Silicate Source Research Articles

Peculiar Optical Characteristics of Different Silicate Source and Synthesis Technique in Europium Doped Li2SrSiO4

In this work, a silicate based phosphor doped with divalent and trivalent europium (Eu(2+/3+)) ions have been synthesized by two different synthesis processes, PGMS (propylene glycol modified silane) assisted sol-gel and solid state reaction (SSR) method. The structural, morphological and optical properties were studied in detail. The emission intensity of Li2SrSiO4:Eu3+ phosphors synthesized through the PGMS assisted sol-gel increased up to 10 times more than that prepared by SSR method. However, Li2SrSiO4:Eu2+ phosphor prepared by PGMS exhibited lower emission characteristics and decreased to one-third of that prepared by SSR. The Eu3+ doped Li2SrSiO4 synthesized by SSR method using different silicate (Si) sources revealed peculiar optical properties and this is the first kind of its report. The obtained results suggest that the optical properties of Eu(3+/2+) doped in silicate host can be regulated by the alteration of Si source besides sample preparation method. These results open a way to understand the mechanisms responsible for such optical properties and to develop new materials for white LEDs.

A reaction model for cement solidification: Evolving the C–S–H packing density at the micrometer-scale

Cement paste is a multiphase material of complex chemistry, of which 60% by volume is typically composed of calcium–silicate–hydrates (C–S–H), the phase that lends the material its strength and stiffness. Moreover, it has been shown that the C–S–H phase is a dispersion of nanometer-sized particles that are characterized by an attractive–repulsive potential and densify in course of the hydration reaction. Herein, we model the nucleation and growth of the nanoparticles as a continuous density field subject to a reaction equation. Using this phase-field approach, we aim to reduce the parameter space present in similar hydration models and create a vehicle to upscale mechanical information from the nanometer-scale to the micrometer-scale. Despite the apparent simplification of the physics at play, we readily reproduce the cement paste reaction kinetics and microtexture—functions of temperature, coarseness of the calcium–silicate source particles, and initial water-to-cement ratio—, and vet them against experimental observations. Presenting results for two-dimensional simulations, we achieve excellent agreement with measurements of hydration heat curves, pore-chord-length and solid-chord-length density functions, distributions of low- and high-density C–S–H products, and elasticity.

The effects of selected minerals on laboratory simulated thermochemical sulfate reduction

Improved understanding of the reservoir mineral effects on the occurrence of thermochemical sulfate reduction (TSR) and possible mechanisms is critical for petroleum exploration and production. A series of hydrous pyrolysis experiments were conducted with and without the presence of montmorillonite, illite and quartz (silicates), calcite and dolomite (carbonates) at 360 °C and 50 MPa for 12 h, 48 h and 312 h. Gaseous compounds, C7–8 light hydrocarbons, n-C9+ and organic sulfur compounds as well as compound-specific δ34S analysis were conducted subsequently. The results show that with the heating time increasing or TSR advancing, n–C9+ decrease gradually accompanied by the increases of TSR products, H2S, alkylbenzenes, benzothiophenes (BTs) and dibenzothiophenes (DBTs). The δ34S values of BTs and DBTs progressively approached that of MgSO4 and the difference in their average δ34S values (Δ34SBT-DBT) decreased. Compared to experiments with the absence of minerals, the experiments with silicates, in particular montmorillonite, are depleted in n–C9+ and enriched in H2S, light monoaromatics and organosulfur compounds and smaller Δ34SBT-DBT. Experiments with carbonates demonstrated the opposite trend. This indicates that silicates, in particular montmorillonite, accelerated TSR rates while carbonates reduced them. Under the experimental conditions, the exchange cations within montmorillonite and on the surface of clays and talc precipitation can increase the concentrations of H+ and hence HSO4− and then catalyze the generation of isoalkanes, which are prone to be oxidized in TSR. This can reduce the activation energy required for the occurrence of TSR. It might therefore be possible for TSR to occur at relatively low temperatures in silicate reservoirs and source rocks in natural environments. Carbonate dissolution can reduce the concentrations of H+ and HSO4−, and hence reduce the TSR rates. This study demonstrates the importance of minerals in regulating the TSR occurrence and suggests that TSR can occur in siliciclasitic as well as carbonate formations in nature. Therefore, the significance of minerals should be considered in the evaluation of TSR and sour gas risk in TSR-altered hydrocarbon accumulations in different geological contexts.

