The origin of the pozzolanic activity of calcined clay minerals: A comparison between kaolinite, illite and montmorillonite

This paper investigates the decomposition of three clayey structures (kaolinite, illite and montmorillonite) when thermally treated at 600 °C and 800 °C and the effect of this treatment on their pozzolanic activity in cementitious materials. Raw and calcined clay minerals were characterized by the XRF, XRD, ²⁷Al NMR, DTG and BET techniques. Cement pastes and mortars were produced with a 30% substitution by calcined clay minerals. The pozzolanic activity and the degree of hydration of the clinker component were monitored on pastes using DTG and BSE-IA, respectively. Compressive strength and sorptivity properties were assessed on standard mortars. It was shown that kaolinite, due to the amount and location of OH groups in its structure, has a different decomposition process than illite or montmorillonite, which results in an important loss of crystallinity. This explains its enhanced pozzolanic activity compared to other calcined clay–cement blends.     

Mineralogical and morphological changes of calcined paper sludge at different temperatures and retention in furnace

This work reports the project of thermal effects in mineralogy and texture of the paper sludge to employ them as pozzolanic material.For this reason, the chemistry and mineralogical composition has been studied, as well as its morphology by XRD, SEM and EDX.The initial sludge has been treated to 700, 750 and 800 °C during 2 and 5 h being observed that initial kaolinite becomes metakaolinite and that its pozzolanic activity with a paper sludge treated to 700 °C for 2 h is comparable to that of a commercial metakaolinite. The transformation of kaolinite after the dehydroxylation is to convert in amorphous metakaolinite. At the temperature mentioned above, calcite from the initial sludge is maintained active.It is concluded that the pozzolanic activity of metakaolinite is strongly related to the crystallinity of the original kaolinite. Well-ordered kaolinite is transformed into more reactive metakaolinite.     

Preparation and characterization of novel organo-clay minerals for Hg(II) ions adsorption from aqueous solution

Novel organo-clay minerals for adsorption of Hg(II) ions were prepared. The clay minerals were grafted with 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE). AEPE-montmorillonite and AEPE-hectorite were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), CHN element analysis (EA) and surface area analysis. The XRD patterns indicated that the chelating agents (AEPE) were mainly grafted on the external surface of montmorillonite while AEPE was grafted on both the external and interlayer surfaces of hectorite. The results from IR, TGA and EA showed a difference in chemical composition of the unmodified and modified clay minerals. The results confirmed that montmorillonite and hectorite were modified with the desired organic groups. The extent of ligand loading depended on the nature of the clay mineral. The AEPE-modified clay minerals were good chelating materials for Hg(II) ions, compared to the unmodified clay minerals. The adsorption capacity for Hg(II) of AEPE-montmorillonite and AEPE-hectorite was 46.1 and 54.7 mg g^− 1, respectively, for solution containing 140 mg L^− 1 Hg(II) ions (pH 4).     

Clay minerals and iron oxides-oxyhydroxides as fingerprints of firing effects in a limestone monument

Different Portuguese limestones-Encarnadão, Amarelo de Negrais, Lioz and Gresoso-have been widely used as building materials and ornamental stones in the architecture of Lisbon (Portugal) area.The aim of the study of those materials was focused on fire-induced stone damage, mainly on identifying the thermal transformations of clay minerals and iron oxides-oxyhydroxides. Taking into account an applied component to the cultural heritage, a special attention was given to one of the most ancient Portuguese monuments — Lisbon Cathedral, specifically its cloister that was severely damaged by a fire that occurred right after the 1755 earthquake.A set of samples collected from outcrops were studied and subjected to artificial heating. The results were compared with those obtained from samples collected in the monument. XRD, SEM-EDS and ⁵⁷Fe Mössbauer spectroscopy were used. The < 2 µm fraction varies significantly between unheated (outcrop samples) and artificially heated samples, particularly in relation to iron oxides-oxyhydroxides. Kaolinite is the dominant clay mineral, followed by illite and smectite. Goethite is the iron oxyhydroxide characteristic of all studied lithotypes both in unheated samples and samples artificially heated to 250 °C. Encarnadão is the exception presenting hematite in those conditions. As firing temperature increases (300 °C–600 °C) disordered hematite appears at the expense of goethite, and smectite is no longer detected. Mössbauer spectra further reveal that Fe²⁺ in silicate minerals is fully oxidized at 600 °C but remains in the carbonate structure up to this temperature. The SEM-EDS analyses show that 2:1 clay minerals have Si and Al as main cations and minor amounts of K and Fe. It also shows that all the clay minerals and Fe-rich particles are always associated to the limestone porosity.. The present results show that the clay minerals, namely illite–smectite mixed-layer and smectite, can be used as indicators of stone provenance used in Lisbon Cathedral Cloister as well as fingerprints of the temperature achieved during the fire of Lisbon Cathedral that most likely was not higher than 350 °C.     

