Hydration characteristics of tricalcium aluminate phase in mixes containing β-hemihydate and phosphogypsum

The tricalcium aluminte phase was prepared from pure chemicals on a laboratory scale. Five mixes were formulated from the prepared C₃A phase, β-hemihydate, phosphogypsum, calcium hydroxide and quartz. Different mixes were hydrated at various time intervals, namely, 6, 24, 72 and 168 h. The kinetics of hydration was measured from chemically combined water and combined lime contents. The phase compositions and microstructures of the hydrated products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA)/TG, scanning electron microscopy (SEM) techniques and FT-IR spectroscopy. This work aimed to study the effect of partial to full substitution of phosphogypsum by β-hemihydate on the hydration characteristics and microstructures of tricalcium aluminte phase. The results showed that the combined lime slightly increases with the increase of amounts of phosphogypsum. The XRD patterns showed the increase in the intensities of monosulphate and different forms of calcium aluminate (C₄AH₁₃ and C₄AH₁₉) with phosphogypsum content. Ettringite is less stable than monosulphoaluminate, so it transformed into monosulpho-aluminate after 24 h, which persisted up to 168 h. The mechanism of the hydration process of C₃A phase in the presence of phosphogypsum proceeds in a similar path as with β-hemihydate. Phosphogypsum reacts with C₃A in the presence of Ca(OH)₂ forming sulphoaluminate hydrates, which are responsible for setting regulation in cementitious system.     

Experimental Studies on Hydration of Partially Sulphated CFBC Ash

The hydration of partially‐sulphated fluidized bed combustor (FBC) ash with water was carried out at laboratory scale. The bottom ash fractions and the as‐received fly ash were hydrated for different lengths of time, at different temperatures between 5°C and 80°C. The free lime and calcium hydroxide content in the samples were analyzed before and after the hydration process. Scanning electron microscopy (SEM) with an energy dispersive X‐ray system (EDX) was employed to investigate the physical characteristics of the samples. X‐ray diffractograms (XRD) were used to obtain information on the phase composition. The current results show that, during the hydration treatment, the unreacted CaO in the partially‐sulphated material can be almost quantitatively converted to Ca(OH)₂ but that the free lime content is not constant. It is also clear that effectiveness of the hydration depends on hydration time and temperature. In addition, the behaviour of different particle size fractions is different and there is evidence that the hydration of CaO is not the only reaction occurring in this system.     

Removal of iron from milk of lime to produce pure precipitated calcium carbonate

ABSTRACT

Milk of lime (MOL) suspension is widely utilized in many industries; however, in some of the applications, such as the production of precipitated calcium carbonate (PCC), the suspension must be of high purity and quality. In other words, the amounts of iron and magnesium should be so little so that the final PCC product would be of high quality. The effects of initial temperature, quicklime particle size, amount of initial solid present, agitation rate, and time on the recovery of hydrated lime and iron/magnesium reduction efficiency were investigated through a central composite design (CCD) of experiments. The iron reduction efficiency and the weight recovery of hydrated lime were obtained as 91% and 98.5%, respectively. The reduction of magnesium was not significant in this stage. Moreover, approximately the whole existing SiO₂ was eliminated.     

Experimental design approach to calcium carbonate precipitation in a semicontinuous process

The production of precipitated calcium carbonate, PCC, by a semicontinuous process of slaked lime carbonation was performed in a bench-scale chemical reactor, fully controlled by means of custom built electronics and software for the personal computer. Calcite crystals, with different characteristic morphologies (rhombohedral, truncated prismatic, scalenohedral, spheroidal or chain-like agglomerates) were produced by varying a range of process parameters, like temperature, supersaturation, gas mixture flow rate, stirring rate and mass concentration of Ca(OH)₂ suspension. In order to identify the effects of the chosen process parameters on the PCC morphology and on the related specific surface area, as well as on the extent of CO₂ conversion, an empirical approach based on the experimental design techniques was employed. A multiple correlation analysis of the obtained data suggests that temperature and conductivity significantly influence the PCC morphology, while CO₂ conversion is principally influenced by stirring rate, conductivity and gas mixture flow rate.     

Some observations on the setting of calcium sulphate hemihydrate plaster in the presence of phosphoric acid and hydrated lime

The effects of additions of phosphoric acid and hydrated lime on the setting of calcium sulphate hemihydrate plasters have been examined. At two levels of P₂O₅ concentration (0.75 and 1.5 wt-% P₂O₅) it has been found that extreme retardation of the plaster setting process occurs with lime additions in the range, Ca(OH)₂/P₂O₅ (weight ratio) 0.5 to 1.5. The chemical nature of this effect has not been identified. Formation of either CaHPO₄. 2H₂O or hydroxyapatite in the paste is considered as deleterious to plaster hydration.     