Effect of ammonia molecules on the separation of pentlandite from serpentine using copper (II) as activator

Abstract Pentlandite, the material containing more than 90% of the world's nickel, is typical purified by froth flotation followed by mixing the concentrate with relatively high levels of MgO silicates. Furthermore, serpentine, a source of magnesium silicate, just like pentlandite, can be activated using cupric ion at the pH from 7 to 10, where copper hydroxides are stable species adsorbed on the mineral surface to promote xanthate adsorption. In this study, we used a copper ammonium complex as the selective activator for the pentlandite flotation; it showed a good adaptability to temperature and pH. Compared with the best activation effect of CuSO4, in the actual mineral flotation, the recovery of pentlandite using a cuprammonium solution as an activator increased by 10.18% with a slight decrease in the MgO level, whereas that of pure pentlandite flotation increased by 14%. Compared with the CuSO4 solution, Cu(NH3)42+ is the main stable species of the copper ammonium solution in mild alkaline solution (pH

Chemical weathering outputs from the flood plain of the Ganga

Transport of sediment across riverine flood plains contributes a significant but poorly constrained fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has important implications for chemical fluxes to the oceans and the impact of orogens on long term climate. We report water and bedload chemical analyses from the Ganges flood-plain, a major transit reservoir of sediment from the Himalayan orogen. Our data comprise six major southern tributaries to the Ganga, 31 additional analyses of major rivers from the Himalayan front in Nepal, 79 samples of the Ganga collected close to the mouth below the Farakka barrage every two weeks over three years and 67 water and 8 bedload samples from tributaries confined to the Ganga flood plain. The flood plain tributaries are characterised by a shallow δ18O - δD array, compared to the meteoric water line, with a low δDexcess from evaporative loss from the flood plain which is mirrored in the higher δDexcess of the mountain rivers in Nepal. The stable-isotope data confirms that the waters in the flood plain tributaries are dominantly derived from flood plain rainfall and not by redistribution of waters from the mountains. The flood plain tributaries are chemically distinct from the major Himalayan rivers. They can be divided into two groups. Tributaries from a small area around the Kosi river have 87Sr/86Sr ratios >0.75 and molar Na/Ca ratios as high as 6. Tributaries from the rest of the flood plain have 87Sr/86Sr ratios ≤0.74 and most have Na/Ca ratios <1. One sample of the Gomti river and seven small adjacent tributaries have elevated Na concentrations likely caused by dissolution of Na carbonate salts. The compositions of the carbonate and silicate components of the sediments were determined from sequential leaches of floodplain bedloads and these were used to partition the dissolved cation load between silicate and carbonate sources. The 87Sr/86Sr and Sr/Ca ratios of the carbonate inputs were derived from the acetic-acid leach compositions and silicate Na/Ca and 87Sr/86Sr ratios derived from silicate residues from leaching. Modelling based on the 87Sr/86Sr and Sr/Ca ratios of the carbonate inputs and 87Sr/86Sr ratios of the silicates indicates that the flood plain waters have lost up to 70% of their Ca (average ∼ 50%) to precipitation of secondary calcite which is abundant as a diagenetic cement in the flood plain sediments. 31% of the Sr, 8% of the Ca and 45% of the Mg are calculated to be derived from silicate minerals. Because of significant evaporative loss of water across the flood plain, and in the absence of hydrological data for flood plain tributaries, chemical weathering fluxes from the flood plain are best calculated by mass balance of the Na, K, Ca, Mg, Sr, SO4 and 87Sr/86Sr compositions of the inputs, comprising the flood plain tributaries, Himalayan rivers and southern rivers, with the chemical discharge in the Ganga at Farakka. The calculated fluxes from the flood plain for Na, K, Ca and Mg are within error of those estimated from changes in sediment chemistry across the flood plain (Lupker et al., 2012, Geochemica Cosmochimica Acta). Flood plain weathering supplies between 41 and 63% of the major cation and Sr fluxes and 58% of the alkalinity flux carried by the Ganga at Farakka which compares with 24% supplied by Himalayan rivers and 18% by the southern tributaries.