Preparation of decolorizing ceramsites for printing and dyeing wastewater with acid and base treated clay

A new type of decolorizing ceramsites for printing and dyeing wastewater were prepared, by means of acid activation, base neutralization, granulation and heating treatment, by using palygorskite, Hangjin2# clay, bentonite or sepiolite as raw materials. The experimental results show that, the decolorizing ceramsites made of palygorskite have the best performances among different clay minerals in treating printing and dyeing wastewater. Its decolorizing amount is 635 mL g^− 1 and the reduction in COD is 81%, after 5 min static treatment at normal temperature. After calcined at 700 °C for 1 h, the loss ratio of decolorizing ceramsites made of palygorskite is less than 5%. Treated with saturated (NH₄)₂SO₄ solution for 10 min, and then calcined at 300 °C for 10 min, the used decolorizing ceramsites can be reused for more than 15 times. The decolorizing effect of the decolorizing ceramsites is mainly attributed to the combined chemical flocculation reaction of various metal ions in the material, with minor physical adsorption involved.     

Microstructural evaluation of a clay ceramic incorporated with petroleum waste

In this work an extensive SEM/EDS microstructural investigation associated with technological properties was carried out on kaolinitic clay bodies incorporating different amounts, up to 20 wt.%, of a bentonite-treated petroleum waste. The clay bodies were fired at temperatures from 850 to 1100 °C and then bend tested until rupture. Fracture surface observation revealed the existence of relatively small BaSO₄ particles that do not significantly contribute to the rupture process. By contrast, larger clay matrix embedded particles and voids exert more influence on the ceramic rupture. Since these particles are also normal features in waste-free clay bodies, the incorporation of bentonite-treated petroleum waste cannot be directly related to the rupture process. Exception occurred above 10 wt.% added waste and temperatures higher than 1000 °C, at which the comparatively higher amount of non-plastic quartz particles plays a role in the decrease of strength.     

Effects of the secondary minerals of the natural pozzolans on their pozzolanic activity

Natural pozzolans have been widely used as substitutes for Portland cement, because of their binding properties. Some of them are natural volcanic rocks which contain secondary minerals such as clays and zeolites corresponding to products of the alteration of the rock. The objective of this study was to document the potential effect of the secondary minerals on the strength development of pozzolanic mortars. We chose to investigate this effect by thermally destabilising these minerals in three different pozzolanic deposits (poz-1, poz-2 and poz-3). We first did a detailed mineralogical study, to identify the occurrence and the nature of the different secondary minerals. Kaolinite is abundant in poz-1 and different types of zeolite were identified in poz-2 and poz-3. Thermal treatments were monitored by X-ray Diffraction (XRD) analysis, in order to document mineralogical transformations. The effect on the pozzolanic activity has been tested by strength measurements on normalised mortars at 1, 7 and 28 days. Strength of all blended cements is enhanced while destabilising secondary alteration minerals. For kaolinite, we showed that a strength improvement occurs as soon as it is destructured, even if it is not transformed in metakaolin. For zeolites, destabilisation takes place at low temperature (350 °C), but as recrystallisation products are easily formed, activation temperature window is narrow. Endly, we have evidence that the presence of calcite in pozzolans has an effect on early strength. Therefore this study is giving new perspectives for a better use of natural pozzolanic materials in the cement industry.     

Centrifuge model tests on clay based engineered barriers subjected to differential settlements

The objective of this paper is to investigate the influence of thickness of the clay barrier and overburden pressure on the integrity of the clay barrier, which is provided in the cover system of low low-level radioactive waste disposal sites. A series of centrifuge tests were performed on model clay barrier subjected to continuous differential settlement at 40 g. The model clay barrier material has been selected in such a way that it envelopes the material characteristics of the clay barriers. The model clay barriers were moist-compacted on the wet side of its optimum moisture content. The performance of the clay barrier as an effective hydraulic barrier was monitored throughout the deformation process and attempts have been made to arrive at strain distribution along the top surface of clay barrier with the help of marker based digital image analysis. A 1.2 m thick clay barrier with a nominal overburden pressure of the order of 12.5 kPa was found to experience multiple cracking extending up to its full depth; whereas with overburden pressure of the order of 25 kPa, it was observed to have suppression of cracks. A catastrophic nature of water breakthrough was registered for a 0.6 m thick clay barrier with an overburden equal to that of a closure system of a low-level radioactive waste disposal site, once settlement ratio attains 0.6. This indicates that provision of thickness of clay barrier equal to 0.6 m may not be adequate.     