Steam reactivation of 16 bed and fly ashes from industrial-scale coal-fired fluidized bed combustors

The hydration behaviour of sixteen ashes, obtained from different commercial-scale fluidized bed combustors, has been investigated. Hydration is important for both ash disposal and reactivation of excess lime present in the ashes for further use in flue gas desulphurization. The techniques used were instrumental and conventional chemical analysis, thermogravimetry and X-ray diffraction. The ashes comprised both fly ash and bottom ash, with particle size less than 2 mm. The ashes were heat treated in air to oxidize free carbon and then hydrated with pressurized steam at about 170 °C, alone and with addition of pure CaO.It has been shown that steam hydration is effective in quantitatively converting CaO to Ca(OH)₂, but in most cases the free lime content (i.e. CaO+Ca(OH)₂), expressed as CaO, decreases and added CaO enters into pozzolanic reactions with coal ash components, in part or even completely. Both the chemical evidence and X-ray phase analyses indicate that hydrated silicates and silicoaluminates are formed. The hydrated ashes are all able to take up additional SO₂ and it appears that the presence of amounts of Ca(OH)₂ detectable by phase analysis is not necessary for such capture.     

Properties and mechanism of a poly(ionic liquid) inhibitor contained bi-functional groups for bentonite hydration

Present polymer inhibitors depend on a major inhibitory group to restrain bentonite hydration, and monomer design is concerned to improve the inhibition and stability through complex copolymerization. Conveniently, a homopolymer (PIL-NH₂) that contained primary amine and cationic imidazolium as bi-functional groups was proposed, aiming to provide two synergistic inhibitory modes. Comprehensive methods were conducted to characterize the chemical structure and inhibitory performance of PIL-NH₂. The ζ potential absolute value of bentonite suspension was decreased by PIL-NH₂ from 28.7–33.3 mV to 4–7 mV, and the increment of bentonite particle size d₅₀ was observable from 1.83892 μm to over 200 μm. With water squeezed out, the lattice spacing d₀₀₁ of hydrated bentonite was reduced from 1.9070 to 1.2683 nm due to PIL-NH₂ intercalation. The ESEM images revealed that inhibited bentonite showed a tight structure with classical dehydration phenomenon, and the hydrogen bond between PIL-NH₂ and bentonite was further confirmed according to the FT-IR result. In mechanism analysis, the electrostatic attraction and hydrogen bond existed simultaneously for PIL-NH₂ to adsorb bentonite. The two adsorption modes from bi-functional groups were synergistic to improve inhibition remarkably. PIL-NH₂ maintained high performance during the whole hydration process, including crystalline hydration, osmotic hydration, and hydrated dispersion.     

The effect of high curing temperature on the reaction kinetics in MK/lime and MK-blended cement matrices at 60 °C

It is well known that the pozzolanic reaction between metakaolin (MK) and calcium hydroxide produces CSH, C₂ASH₈ (stratlingite), C₄AH₁₃ and C₃ASH₆ (hydrogarnet). However, the presence or absence of these hydrated phases depends on different parameters, such as curing temperature, matrix used, etc. This paper shows the results of a study in order to know the effect of high curing temperature (60 °C) on the kinetics of the pozzolanic reaction in different matrices. MK/lime (calcium hydroxide) and MK-blended cement matrices were studied in samples stored and cured at 60 °C and up to 123 days of hydration. The nature, sequence and crystallinity of the hydrated phases were analysed using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques.Results showed that the sequence and formation of the hydrated phases was different in both matrices cured at 60 °C. In an MK/lime matrix, C₂ASH₈, C₄AH₁₃ and C₃ASH₆ were the main hydrated phases; while in an MK-blended cement, stratlingite was the sole hydrated phase issued from pozzolanic reaction. The DTA and XRD data also reveal an important fact: there is no evidence of the presence of hydrogarnet in blended cements.     

Study of hydrated phases present in a MK-lime system cured at 60 °C and 60 months of reaction

This research presents the experimental results of a study carried out to determine the effect of curing temperature on the reaction kinetics in a metakaolin/lime mixture cured at 60 °C and after 60 months of hydration. The stabilities of hydrated phases formed during the pozzolanic reaction in these working conditions were evaluated. The results obtained in current paper showed that metastable hexagonal phases (C₂ASH₈ and probably C₄AH₁₃) coexist with stable cubic phase (hydrogarnet) in the absence of lime. Also, there is evidence of the possible presence of a calcium aluminum silicate hydroxide hydrate (vertumnite).     