Evaluating an Eco-Olivine Nanosilica as an Alternative Silica Source in Alkali-Activated Composites

In this study, an eco-olivine nanosilica is produced from natural olivine, and then used as an alternative silicate source in synthesizing the silicate-based alkali activator. The effects of silicate origin and dosage on activator characteristics, fresh behaviors, gel structure, strength, porosity, and shrinkage are investigated. The results indicate that the soluble silica content in the alternative activators is above 98%; compared with the traditional waterglass-based mixes, the nanosilica-derived ones exhibit similar behaviors in the early age reaction, gel structure, and mechanical properties. As some important engineering properties, nanosilica-based mixtures also exhibit increased workability, prolonged setting time, slightly increased porosity, and shrinkage. Thus, by using this alternative silica source in alkali-activated materials, a more sustainable product with less cost and carbon emission can be achieved, and with comparable key performances.

New data on the continental deposits from the Cao Bang Basin (Cao Bang-Tien Yen Fault Zone, NE Vietnam) – biostratigraphy, provenance and facies pattern

The Cao Bang Basin is the northernmost of the basins related to the Cao Bang-Tien Yen Fault Zone in northern Vietnam. The basin is filled with a thick series of continental deposits. However, the exact age of the sedimentary basin infill has been under discussion for a long time. Because of new published data, the authors have decided to revisit this basin. Palynological data has allowed us to assign the Cao Bang Basin infill to the Lower Oligocene PC1 complex of the Shangcun Fm. (southern China). Among the saccate grains of gymnosperms, the domination of Cathaya and Pinus was observed, whereas angiosperms are represented by Carya , Celtis , Hammamelidaceae, Ulmus and also Pterocarya , Quercus , the Castanea – Castanopsis – Lithocarpus group, and the Loranthaceae. Among pteridophytes occur Laevigatosporites , Osmundaceae, and Pteris . The sedimentological features of the Cao Bang Basin are distinct from those of other basins from the Cao Bang-Tien Yen Fault Zone. The basin is filled with a wide variety of clastic deposits, from some of coarse-grained, alluvial-fan origin, through sandy beds of fluvial origin up to fine, organic-rich lacustrine deposits. The coarse-grained lithofacies are built of clasts derived mainly from local sources. The sandstones from the basin equally are submature or immature. They contain a lot of lithoclasts, the composition of which depends on the sample location within the basin. The potential source area is composed of older sedimentary units and of granitic rocks. The geochemical samples studied reflect the geochemical composition of silicic source rocks with only a minor contribution of basic components. The succession that fills the basin is interpreted as a typical fill for relatively long-lasting evolving half-graben or strike-slip basins. Moreover, the basin is partly occupied by a subsequent present-day sedimentary basin of Quaternary age.

VARIABLE FACTORS CONTROLLING AMORPHOUS-ZEOLITE PHASE TRANSFORMATION IN METAKAOLIN BASED GEOPOLYMER

Metakaolin based geopolymer binder was synthesized using variable factors such as Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O molar ratios as well as curing time. Metakaolin was used as an alumino silicate source incorporating with strong alkali activators; sodium hydroxide solution to undergo polycondenation and hence a formation of hardened geopolymeric binder. This work was aimed to investigate the effect of Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O ratios, as well as curing time on phase transformation. The chemical composition, phase development and microstructure of reaction products were examined by X-ray fluorescence, X-ray diffraction analysis and scanning electron microscopy with respect to their final compressive strength. Increasing the molar ratios of Na2O/Al2O3 and H2O/Na2O tended to favour the transformation of amorphous gel to various types of crystalline zeolite. Sodalite (pseudo-phase zeolite) and zeolite belonging to the Faujasite group, as well as zeolite X and A were identified in these mixtures. Based on this study, systems that favoured the formation of zeolitic products tended to possess lower strengths.

Direct Crystallization of Layered Silicates on the Surface of Amorphous Silica.

We overview the entire surface coverage of amorphous silica with layered silicates (smectite-like) using the direct crystallization technique under hydrothermal conditions. Various shapes (e. g., microspheres, fibers, microcapsules) have been available as the silica substrates, which involve partial dissolution triggered by the hydrolysis of urea for supplying a source of the layered silicates. Other sources (e. g., Li+ , Mg2+ , Al3+ ) have been supplied from aqueous solutions, participating the heterogeneous nucleation of the layered silicates. Because of partial dissolution of the silica substrates, the original shape has been preserved after the hydrothermal reactions. Cation exchange reactions, usually observed for conventional smectite systems, occurred by replacing the interlayer cations in the silicates with metal (a rare-earth ion) and organic (a quaternary long-chain alkylammonium, methylene blue, and a tris-chelated complex) cations. We have discussed the mechanism of the crystallization and displayed some applications of the hybrids (e. g., chiral discrimination, filtering, and magnetic recovery).

