CaO SiO2 H2O Research Articles

Mechanochemical activation of waste glass in alkaline composite cements with blastfurnace slag: A sustainable approach to recycling

This study investigates alkali-activated cements incorporating blast furnace slag and urban waste glass across varying slag:glass ratios (100:0, 25:75, 75:25, 0:100). Activation with 4 % and 12%Na2Oeq, coupled with two curing regimes (24h at 60 °C, followed by either 60 °C or 20 °C for 28 days), was explored. Prior to each formulation, part of the waste glass underwent mechanochemical ball milling with the required water and alkalis, to enhance the glass reactivity and to pre-dissolve SiO2 and other species to obtain a waterglass-like material in an environmentally friendly manner. Optimal strengths were achieved for slag binders (27 MPa) with 4%Na2O and 20 °C curing, while waste glass binders (48 MPa) required 12%Na2O and 60 °C. Composite formed cementitious products of CaO-(Al2O3)–SiO2–H2O gel, calcite, and hydrotalcite as reaction products, while 100 % waste glass binders displayed cementitious products of CaO–SiO2–H2O intermixed with silica gel. This approach holds promise for reducing environmental impact and promoting the efficient widespread and straightforward reuse of urban waste glass in construction materials.

Phase stabilities and thermodynamic properties of crystalline phases in CaO–SiO2–H2O above 100 °C

The current study revises the phase diagram of the CaO–SiO2–H2O system at saturated steam pressure between 100 and 250 °C by combining experimentally observed phase relations, solubility data and thermodynamic restrictions. A new self-consistent thermodynamic dataset for crystalline C-S-H phases is reported. The dataset revises known thermodynamic properties and derives values for phases for which no data have been published up to now. Chemical compositions and molar volumes of the phases were updated based on recent crystal structure refinements. Additionally, thermodynamic properties are deduced for hydroxylellestadite, scawtite and spurrite as they are relevant for autoclaved concrete and metamorphism/metasomatism of rocks. Portlandite dominates together with hillebrandite, xonotlite and low-quartz the equilibrium phase assemblage up to 250 °C. If the formation of the latter three phases is delayed by kinetic reasons, numerous metastable phases can occur.

Evaluation of Pozzolanic and Alkali-Activated Reactivity of Low-Purity Calcium Bentonite.

Alkali-activated cement (AAC) is a sustainable building material with low carbon emissions, but it has a growing demand for raw materials. In this study, the potential of low-purity modified calcium bentonite (CB) as a raw material for AAC was evaluated. The thermodynamic changes and pozzolanic properties of calcined CB were determined using X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), zeta potential, and a strength activity index (SAI) test. The compressive strength test, scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), and Fourier-transform infrared (FTIR) spectroscopy were performed to examine the compatibility between CB and AAC. It was revealed that CB is a low-purity clay with low-pozzolanic activity. Calcination enhanced its pozzolanic activity, and the optimum temperature is 750 °C. The incorporation of modified CB improved the mechanical properties of AAC, and low-temperature modified CB had better compatibility with AAC than the high-temperature modified CB. Calcination at 150 °C had little effect on the structure of CB, and the water absorption of montmorillonite increased the ion concentration, increasing the rate and degree of hydration. Furthermore, low-temperature calcination had a dissolution-precipitation effect, resulting in leaf-like CaO·SiO2·H2O (C-S-H) gels, whereas the high-temperature calcination of CB was very reactive, resulting in flower-like C(N)-S-H gels.

Study of the Hydration Kinetics of Cement Paste by Measuring Electrical Conductivity