Improved evidence for the existence of an intermediate phase during hydration of tricalcium silicate

Tricalcium silicate (Ca₃SiO₅) with a very small particle size of approximately 50 nm has been prepared and hydrated for a very short time (5 min) by two different modes in a paste experiment, using a water/solid-ratio of 1.20, and by hydration as a suspension employing a water/solid-ratio of 4000. A phase containing uncondensed silicate monomers close to hydrogen atoms (either hydroxyl groups or water molecules) was formed in both experiments. This phase is distinct from anhydrous tricalcium silicate and from the calcium-silicate-hydrate (C-S-H) phase, commonly identified as the hydration product of tricalcium silicate. In the paste experiment, approximately 79% of silicon atoms were present in the hydrated phase containing silicate monomers as determined from ²⁹Si{¹H} CP/MAS NMR. This result is used to show that the hydrated silicate monomers are part of a separate phase and that they cannot be attributed to a hydroxylated surface of tricalcium silicate after contact with water. The phase containing hydrated silicate monomers is metastable with respect to the C-S-H phase since it transforms into the latter in a half saturated calcium hydroxide solution. These data is used to emphasize that the hydration of tricalcium silicate proceeds in two consecutive steps. In the first reaction, an intermediate phase containing hydrated silicate monomers is formed which is subsequently transformed into C-S-H as the final hydration product in the second step. The introduction of an intermediate phase in calculations of the early hydration of tricalcium silicate can explain the presence of the induction period. It is shown that heterogeneous nucleation on appropriate crystal surfaces is able to reduce the length of the induction period and thus to accelerate the reaction of tricalcium silicate with water.     

Hydration kinetics and microstructure development of normal and NaAlO2-activated Al-doped β-C2S pastes

Sodium aluminate (NaAlO₂) can be used to accelerate hydration of Portland cements. Here, we compare the use of NaAlO₂ solutions with deionized water on the hydration of Al-doped β-C₂S. The microstructure development of hydration product C-S-H obtained from hydrated Al-doped β-C₂S was investigated. NaAlO₂ significantly improved the hydration kinetics of Al-doped β-C₂S due to the enhancement in the precipitation of calcium hydroxide and stimulation of C-(A)-S-H nucleation and increased the early-age mechanical strength of Al-doped β-C₂S pastes. NaAlO₂ promoted the incorporation of Al in the C-(A)-S-H structure and accelerated the formation of C-(A)-S-H phases containing tetra-, penta- and hexa-coordinated Al in its composition. The alkali cations modified the electrostatic equilibrium of the C-(A)-S-H, thus promoting the development of the nano-mechanical properties.     

A new approach on the hydration mechanism of tricalcium silicate

A new interpretation on the hydration mechanism of the tricalcium silicate is given. This interpretation is dependent on the total released lime extraction, free, interlayer and “bound” limes, by the modified Franke's method in which the lithium chloride is used as accelerator and to increase the solubility of the complex formed (1). The chemical studies as well as the infrared spectra of the hydrated tricalcium silicate after complete hydration (3.5 years) is identical with the natural and synthetic mineral tobermorite, 5CaO.6SiO₂.5H₂O and is far from the tobermorite-like structure, 3CaO.2SiO₂.3H₂O as stated earlier. The hydration mechanism is divided into five stages which are discussed in full detail.     

Alkali activation of a novel calcium‐silicate hydraulic binder with CaO/SiO2 = 1.1

A quasi‐amorphous low‐calcium‐silicate hydraulic binder, with an overall CaO/SiO₂ (C/S) molar ratio of 1.1, was produced. This cementitious material was then hydrated with aqueous solutions containing 3 wt% alkalis (either NaOH, Na₂CO₃ or Na₂SiO₃). The evolution of the hydration processes of the samples were monitored by compressive strength testing, XRD, FTIR, ²⁹Si and ²⁷Al MAS NMR, isothermal calorimetry and TGA. It was found that the nearly exclusive hydration product formed was a C‐S‐H phase with a semi‐crystalline structure. More importantly, the paste prepared with the Na₂SiO₃ solution developed compressive strength values similar to those of ordinary portland cements (OPC) with faster early age kinetics. In addition, the isothermal calorimetry results indicated that these new hydraulic binders present much lower heat of hydration values compared with a traditional OPC. The results presented here open the possibility of producing cement with a compressive strength comparable to that of OPC but with lower CO₂ emissions during the production process and with lower hydration heat related problems during the production of concrete structures.     