River and Submarine Groundwater Discharge Effects on Diatom Phytoplankton Abundance in the Gulf of Alaska

The Gulf of Alaska is a highly productive ecosystem that supports fisheries and subsistence harvesting of marine resources. The highly productive summer season begins with a bloom that is dominated by diatoms. Both river and submarine groundwater discharge have been recognized as substantial terrestrial nutrient (nitrate and silicate) sources to the Gulf’s coastal waters. Here, the response of in-situ phytoplankton to groundwater and river water additions was evaluated via a bioassay incubation experiment. Special attention was given to diatom genera, as previous studies have shown that submarine groundwater discharge preferentially induces growth of diatoms. The abundance of Pseudo-nitzschia spp., Chaetoceros spp., and Leptocylindrus spp. increased significantly in groundwater and river water containing treatments. Although groundwater and river water are both rich in nitrate and silicate, groundwater treatments with a higher salinity favored a higher relative abundance of Pseudo-nitzschia spp. Conversely, in the highest river water concentration treatments with lower salinity, relative abundances of Pseudo-nitzschia spp. decreased, while Chaetoceros spp. and Leptocylindrus spp. increased. Total abundances of all three genera increased in the lower salinity treatments. These findings could portend changes in the phytoplankton community composition in the Gulf of Alaska as the climate warms and river discharge increases in the coming decades. Furthermore, the findings support previous assertions that submarine groundwater discharge, with higher salinity than river water, is a preferable source of nutrients to the genus Pseudo-nitzschia.

Interaction of mantle magmas and fluids with crustal fluids at the 1894 Ma Montviel alkaline-carbonatite complex, Canada: Insights from metasomatic and hydrothermal carbonates

Alkaline and carbonatite rocks are relatively rare but offer the opportunity to study the contribution of fluids in the genesis of mantle and crustal rocks because they are commonly affected by metasomatism. Carbonate minerals represent versatile archives of mantle and crustal magmatic-hydrothermal processes because they can have magmatic, metasomatic or hydrothermal origins and because they host the trace elements, stable and radiogenic isotopes required to unravel their petrogenesis. Previous studies have shown that the 1894Ma Montviel alkaline‑carbonatite complex was emplaced through four injections of volatile-saturated, mantle magmas which evolved through fractional crystallization, mixing of mantle and crustal fluids and metasomatism. Trace element analyses and δ18O, δ13C, 87Sr/86Sr and 143Nd/144Nd isotope compositions of metasomatic and hydrothermal carbonates further support that each magma injection was accompanied by a volatile phase. Variations in trace element concentrations suggest that the carbonatite might have exsolved from a metasomatized mantle or hybrid silicate‑carbonatite magma, and that the fluid composition evolved towards higher REE and lower HFSE with increasing degree of segregation of the carbonatite magma and the silicate source. A strong correlation between the C-O-Sr isotopic systems show that mantle fluids mixed with crustal fluids, increasing the 87Sr/86Sr from mantle to crustal values, and driving the C and O isotopic ratios towards respectively lighter and heavier values. The Sm/Nd isotopic system was weakly coupled with the other isotopic systems as depleted mantle fluids mixed with crustal fluids and metasomatized the crystallizing magmas, thereby redistributing the REE and affecting their Sm/Nd ratios. The Nd isotopes suggest that the mixed mantle/crustal fluids redistributed the rare earth elements, producing ultra-depleted (εNd=+10), normally depleted (εNd=+4) and slightly enriched (εNd=−2) isotopic compositions.

“Undatable, unattractive, redundant”? The Rapavi silcrete source, Saint-Pierre-Eynac (Haute-Loire, France): Challenges studying a prehistoric quarry-workshop in the Massif Central mountains

Silcretes are not widely discussed archaeologically as a prehistoric lithic resource in Europe, despite there being many years of geological research into their formation, and in some regions, a long record of their use in prehistory. In the Massif Central, south eastern France, various silcrete sources represent some of the largest and best quality siliceous stone sources within this otherwise volcanic area. Field research within a wider programme of landscape archaeology and lithic sourcing identified a quarry-workshop for one of these silcrete sources, at Rapavi, Saint-Pierre-Eynac (Haute-Loire).Archaeological and geological results are reported, discussing the technological behaviour uncovered at this locale and its wider regional connections, including the presence of imported flint artefacts. Additionally, the Rapavi silcrete provides an example of the challenges encountered when analysing raw materials with idiosyncratic fracture properties, and attempting to disentangle quarry-workshop palimpsests.