The hardening cement paste is a highly concentrated polyelectrolyte that changes over time. From this point of view, the main electrochemical characteristics of the hydration process of cement paste can be the electrical conductivity and electrode potentials of metals lowered into cement pastes. In the process of setting and hardening of cement dough, complex physical and chemical processes of interaction of clinker minerals with water occur. In this case, new chemical compounds are formed, the concentration of Ca2+, Mg2+, A13+, Si4+, Na+, H+, OH− ions in the liquid phase changes, and the amount of free water decreases. Therefore, the electrical conductivity of the cement paste will change during the hydration of Portland cement minerals. The kinetics of hydration of cement paste is studied by measuring the electrical conductivity depending on the water-cement ratio and in the presence of additives SP C-3 and PDO-M. The parameter of electrical resistance of cement paste was studied using a digital voltmeter of type B7-27A/1. In experimental studies, we used additive-free sulphate-resistant Portland cement with an activity of 41.0 MPa with a normal density of 0.24 and pure clinker minerals – C3S и β-C2S. After preparation, the cement paste was placed in the assembled cell and compacted by vibration. Graphs of changes in the complex electrical resistance of cement pastes are obtained, which show that the curves have an oscillatory character. It was found that with an increase in the water-cement ratio, the vibrational nature of the interaction of water with Portland cement minerals weakens. When the water-cement ratio is equal to 0.42, there are no fluctuations in electrical conductivity. This can be explained by the fact that when the amount of water in the cement paste increases, the influence of cations on the degree of its ordering decreases. Curves of changes in the electrical conductivity of cement pastes obtained at W/C=0.26, without additives, as well as in the presence of SP C-3 and PDO-M additives. It can be seen that in the presence of SP C-3, fluctuations in electrical conductivity are weakly expressed. It can be assumed that SP C-3 has a disordering effect on the water in the cement paste. The addition of PDO-M, on the contrary, orders the structure of water, there are fluctuations in electrical conductivity. It is shown that in the CaO–SiO2–H2O system, a state of instability develops at certain ratios of the rate constants of individual stages of the hydration process. SP C-3, adsorbed on the adsorption centers of polysilicon acid, slows down some processes, which accelerates others, and the difference between the speed constants of individual stages increases.

Synthesis of Modifiers in the System Cao-Sio<sub>2</sub>-H<sub>2</sub>O and their Influence on the Non-Autoclave Silicate Materials’ Properties Using Aluminosilicate Binder

Improving the hydration hardening building materials’ operational properties is possible due to the stable macro-, micro-and nanostructures’ creation by the cementing material crystalline aggregate directed modification, which can be achieved through the use of various additives acting as crystallization centers. In the course of the studies, the synthesized modifier effect nature represented by the system was revealed CaO-SiO2-H2O (CSH) on the properties of non-autoclave silicate materials using aluminosilicate binder of various compositions. It has been established that the use of an aluminosilicate binder together with the addition of a CSH modifier increases the presence of a crystalline phase at all stages of hardening, as well as intensifies the synthesis of low-basic calcium hydro silicates with a higher crystallization degree in the system CaO-SiO2(Al2O3)-H2O represented by lime and clay rocks. This contributes to the micro-reinforced crystalline framework formation of the neoplasms with increased strength. Due to this, the pore space decreases and the amount of synthesized crystalline substance increases, which helps to increase the water resistance of the samples in all compositions. The samples using an aluminosilicate binder and the addition of a CSH modifier achieve maximum strength with a CaO content of not more than 10 wt. % The optimal CSH modifier addition is up to 1.5 wt. %, with 7 wt. %, CaO content in a mixture increase in strength is up to 6%.