Synthesis of highly stabilized mesoporous molecular sieves using natural clay as raw material

Mesoporous molecular sieves were hydrothermally synthesized from natural clay and sodium silicate by using cetyl trimethyl ammonium bromide (CTAB) as a template. The samples were characterized by means of XRD, TEM, TPR, FT-IR and N₂ physical adsorption. The results show that well-ordered and highly stabilized mesoporous molecular sieves were obtained by adjusting the proportion of raw materials. The pore structure of the as prepared mesoporous molecular sieves was not damaged after calcination at 850 °C for 3 h or hydrothermal treatment at 100 °C for 10 days. The mesoporous ordering became better after hydrothermal treatment, but the pore shrank and the surface area decreased after thermal treatment. The stability of the as prepared mesoporous molecular sieves depends on the stability of the clay present in mesoporous pore walls. The presence of clay in the walls can be certified by the regular pore channel images as determined by TEM and XRD analyses.     

Bleaching of kaolins and clays by chlorination of iron and titanium

The quality of the clays and over all kaolin is measured in function of iron content, since this element gives an undesirable reddish color to this type of minerals. The use of chlorination for iron and titanium removal from different clay and kaolin minerals, used in Argentinian ceramic industry, has been investigated to establish reaction mechanisms, precautions and optimal conditions to bleach the clays. The method consists of the calcinations of the pellets in a flow of chlorine gas at temperatures between 700 and 950 °C, to remove the iron and the titanium by volatilization of the respective chlorides. Isothermal and non-isothermal chlorination assays were made and the effects of the temperature, reaction time, and carbon content in the sample over the bleach of the minerals and the phase transformations suffered by these minerals during the chlorination step were investigated. The removed amounts of iron and titanium were determined by X-ray fluorescence, the phase transformations were followed by X-ray diffraction and the samples' bleaching was established making an analysis of the space of the color by the spectrophofotometric method CIELAB. Also, a thermodynamic analysis of the system using HSC Chemistry for Windows software was made. The experimental results show that for red clay with high colloidal iron content, the optimum working temperature for the iron quantitative removal, without aluminium loss, is close to 850 °C, in controlled atmosphere free of O₂ and H₂O; lower temperatures require long reaction times. The extraction of titanium is not quantitatively achieved, as that of iron, even for long reaction times, because this element is present in different structures, with the consequent variation of its reactivity. The content of carbon and organic matter in the clays, in the case of the San Luis red clay, is enough for extraction improvement. However, the optimum concentration of carbon can vary depending on the type of clay and the amount of Fe₂O₃ in the mineral. The formation of phases such as α-Al₂O₃, a product of chlorination, increases the piece hardness. The methodology used in this work allows for obtaining a great improvement in the color of the final product of the firing, achieving ceramic materials with notable whiteness, even with red clays.     

Thermal analysis of high‐performance mortar containing burnt clay shale as a partial portland cement replacement in the temperature range up to 1000 °C

Clay shale is a specific type of material that contains a large amount of kaolinite. Burnt clay shale belongs to a large group of pozzolans, and its pozzolanic properties are activated after burning at temperatures similar to those when kaolinite is transformed into metakaolin. In this study, fine powder of burnt clay shale was used for the design of a high‐performance mortar as a partial replacement for portland cement up to 60 wt.%. The prepared specimens were subjected to a thermal analysis by using differential scanning calorimetry, thermogravimetry, and thermodilatometry. The investigation was performed in the temperature range 25–1000 °C. The basic physical and mechanical properties were studied as well. It was demonstrated that it is possible to design and produce a high‐performance mortar containing fine burnt clay shale powder and that an appropriate amount of this replacement is up to 20 wt.%. Copyright © 2016 John Wiley & Sons, Ltd.     

