hardness of the Eti-film can reach ~ 11 GPa (~ 1000 HV0.01). The better wear resistance of the Eti-film as compared to the currently widely used Sul-film is attributed to the low porosity and less hydrated alumina content of the Eti-film. Moreover, the corrosion resistance of the Eti-film has been found to be 10 times higher than that of the Sul-film. In general, our results suggest a possibility of replacing the pollution-carrying anodizing methods currently used in the industry with the etidronic acid anodizing.

The Mechanical Properties of Fly-Ash-Stabilized Sands

The stabilization of soil through the addition of fly ash has been shown to be an effective alternative for improving the strength and stiffness of soil through the resulting chemical reactions. The chemical reaction that occurs dissociates the lime (CaO) in the fly ash, and the establishment of cementitious and pozzolanic gels (consisting of calcium silicate hydrate (CSH) gel and calcium aluminate hydrate (CAH) gel) binds the soil particles and increases the strength and stiffness of the soil. Investigations into the mechanical properties of sands stabilized with fly ash (fly-ash-stabilized sands) were conducted through a series of unconfined compressive strength (UCS) and direct shear strength tests for various fly ash percentages, curing times, grain sizes, degrees of saturation during sample preparation, and content of fines. It was found that the mechanical properties—UCS and direct shear strength (DSS)—of fly-ash-stabilized sands increased with both increasing fly ash content in the specimen and curing time, but decreased with increasing grain size, degree of saturation during sample preparation, and content of fines. The results indicated that fly-ash-stabilized sands required more than a month to attain their optimum performance with regard to binding sand particles.

Solid-State Recrystallization Pathways of Sodium Aluminate Hydroxy Hydrates.

Crystallization of Al3+-bearing solid phases from highly alkaline Na2O:Al2O3:H2O solutions commonly necessitates an Al3+ coordination change from tetrahedral to octahedral, but intermediate coordination states are often difficult to isolate. Here, a similar Al3+ coordination change process is examined during the solid-state recrystallization of monosodium aluminate hydrate (MSA) to nonasodium bis(hexahydroxyaluminate) trihydroxide hexahydrate (NSA) at ambient temperature. While the MSA structure contains solely oxolated tetrahedral Al3+, the NSA structure is a molecular aluminate salt solely based upon monomeric octahedral Al3+. Spontaneous recrystallization of MSA and excess sodium hydroxide hydrate into NSA over 3 days of reaction time was clearly evident in X-ray diffractograms and in Raman spectra. In situ single-pulse 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and 27Al multiple quantum (MQ) MAS NMR spectroscopy showed no evidence of intermediate aluminates, suggesting that transitional states, such as pentacoordinate Al3+, are short-lived and require spectroscopy with greater time resolution to detect. Such research is advancing upon a detailed mechanistic understanding of Al3+ coordination change mechanisms in these highly alkaline systems, with relevance to aluminum refining, corrosion sciences, and nuclear waste processing.

Influence of sulfur trioxide in clinker on the hydration heat and physical properties of Portland cement

The cement industry has achieved stable usage of waste gypsum board in clinker production. However, few studies have focused on the heat of hydration and physical properties of cements with high-SO3 clinker with different C3A and gypsum contents. Clinker with different amounts of SO3 and coexisting C3A was prepared in an electric furnace. Cement samples were then prepared by adding gypsum. Increasing the clinker SO3 content accelerated the alite reaction and caused aluminate hydrate reaction to occur later. Compressive strength at 3 days increased with SO3 content whereas that after 28 days decreased. The drying shrinkage strain of mortar decreased with increasing SO3 content of the clinker and gypsum at each C3A content. The decrease of drying shrinkage attributed to SO3 in the clinker was smaller than that caused by gypsum SO3. As the clinker SO3 content increases, cement physical properties can be maintained by adjusting the gypsum content.