НЕАВТОКЛАВНЫЕ СИЛИКАТНЫЕ КОМПОЗИТЫ С ПРИМЕНЕНИЕМ НЕТРАДИЦИОННОГО СЫРЬЯ И КРИСТАЛЛИЧЕСКОГО НАПОЛНИТЕЛЯ

In the construction of buildings and structures, many wall materials are used, including silicate products of various functional purposes. In traditional production technology of such materials, the hardening process occurs due to the formation of a crystal structure in the CaO-SiO2-H2O system. There are various ways to modify the crystalline growth of the cementing substance, one of which is the use of various kinds of crystal seedings, in particular the use of natural and synthetic calcium hydrosilicates. The purpose of the experiments is to study the possibility of improving the performance properties of non-autoclave silicate composites by modifying the structure formation in the "lime-non-traditional aluminosilicate raw materials" system, which consists in the crystal-chemical regulation of the structure formation processes with a synthetic crystal filler CaO-SiO2-H2O (C-S-H). The use of synthetic crystalline filler C-S-H synthesized by hydrothermal synthesis in an autoclave at a pressure of 1 MPa and a temperature of 175 °C from a mixture of Ca(OH)2 and crystalline silica in a ratio different C/S=1 in the technology of non-autoclave silicate materials on the basis of alternative aluminium raw material allows to increase the operational indicators resulting products to 18 % or more. The optimal content of CaO and crystal filler C-S-H at which the maximum strength characteristics are provided is 8 % and 2.5 %, respectively, which allows to develop optimal compositions of raw materials for the technology of producing high-density non-autoclave silicate materials based on non-traditional aluminosilicate raw materials with a compressive strength of at least 20 MPa and more, with an average product density of no more than 2000 kg /m3.

Hydrothermal reaction of K‐feldspar powder in NaOHCa (OH)2 mixed medium

AbstractThe hydrothermal reactions between K‐feldspar powder and NaOHCa (OH)2 mixed alkaline medium at 240°C were investigated. The effects of Na ratio (xNa, Na2O/(Na2O + CaO) in mole) on the phase and composition change in solid products were explored in detail. Results showed that increase in xNa led to the orderly formation of compounds following the sequence of CaOSiO2H2O → Na2OCaOSiO2H2O → Na2OAl2O3SiO2H2O, and K+ was accessible into solution after reaction. The new products after reaction was tobermorite, pectorite, nepheline hydrate, analcime, and hydroxycancrinite, depending on different xNa. The hydrothermal reaction was deduced to be a coupled dissolution and reprecipitation process, which was governed by the chemical equilibrium with the change in solution chemistry.

Microstructure and characterization of aluminum-incorporated calcium silicate hydrates (C-S-H) under hydrothermal conditions.

The phase assembly and microstructure of the aluminum-incorporated CaO–SiO2–H2O system, which is technologically important in autoclaved building materials, catalysis and waste management, were investigated using XRD, SEM, FTIR and NMR depending on aluminum addition, reaction temperature and curing time. The content of each phase was obtained using the MAUD program based on the Rietveld refinement. The results revealed that the formation of the tobermorite phase was promoted at Al/(Al + Si) ≤ 0.03, and subsequently retarded by higher aluminum addition, which was corroborated by the presence of more low polymerized and cross-linked (alumino)silicate chains. The phase purity decreased with increasing aluminum addition. Aluminum changed the morphology of tobermorite from plate-like to lath-like and fibrous. About a quarter of the (alumino)silicate chains in the C–S–H structure were linked though a Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Si–O–Al configuration, and this proportion was almost independent of aluminum addition. Furthermore, only Al[4] substituted for silicon in the aluminum incorporated C–S–H, while Al[6] just exited in the hydrogarnet phase. This work is beneficial for understanding the implication on micro-properties of by-products or admixtures containing aluminum in concrete.

Study on the phase evolution of calcium oxide–silicon dioxide–water system incorporating sucrose under hydrothermal conditions

The effect of sucrose on the reaction mechanism and product properties of the calcium oxide–silicon dioxide–water (CaO–SiO2–H2O) system under hydrothermal conditions was investigated. The reaction products with varying calcium oxide to silicon dioxide molar ratio (Ca/Si), concentration of sucrose, reaction temperature and time were characterised by X-ray diffraction, thermogravimetric/differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy and acid-insoluble residue analysis. The concentration of Ca2+, pH value and viscosity of the reaction solutions were determined and the influence mechanism of sucrose was then clarified. The results reveal that the phase composition and morphology of the products changed due to the incorporation of sucrose. There was a critical concentration of sucrose (e.g. the sucrose/calcium oxide molar ratio was 0·05 at Ca/Si = 0·8) for the formation of tobermorite. Sucrose promoted the formation of tobermorite when its concentration was lower than the critical value, and retarded the formation once its concentration exceeded the critical value. The morphology of tobermorite changed from plate-like to lath-like or lance-like as the concentration of sucrose increased. In the presence of sucrose, the transformation of tobermorite into xonotlite was inhibited at Ca/Si = 1·0 and the tobermorite phase remained stable even at higher reaction temperatures or for longer reaction times.