Behaviour of silicon released during alteration of nuclear waste glass in compacted clay

Long term integrated in situ experiments are performed in the HADES underground research facility (Mol, Belgium) in order to study the coupled reactivity between the different components of an underground repository for vitrified high level radioactive waste (HLW): glass, compacted clay, and stainless steel containers, at 90 °C and under gamma irradiation. Studies pertaining to the behaviour of silicon, a major element released during glass alteration, are presented here. Data collected from the integrated experiment, from simplified tests, and from modelling are put together, giving complementary information. The integrated experiment is used to investigate overall reactivity, whereas diffusion experiments coupled with modelling focused on the precipitation of silica in clay media. In the integrated in situ experiment, a bentonite clay (FoCa7) mixed with 5 wt.% of powdered glass frit was put in contact with U/Th-doped SON68 reference glass specimens for 1.2 years. One of the samples was thoroughly analysed by ESEM, SEM-EDS, TEM, XRD, and FTIR. The observations showed that after combined heating and γ irradiation, the glass/clay interface is still fully reactive. At this interface, the most important process occurring is silica precipitation either as spherical flaky nodules, or in the form of coatings on the clay surface, as chalcedony and amorphous silica. In parallel, numerical modelling was used to investigate the interactions between silica and clay material in a simplified experiment using diffusion cells. A fit between calculation results and silica migration measurements in diffusion cells was performed, including sensitivity tests with regard to the diffusion coefficients and the precipitation rate of amorphous silica. A satisfactory agreement is reached with the experimental results, using a set of reasonable fitting parameters for the FoCa7 clay material. Surface area values for silica precipitation are found to be very high, i.e. 2% of the total surface area of the clayey material indicating that silica precipitation occurs on surfaces other than those of pure silica minerals.     

Shelf life stability of diatomites

Diatomites or diatomaceous earths are biogenic sediments mainly consisting of porous siliceous algae skeletons. Some diatomites are industrially processed and used for different applications.For some applications and sometimes for the authorization of products for new applications it is important to prove the ‘shelf life stability’ which means that the properties of the product should not change upon storage.Freshly prepared silica gel is known to alter upon aging e.g. with respect to the specific surface area and porosity, respectively. This can be relevant for diatoms as well since recent skeleton consist of XRD amorphous silica which – upon aging – crystallizes, finally forming microcrystalline silica. The age of most of the diatomites being mined is higher than 1 million years. Accordingly, one could expect that storage for some years does not alter the diatomite properties. However, it is at least conceivable that prolonged open storage (exposure to air) can lead to the adsorption of molecules from air which in turn would block reactive adsorption sites.The aim of this study is to assess the shelf-life-stability of diatomite and perlite based on long term tests. Therefore, two different diatomite samples and a perlite sample (XRD amorphous volcanic alumosilicate glass) were stored at different conditions for 1 year. All materials were investigated with respect to structural aspects as well as adsorption capacity.Using the common mineralogical methods X-ray diffraction (XRD) and infrared spectroscopy no structural changes could be observed. In contrast, the determination of the amount of soda soluble silica turned out to be a valuable tool for the identification of structural changes. Using this method a slightly higher reactivity of the perlite surface after storage at 60 °C in water was found which possibly can be explained by the beginning devitrification process. At the same conditions, the diatomite sample which mostly consists of XRD amorphous silica showed a slight decrease of the amount of soda soluble silica which was interpreted as the beginning (re)crystallization process. The diatomite sample containing swellable clay minerals lost some water adsorption capacity upon extensive drying.Systematic changes of material properties after 1 year storage could only be observed under exaggerated conditions (120 °C or storage in water at 60 °C). It is concluded that the investigated materials are stable under common storage conditions.     

Moulding properties of a Nigerian silica–clay mixture for foundry use

The moulding properties of Igbokoda silica sand, bonded with Ijero–Ekiti clay, were investigated. American Foundrymen Society (AFS) standard cylindrical samples dimensioning Ø50 mm × 50 mm in height were prepared from various sand–clay ratios with 5% tempering water, by applying three ramming blows of 6.666 g each from a height of 50 mm as required for foundry sands. The samples were subjected to various physical and mechanical tests. These include permeability, green compression strength, and dry compression strength tests. Green shear strength, dry shear strength, field mould strength, shattered index and refractoriness tests were also carried out on the samples. Samples containing 23–32% clay were found to possess adequate permeability, good strength and refractoriness suitable for casting of both ferrous and non-ferrous alloys.     