Research on Morphology and Growth Mechanism of Xonotlite Crystal

The xonotlite crystals were synthesized via the hydrothermal synthesis manner from CaO and SiO2 as the raw materials with their Si/Ca molar ratio of 1.0. The morphologies and their growth mechanisms of xonotlite crystal were explored by taking the various siliceous sources into account in this paper. These products were characterized by XRD and SEM techniques to investigate their crystalline phase and morphology. The results indicated that siliceous material had little influence on the crystalline phases of xonotlite crystals but posed a great impact on their morphologies. Based on the various morphologies characteristics of the products, the growth mechanism for the morphologies of spherical particles and fibers were further revealed according with the in situ transformation mechanism and dissolution-precipitation mechanism respectively.

Sintering behavior of waste serpentine from abdasht chromite mines Abdasht chromite mines and kaolin blends

Abstract The aim of this study is investigating on sintering behavior of Abdasht waste serpentine and kaolin blends. According to this, three formulations of dry milled waste serpentine with 25%, 50% and 75% high grade kaolin were wet milled. The slurries were then dried, sieved and uniaxially pressed at 100 MPa and fired for 2 h soaking time at temperatures between 1100 and 1400 °C. Sintered samples were investigated by bulk density, apparent porosity, water absorption, linear shrinkage and phase changes with raising temperatures in order to characterize their sintering process. It was revealed that all samples were starting to melt at 1350 °C and the sintering was completed for all specimens at 1300 °C. The only phases of fully sintered samples were cordierite and enstatite. Cordierite concentration, however, increased with enhancing kaolin percentage in composition. The results of this study can introduce Abdasht waste serpentine as magnesium silicate source into the ceramic industries and may help to solve environmental problems caused by several million tones wastes in Abdasht chromite mines.

Identifying the main sources of silicate in coastal waters of the Southern Gulf of Valencia (Western Mediterranean Sea)

Summary Silicon is a major nutrient for siliceous primary producers, which can become a potential limiting nutrient in oligotrophic areas. Most of the silicon inputs to the marine environment come from continental discharges, from both superficial and ground waters. This study analyses the main sources of silicon and their dynamics along the southernmost 43 km of shoreline in the Gulf of Valencia (Western Mediterranean Sea). The salinity and silicate concentration in the different compartments (springs, freshwater wells, beach groundwater, surf zone and coastal waters) in this coastal area were determined. In addition, chlorophyll a and phytoplankton community were analyzed in the surf zone and coastal waters. Silicate concentrations in freshwater wells ranged between 130 and 150 μM, whereas concentrations of this nutrient declined to 49 μM in freshwater–seawater mixture transects. At the same time, there was a positive gradient in silicate for both freshwater and coastal waters southward. An amount of 18.7 t of dissolved silicate was estimated in the nearest first kilometre nearest to the coastline, 6 t of this silicate belonged to the background sea level. On the other hand, the sum of the main rivers in the area supplies 1.6 t of dissolved silicate per day. This implies that a large amount of the remaining 11.1 t must derive from submarine groundwater discharges, which would thus represent 59% of the coastal dissolved silicate budget. Overall, it is suggested that a subterranean transport pathway must contribute considerably to silicate concentrations throughout this zone, which is characterized as permeable.

Investigation on a green olivine nano-silica source based activator in alkali activated slag-fly ash blends: Reaction kinetics, gel structure and carbon footprint

In this paper, a green olivine nano-silica is synthesized and applied as an alternative silicate source to prepare alkali activators, and a commercial waterglass based silica source is studied as a reference. The synthesis route and characterization of olivine nano-silica is presented. The effects of silicate origin and dosage on activator characteristics, reaction kinetics, gel structure and strength are investigated and the CO2 footprint is evaluated. The results show that increasing the activator modulus significantly increases the high crosslink Q contents in the alkali solution, nano-silica based ones exhibit slightly higher percentages of Q3 sites. Nano-silica based mixes exhibit comparable properties regarding the reaction intensity, chemically bound water content and strength. Gel compositions of both nano-silica and waterglass based samples are characterized in detail by using solid 29Si and 27Al MAS NMR. Moreover, replacing commercial waterglass by this alternative silicate source reduces the CO2 emission between 20.4% and 29.0%.