Water-bearing, high-pressure Ca-silicates

Water-bearing minerals provide fundamental knowledge regarding the water budget of the mantle and are geophysically significant through their influence on the rheological and seismic properties of Earth's interior. Here we investigate the CaO–SiO2–H2O system at 17 GPa and 1773 K, corresponding to mantle transition-zone condition, report new high-pressure (HP) water-bearing Ca-silicates and reveal the structural complexity of these phases. We document the HP polymorph of hartrurite (Ca3SiO5), post-hartrurite, which is tetragonal with space group P4/ncc, a=6.820(5), c=10.243(8) Å, V=476.4(8) Å3, and Z=4, and is isostructural with Sr3SiO5. Post-hartrurite occurs in hydrous and anhydrous forms and coexists with larnite (Ca2SiO4), which we find also has a hydrous counterpart. Si is 4-coordinated in both post-hartrurite and larnite. In their hydrous forms, H substitutes for Si (4H for each Si; hydrogrossular substitution). Fourier transform infrared (FTIR) spectroscopy shows broad hydroxyl absorption bands at ∼3550 cm−1 and at 3500–3550 cm−1 for hydrous post-hartrurite and hydrous larnite, respectively. Hydrous post-hartrurite has a defect composition of Ca2.663Si0.826O5H1.370 (5.84 weight % H2O) according to electron-probe microanalysis (EPMA), and the Si deficiency relative to Ca is also observed in the single-crystal data. Hydrous larnite has average composition of Ca1.924Si0.851O4H0.748 (4.06 weight % H2O) according to EPMA, and it is in agreement with the Si occupancy obtained using X-ray data collected on a single crystal. Superlattice reflections occur in electron-diffraction patterns of the hydrous larnite and could indicate crystallographic ordering of the hydroxyl groups and their associated cation defects. Although textural and EPMA-based compositional evidence suggests that hydrous perovskite may occur in high-Ca-containing (or low silica-activity) systems, the FTIR measurement does not show a well-defined hydroxyl absorption band for this phase, implying the water content, at least in the quenched glass, is below the limit of detection (100–1000 ppm). We conclude that at high pressure, as at ambient pressure, some calcium silicates have a high affinity for H2O and high dehydration temperatures. The thermal stability of these hydrous phases suggests that they could exist along a typical mantle geotherm and thus they might be relevant for understanding the mineralogy and water content of Earth's mantle.

Crystallization of calcium silicate at elevated temperatures in highly alkaline system of Na 2 O–CaO–SiO 2 –H 2 O

Abstract In Na2O–CaO–SiO2–H2O system, systematic investigations of phase and morphology of calcium silicate in hydrothermal conditions were concisely conducted for high-value utilization of silicon resource in high-alumina fly ash (HAFA). The results show that crystal composition and phase may be affected by relatively low concentration of NaOH, and sodium ions are rearranged into the structure to form NaCaHSiO4 and Na2Ca3H8Si2O12 with different C/S ratio at high concentration of NaOH. In addition, phases in wollastonite group possess the morphology of nanofiber. Formation of nanofiber is attributed to the difference of surface energies between axial and radial direction, and higher temperatures lead to easier growth along radial direction. The preparation of C–S–H with different phases and morphologies can guide for the application of silicate solution with high alkalinity with different purposes.

Synthesis of Calcium Silicate Hydrate in Highly Alkaline System

Synthesis of calcium silicate hydrate (C‐S‐H) was conducted over the range of 50°C–90°C and C/S ratio of 0.86–2.14 in the highly alkaline Na2O–CaO–SiO2–H2O system for silicon utilization in high alumina fly ash. Structural change in C‐S‐H formed in the highly alkaline system was investigated using XRD and 29Si MAS NMR spectra. X‐ray photoelectron spectroscopy was used to confirm the amount of sodium ions in C‐S‐H. Conversion of Si may reach 99% under optimum conditions. A higher degree of polymerization of silicate was obtained at lower temperature and C/S ratio. Na+ was confirmed to exist as Na–OSi and Na–OH. The amount of Na+ is the least at C/S ratio of 1.43, which conform to the prediction of topological constraint theory. High Ca/Si ratio leads to the increasing in Na+ combined in the interlayer. Increasing in the Na+ concentration in the system also increases the amount of Na+ combined in the interlayer and reduces the polymerization. Ion exchange was proven to be an effective way to remove Na+ combined in the interlayer of C‐S‐H.