The Influence of Slate Waste Activation Conditions on Mineralogical Changes and Pozzolanic Behavior

The transformation process of clay minerals into pozzolanic products for use as active additives in cement matrixes has been closely studied by both the scientific community and the cement industry. Sourcing these additions from recycled waste products is widely prioritized in environmental policies, because of their associated environmental benefits. This article reports an exhaustive analysis of slate waste in Spain, for its eventual use as an alternative source of pozzolans, based on activated phyllosilicates. The analysis examines the effect of activation conditions on mineralogy and the formation and evolution of the hydrated phases that form during the pozzolanic reaction at 28 days. The results show good pozzolanic activity in the starting slate wastes activated between 800°C and 1100°C of temperature for 2 h of retention, on which basis 1000°C and 2 h were recommended as the more suitable activation conditions for these activated slate wastes (ASW), as total destruction of all phyllosilicates is ensured under those conditions. The hydrated phases formed during the pozzolanic reaction in the ASW/Ca(\OH)₂ system were calcium silicate hydrate gels (CSH), randomly interstratified chlorite (Cl) /smectite with variables containing chlorite/smectite ratios 30% chlorite, 40% chlorite, 50% chlorite, 60% chlorite, respectively, and monosulfoaluminate (C₃A·SO₄Ca·12H₂O) directly related to the activation process used in this article. All of these findings support the viability of slate waste for use as a pozzolanic addition.     

Pozzolanic reactions of six principal clay minerals: Activation, reactivity assessments and technological effects

Six standard clays, before and after calcination at 3 or 4 temperatures and being mixed with Ca(OH)₂ [CH] in the presence of simulated cement pore solution, and with ordinary Portland cement, respectively, were studied in detail. Chemical compositions of most clays conform well to the requirement in ASTM C 618. Water demand of clay-containing mortar varies, depending on the crystal chemistry of raw clays, and on the specific surface area of calcined clays. Measurements of XRD background or alkali soluble Si are rapid methods in evaluation of the pozzolanic activity of clays. Compressive strength of mortars based on the raw clays is affected by structure of clays. Calcination increases the pozzolanic activity of clays and the compressive strength of the Portland cement — clay mortars. A close correlation exists between compressive strength of mortars and particle size distribution of the dehydroxylated clays. The most common reaction products of clay — CH mixtures are C-S-H² and C₄AH_x, while C₂ASH_j8 and C₃AH₆ were also detected with clays rich in Al.     

Influence of firing temperature on the ceramic properties of clays from Campos dos Goytacazes, Brazil

The characteristics of two clays used for red ceramic production in Campos dos Goytacazes, southeast of Brazil, were investigated. Characterization was performed by X-ray diffraction, chemical analysis, as well as granulometric and plasticity measurements. Technological properties related to water absorption, linear shrinkage and flexural strength were obtained from specific tests. Samples were prepared by pressing clay bodies at 20 MPa, which were then fired at temperatures from 850 to 1200 °C. The results showed that both clays from three deposits are highly plastic due to the elevated percentage of kaolinitic clay minerals. The samples richer in clay minerals present higher linear shrinkage as well as a decrease in water absorption in comparison to the others. The effectiveness of these clays for red ceramic production has been discussed in terms of the Winkler diagram.     

Adsorption and intercalation of tetracycline by swelling clay minerals

Many of commonly used pharmaceuticals are hydrophilic. In aqueous solutions, these molecules will have strong interaction with swelling clay minerals, resulting in intercalation of these compounds in the interlayer of the minerals and retention by these minerals. In this research, we studied the intercalation of tetracycline (TC) into swelling clay minerals as represented by montmorillonite and rectorite of different surface charges and different charge densities. The maximum interlayer expansion was seen at a higher pH when the TC molecules adopted an extended conformation, even though, the amount of TC intercalated at higher pH is lower. Under pH 4–5, the intercalated TC produced an interlayer gallery height of 9.2 Å compared to 10.3 Å produced at pH 8.7 in the interlayer space of rectorite. Due to TC intercalation, the full width at half maximum (FWHM) becomes much broader, suggesting that the number of fundamental layers stacking along c axis decreased to 2–3. Depending upon the nature of the swelling clay minerals and the TC concentration, the intercalation process can be transitional, involving in the occurrence of mechanical mixtures, materials of intermediate layer thicknesses, and/or mixed layering of different ordering states.     

Synthesis and characterization of low-charge sodium fluorophlogopite mica-type clay minerals

This work reports a solid-state synthesis of low-charge mica-type clay minerals using a fluoride flux method. Synthesized mica-type clay samples yielded hydrated phases with d-spacings in the range of 12.1–12.5 Å as revealed by powder X-ray diffraction (XRD). ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy revealed the Si and Al nearest neighbor environments as well as the presence of impurities such as amorphous silica and forsterite in these samples.     

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Na_x(Al_(2 − x)Mg_x)Si₄O₁₀(OH)₂), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.