Hydration of low porosity slag-lime pastes

Slag-lime pastes of low porosity (water/solid ratio of 0.20) were hydrated from 6 hours to 180 days at 20°C. The kinetics and mechanisms of the hydration process were studied from the results obtained in this investigation. The depth of the hydrated layer on the slag particles is found to be thin indicating that the hydration reaction is very slow. The molar compositions of the formed hydrates could also be calculated from the free lime, nonevaporable water and uncombined slag contents. A high lime product (molar C/S+A ratio of 2.5–2.6) is formed during the early stage of the hydration process, then the molar C/S+A ratio drops to a value of 1.5 and finally rises to a value of 1.7 at 180 days. The surface areas and pore volumes of hydrates were determined from water and nitrogen adsorption measurements. For water vapor adsorption, the water molecules in the adsorbed phase seem to be highly oriented in an ordered array. This effect might be associated with the polar character of water molecule, when adsorbed on an ionic surface like high lime hydrate. The results of x-ray diffraction and SEM observations indicate only the formation of ill-cyrstallised hydration products.     

Sulphur dioxide removal using South African limestone/siliceous materials

This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical composition and structural properties of the sorbent with regard to SO₂ removal in dry-type flue gas desulfurization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55-75 °C) and hydration period (4-10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe₂O₃ and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO₂ absorption capacity in terms of mol of SO₂ per mol of sorbent (0.1403-0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous structure with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents.     

硅钙基固废原料配比及蒸养条件对硅酸钙板力学性能的影响

研究协同利用硅钙渣、粉煤灰、水泥和脱硫石膏制备硅酸钙板时,原料配比、蒸养条件对硅酸钙板力学性能、水化产物的影响,并利用XRD、IR和SEM表征了原料的协同水化历程和水化产物的微观结构和表面形貌.试验结果表明:最佳原料配比为硅钙渣60%、粉煤灰24%、水泥10%和脱硫石膏6%;最佳蒸压养护条件为蒸养温度180℃,恒温蒸养时间8 h,硅酸钙板抗折强度满足国家标准强度的D1.3的Ⅱ级要求;随着蒸养温度升高,原料水化依次生成C-S-H凝胶、托贝莫来石和针状硬硅钙石,大量托贝莫来石和硬硅钙石的生成使得硅酸钙板的强度得以提升.     

Studies on the Stability of Calcium Sulphoaluminate

High calcium sulpho-aluminate (3CaO. Al₂O₃. 3CaSO₄. 31H₂O—C.S.A.) is a deterioration product of Portland cement found in concrete. It is formed by the attack of sulphate solutions on two of the Portland cement components: hydrated calcium Aluminate and lime. These phenomena fostered a study of the synthesis and stability of calcium sulpho-aluminate. Results of the experiments on stability of calcium sulphoaluminate in different media are discussed. Measures for preventing the formation of C.S.A. in Portland cement are described.     

Studies on the Stability of Calcium Chloraluminate

Calcium chloraluminate (3CaO · Al₂O₃ · CaCl₂ · 10H₂O) is a deterioration product of Portland cement found in concrete. It is formed by the attack of chloride solutions on two of the Portland cement components: hydrated calcium aluminate and lime. A study of the synthesis and stability of calcium chloraluminate in different media are described. Calcium chloraluminate enhances the growth of high calcium sulphoaluminate crystals (3CaO · Al₂O₃ · 3CaSO₄ · 31H₂O).     

Study on the replacement of the hydrated lime by kaolinitic clay in mortars

ABSTRACT

Mortars are cement-based materials used mainly to coat and settle construction blocks. In addition to cement, their composition usually includes hydrated lime, sand, and water. The hydrated lime is important to improve the mortar workability. However, lime has a high commercial cost, and its production causes emission of CO₂, a major responsible for global warming. Therefore, the purpose of this work was to investigate the possibility of total or partial replacement of hydrated lime in mortars by a kaolinitic clay with ideal plasticity parameters. Clay amounts of 0, 25, 50, 75, and 100?wt-% were used as replacement of hydrated lime in mortars. The results showed that with up to 50?wt-% of hydrated lime replacement, it is perfectly feasible to fulfil with technological parameters of standards.     

Hydration of tetracalcium aluminoferrite in presence of lime and sulfates

The first part of the paper describes conduction calorimetric and SEM studies of the initial first hour hydration of C₄AF and C₃A with water and saturated solutions of lime, gypsum and gypsum with lime. Lime accelerates while gypsum and gypsum with lime strongly retard the hydration of C₄AF. In case of C₃A, the effect is less pronounced. The second part deals with the hydration of C₄AF at later stages in presence of various additives and the same results as above are obtained. Anhydrite has very little influence while the presence of C₃A reduces the effect of gypsum and hemihydrate on hydration of C₄AF. A detailed investigation of the hydration process by means of X-RD, DTA, SEM and calorimetry has also been made.