Tobermorite Synthesis Under Hydrothermal Conditions

The influence of different hydrothermal conditions and starting materials on a preparation of tobermorite was examined. Tobermorite is a mineral, which, thanks to its composition and properties, is an important phase in aerated concrete and it can be also used as an ion exchanger. Tobermorite crystallizes together with other calcium silicate hydrates from the system CaO–SiO2–H2O during hydrothermal reactions. Paper deals with the key role of starting materials pre-treatment and its influence on the hydrothermal synthesis process. The other important parameters for a synthesis of pure tobermorite as conditions of the reaction, especially the temperature of hydrothermal reaction and curing time were also studied. Phase composition of prepared samples was characterized by XRD and was confirmed by TG-DTA. For overall product characterization, the SEM was used.

Thermodynamic modelling of phase equilibria in cement systems: multiple sublattice model for solids in equilibrium with non-ideal aqueous phase

The thermodynamics of the CaO–SiO2–H2O (C–S–H) gel phase present in cement systems has traditionally been modelled in terms of one or more stoichiometric solid phases. We present a model for the C–S–H gel phase based on non-ideal solution of ions and/or molecular species on a number of sublattices similar to that used extensively in National Physical Laboratory’s MTDATA software for modelling oxides and alloy phases. In application to the C–S–H system, aqueous phase compositions are represented within experimental uncertainties, and measured pH values are predicted exceptionally well although not included in the model parameterisation. Extensions to the base C–S–H model to account for the presence of Al2O3, modelling of the analogous M–S–H phase in MgO containing systems and an application to nuclear waste disposal technology where the portioning of uranium between the gel, aqueous and other phases is calculated, are also discussed.

Analyses of the surfaces of concrete by Raman and FT‐IR spectroscopies: comparative study of hardened samples after demoulding and after organic post‐treatment

AbstractConcrete surfaces were studied by two spectroscopic techniques, FT‐IR (in ATR mode) and Raman, to establish a nondestructive method to analyze the distribution of hydrated and organic phases. The surface composition of ordinary clinker, polished concrete, concrete after demoulding, and coated concrete as used in building construction was studied. The clinker's mineral phases and the polished concrete were first analyzed by Raman spectroscopy to determine a spectrum database of the specific phases located on the surface of the concrete. Then, both spectroscopic techniques were used to analyze, directly, the surface of hardened concrete after demoulding. No impact of roughness or porosity was highlighted using Raman spectroscopy; many cementitious, or hydrated phases (alite, belite, tricalcium aluminate, ferrite, portlandite and ettringite) were clearly identified. FT‐IR in ATR mode only identified some hydrated phases: portlandite and CaOSiO2H2O (CSH), but organic residues from the demoulding oil were characterized by this technique. Furthermore, the convenience of using these techniques together was tested by analyzing the composition of concrete surfaces coated by different organic post‐treatments. FT‐IR spectroscopy was useful to identify the main organic groups at the concrete surface, whereas Raman spectroscopy was especially able to characterize the mineral/hydrated phases under a thick post‐treatment layer (constituted of polyester varnish). Due to their own specificities, these complementary techniques should be used together to easily identify all the mineral phases and organic residues/coatings on concrete surfaces. Copyright © 2010 John Wiley & Sons, Ltd.

Novel fast-setting calcium silicate bone cements with high bioactivity and enhanced osteogenesis in vitro

Novel calcium silicate cements (CSCs) consisting of sol-gel-derived calcium silicate powder as a solid phase and ammonium phosphate solution as a liquid phase were developed. The diametral tensile strength, morphology, and phase composition of various cements were evaluated before and after immersion in physiological solution, in addition to setting time. The proliferation and osteogenetic expression of human osteosarcoma cell line U2OS for the cement specimens were also analyzed. The results indicated that the simultaneous formation of apatite and calcium silicate hydrates (CaO–SiO2–H2O, C–S–H) may endow CSCs with unique properties after mixing with a phosphate buffer solution. These CSCs were not only fast-setting within 9 min, as well as highly bioactive, but they also enhanced proliferation and differentiation of U2OS cells. The development of fast-setting CSCs with high bioactivity may thus have numerous applications as biomaterials.