Synthesis of titanium containing MCM-41 from industrial hexafluorosilicic acid as epoxidation catalyst

The industrial by-product hexafluorosilicic acid was investigated as silicate source for titanium containing MCM-41 (Si/Ti–MCM-41) synthesis. An extended and detailed study, which includes the effects of several factors such as the state and content of Ti, surfactant/Si ratio and template removal technique on the physicochemical properties and catalytic activity of Si/Ti–MCM-41, is presented; The Si/Ti–MCM-41 was also characterized by powder X-ray diffraction, N2 adsorption-desorption, fourier transform infrared spectroscopy, ultraviolet-visible spectrophotometer, thermal gravimetric measurements, X-ray fluorescence and scanning electron microscopy. The results show that the specific surface area and pore volume of the molecular sieve reach 1040m2g−1 and 0.74cm3g−1 under the hydrothermal conditions of the Si/Ti mole ratio equal to 60, CTAB/Si mole ratio to 0.81, hydrothermal temperature at 343K and time for 3h. The catalytic performance shows that samples have higher activity and selectivity for cyclohexene epoxidation to produce cyclohexene oxide. With the reaction temperature 333K and mole ratio of cyclohexene/tertiary butyl peroxide hydrogen equal to 1, the highest cyclohexene conversion and epoxide selectivity is 79.23% and 95%, respectively. The catalyst activity has not obvious change with two times recycles.

Two-Dimensional Porous SiO2 Nanostructures Derived from Renewable Petal Cells with Enhanced Adsorption Efficiency for Removal of Hazardous Dye

Diverse microstructures and morphologies from plant cells inspire us with great opportunities for creating novel nanomaterials. In this work, a biomorphic mesoporous SiO2 with unique two-dimensional (2D) nanostructure was feasibly fabricated by employing renewable petal cells as bioscaffolds. During the structure formation, the hydrolyzed siliceous species initially adhere to and then penetrated the cell walls, forming the composite of siliceous species/cells. This is followed by shrinkage and deformation of the cell skeleton in a subsequent drying process. Then, the special 2D SiO2 with abundant internal mesopores was acquired after careful removal of the cells. The concentration of siliceous source (tetraethyl orthosilicate, TEOS) in the impregnation/infiltration steps is a key factor for the replication of the biological morphology for SiO2. The sample prepared with CTEOS of 0.05 mol L–1 can duplicate well the biomorphology of petal cells, which has a BET surface area of 177 m2 g–1 and pore size rangin...

The utilization of Şaphane alunite in geopolymer production

The aim of this work is to research the usage of alunite from the alunite deposits of Şaphane-Kutahya (Turkey) as a raw material in the production of a geopolymer material. Alunite samples were characterized and prepared to use in body recipes. Manufacturing of geopolymer was conducted by mixing alumina silicate sources, silica fume, and sodium silicate solution. The geopolymer samples were cured at 60 °C for 24 h. Chemical and microstructural characterization of materials prepared from representative samples was carried out using X-ray fluorescence (XRF), X-ray diffraction (XRD), 27Al-magic angle spinning nuclear magnetic resonance (27Al MAS-NMR), and scanning electron microscopy (SEM), techniques, respectively. The compressive strength of the geopolymer materials was obtained from the samples after 7 and 28 days. Compressive strength measurements show a maximum strength of almost 48 MPa after 28 days. As a result of this study, it has been shown that alunite minerals can be utilized as alternative raw materials for production of geopolymer mortars.

THAUMASITE NON-SULPHATE ATTACK AT AMBIENT TEMPERATURE AND PRESSURE

Abstract One of the products of concrete sulphate corrosion, besides gypsum and ettringite, is thaumasite. The thaumasite is a very dangerous, non binding crystalline phase, which is forming at the expense of C-S-H phase. There was a conviction that the conditions required for the formation of thaumasite in concrete are: source of calcium silicate, sulfate ions, carbonate ions and a very wet, cold (below 15°C) environment. The corrosion of concrete caused by the external source of sulphate ions during which thaumasite is formed was called thaumasite sulphate attack (shortly TSA). While the TSA is recognized, the thau- masite non-sulphate attack (T n S A ) must be highlighted, because is also possible. The purpose of this work is to show that thaumasite, or solid solutions of Ett-Th (ettringite with thaumasite) are able to form in hardened cement paste without external source of sulphate ions, at ambient temperature and pressure (25±2°C (298.15 K) and 102±1 kPa). The experiment appeared on thaumasite formation in corroded specimen made of CEM I (Portland cement) and of CEM III (slag cement) after 4 days of immersion in saturated water solution of NH4Cl.