Wollastonite from Hydrothermally Synthesized Calcium Hydrometasilicate Obtained from Various Modifications of Silica

A hydrothermally CaO–SiO2–H2O system was investigated at 150°–200°C, 2.5 h (CaO:SiO2=0.95) using various modifications of SiO2 in the presence of a mineralizer. Synthetic (stabilized) γ‐tridymite is the most reactive among SiO2 modifications. In the reaction mixture, the optimal concentration of the mineralizer (KOH) is 2% (versus the solid phase). The binding degree of CaO with SiO2 practically is 100% at 150°C. It is impossible to synthesize CSH free of C2SH on the basis of β‐cristobalite and β‐quartz under the investigation conditions without the use of mineralizer. The calorimetric effects as well as heat of de‐hydration of hydrosilicates were determined during their transformation into wollastonite. The entropy change at the peaks has been calculated.

Some studies on the reaction between fly ash and lime

The reaction between fly ash (FA) and lime is extensively exploited for the manufacture of building bricks, blocks and aggregates. To get a better idea of this reaction, FA from different sources were mixed in different ratios with lime and compacted. The compacts were treated both by ordinary water and hydrothermal curing to promote lime bearing hydrate bond formation e.g. CaO- SiO2-H2O (C-S-H), CaO-Al2O3-H2O (C-A-H) etc. The decrease in free lime content in these compacts was measured as a function of curing time and curing process. This drop in this content was correlated to the chemical composition of the fly ashes. The mathematical relationships between free lime remaining in the compacts after its maximum decrease in concentration and lime binding modulus (a ratio between the amount of added lime and the total amount of lime binding constituents present in FA) for both types of curing were developed. Further, the rate of decrease in free CaO content under both types of curing conditions was compared from kinetic study. From this study the orders of the reactions and rate constants were found out.

Effect of Silicate Anion Structure on Fixation of Chloride Ions by Calcium Silicate Hydrates

Calcium silicate hydrates, CaO–SiO2-H2O (C-S-H), were studied as a chloride fixation material. C-S-H of two different CaO/SiO2 ratios were synthesized and burned with calcium chloride in a temperature range from 600° to 1000°C. Minerals with a chemical composition of CaO·SiO2·CaCl2 and 9CaO·6SiO2·CaCl2 were identified by X-ray diffraction analysis. Comparing the diffraction intensity, it was found that the most efficient chloride fixation was attained when burned at 800°C. Changes in the morphology of silicate anion associated with burning and fixation of the chloride were studied in terms of chloride fixation capability using the trimethylsililation technique. It was confirmed that some silicate anions formed a glassy infinite chain where the chloride ions were fixed as a solid solution.

Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End‐Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solutions

Solubility in the fully hydrated CaO–SiO2–H2O system can be best described using two ideal C‐S‐H‐(I) and C‐S‐H‐(II) binary solid solution phases. The most recent structural ideas about the C‐S‐H gel permit one to write stoichiometries of polymerized C‐S‐H‐(II) end‐members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)2·6SiO2·5H2O and 10Ca(OH)2·6SiO2·6H2O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients nSi and nCa. Thermodynamic solid‐solution aqueous‐solution equilibrium modeling by applying the Gibbs energy minimization (GEM) approach shows the best generic fits to the available experimental solubility data at solid 0.8 < Ca/Si < 2.0 if both stoichiometry and thermodynamic constants of the end‐members are normalized to nSi= 1.0 ± 0.3. Recommended stoichiometries and thermodynamic data for the C‐S‐H end‐members provide a reliable basis for the subsequent multicomponent extension of the ideal C‐S‐H solid solution model by incorporation of end‐members for the (radio)toxic elements or trace metals.