Calcium Silicate Hydrate Phases Research Articles

Impact of different hydrated cementitious phases on moisture-induced damage in lime-stabilised subgrade soils

Durability is a major concern for all stabilised materials as it can reduce the structural capacity of pavement layers. One of the primary factors influencing the long-term performance of stabilised subgrade layers is moisture. The structural properties of these engineered materials starts to deteriorate with ingress of external water. The detrimental effect of soaking depends on the level of pozzolanic reaction achieved by the stabilised layer prior to water intrusion. Although the affinity for water is reduced after stabilisation, some amount of water is held within the matrix by the precipitated Calcium Silicate Hydrate (C-S-H) phases due to their inherent water adsorption capacity. The extent of damage in turn depends on the type and concentration of different cementitious phases due to the structural, compositional and morphological differences. This study tries to determine the type of hydrated cementitious phase that is more resilient to moisture-induced damage. Results indicate the presence of C-S-H II phase to be beneficial for durability of lime-treated soil when compared to C-S-H I phase.

Effect of alkali-activated metakaolin cement on compressive strength of mortars

This paper reports the compressive strength and microstructure characteristic of alkali-activated metakaolin (MK) cement under two curing methods. MK was used to replace part of Portland cement (PC) at 70, 80, 85, 90, 95 and 100% by mass of binder. Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used as activating agents (Na2SiO3 to NaOH ratio of 0.67). NaOH with 10 molar concentrations, liquid alkali/binder ratio of 0.8 and water base was used. Mortar and paste specimens were cured at 23±2°C (55% RH) and 60°C (95% RH). The results indicated that calcium silicate hydrate (C–S–H) and calcium carbonate (CaCO3) phases were detected by thermogravimetry (TGA). Setting time and drying shrinkage decreased when metakaolin replacement level increased, the compressive strength of alkali-activated metakaolin cement increased with the increase of PC content and increased with temperature. The curing of specimens at 60°C (95% RH) enhanced the compressive strength, matrices appeared denser than when cured at 23±2°C (55% RH) and phases of (C, N)-A-S-H gel and or C–S–H gel can be seen clearly.

Effect of CaO on the reaction process of MgO-SiO2-H2O cement pastes

As a potential cementitious material for nuclear waste immobilization magnesium silicate hydrate (M-S-H) phase was prepared by MgO and silica fume at room temperature. Attributable to the slow hydrolysis rate of MgO, the strength of M-S-H paste is lower than Portland cement in 3days. This work has demonstrated that CaO content has a direct influence on the chemical environment in pore solution and phases transformation in CaO-MgO-SiO2-H2O (C/M/S/H) system. Appropriate CaO content (with optimal Ca/Si molar ratio at approximately 0.02) can accelerate M-S-H gel formation in high W/C paste contributing to early age strength improvement. Ca/Si molar ratio should be controlled at a range between 0.02 and 0.1, because the formation of the calcium silicate hydrate (C-S-H) phase is much is easier, which results in the inhibition of the M-S-H formation.

In situ interactions between Opalinus Clay and Low Alkali Concrete

A five-year-old interface between a Low Alkali Concrete (LAC) formulation (CEM III/B containing 66% slag and 10% nano-silica) and Opalinus Clay (OPA) from a field experiment at Mont Terri Underground Rock Laboratory in Switzerland (Jenni et al., 2014) has been studied to decipher the textural, mineralogical and chemical changes that occurred between the two reacting materials.Reactivity between LAC concrete and OPA is found to be limited to a ∼1 mm thick highly porous (ca. 75% porosity) white crust developed on the concrete side. Quantitative mineralogical mapping of the white crust using an electron microprobe and infrared spectroscopy on the cement matrix provides evidence of a Mg-rich phase accounting for approximatively 25 wt % of the matrix associated with 11 wt % of calcite, calcium silicate hydrate (C-S-H) and other cement phases. EDX analyses and electron diffraction combined with transmission electron microscopy of the Mg-rich phase provide evidence for a tri-octahedral 2:1 phyllosilicate with mean composition:(Ca0.5±0.2) (Mg2.0±0.4, Fe0.2±0.1, Al0.5±03, □0.3±0.3) (Al0.9±0.2, Si3.1±0.2) O10 (OH)2, where □ represents vacancies in the octahedral site.Apart from this reactive contact, textural, mineralogical and chemical modifications at the contact with the LAC concrete are limited. OPA mineralogy remains largely unmodified. X-ray micro-fluorescence and EPMA mapping of major elements on the OPA side also provides evidence for a Mg-enriched 300–400 μm thick layer. The cation exchange capacity (CEC) values measured in the OPA in contact with the LAC concrete range between 153 and 175 meq kg−1 of dry OPA, close to the reference value of 170 ± 10 meq kg−1 of dry OPA (Pearson et al., 2003). Changing cation occupancies at the interface with LAC concrete are mainly marked by increased Ca, Mg and K, and decreased Na. Leaching tests performed on OPA with deionized water and at different solid to water ratios strongly suggest that Cl and SO4 have either conservative behaviour or are constrained by the solubility of a precipitated sulfate phase. The Cl and SO4 concentrations measured at 2 cm from the interface are close to concentrations of undisturbed OPA pore waters (SO4: 4.5 ± 1.5 mmol kg−1 of dry OPA; Cl: 7.5 ± 2.1 mmol kg−1of dry OPA), and increase towards the interface with the concrete. The SO4 to Cl ratio also increases towards the interface, suggesting that the increasing anion concentrations are not related to porosity variations but rather to a concentration gradient and sulfate phase precipitation near the interface.

Analysis of barium retention mechanisms on calcium silicate hydrate phases

Calcium silicate hydrate (C-S-H) phases mostly contribute to contaminant retention in cement. The adsorption mechanisms of Ba on C-S-H phases with four different Ca/Si ratios were investigated by batch sorption experiments. The sorption kinetics, the effects of the solid to liquid ratio and the presence of alkalis in solution were analysed. Sorption isotherms in a wide range of Ba concentrations (from 10−10 to 1·10−1M) were performed.To understand the underlying retention mechanisms, the effects of Ba adsorption on surface potentials were additionally examined. The main objective of this study was to develop a sorption model able reproducing not only the Ba sorption behaviour, but also the C-S-H surface potential before and after Ba uptake. The reactions and model parameters selected for satisfying these conditions are discussed in the paper. A three-site model, with weak and strong silanol sites and one exchange site, which satisfactorily fulfils the initial aims, is proposed.

Influence of the Ca/Si ratio on the compressive strength of cementitious calcium–silicate–hydrate binders

Calcium–silicate–hydrate phases have been synthesized with Ca/Si ratios of 0.83–1.50 and it is demonstrated that the compressive strengths of the C–S–H pastes increase for decreasing Ca/Si ratio for all samples and testing ages.

Confocal Raman microscopy as a non-destructive tool to study microstructure of hydrating cementitious materials

Non-destructive investigation of hydrated cement microstructure is still a big challenge. In this study, we present non-destructive image and structural analysis of hydrated monoclinic C3S clinker by means of confocal Raman microscopy (CRM), which to the best of our knowledge is used for studying hydrating cementitious systems for the first time. In detail, CRM enabled structural and spatial characterization of calcium-silicate-hydrate phases (C-S-H), portlandite (CH) and reactants on the micrometer scale without the need for extensive sample preparation. Two complementary data analysis tools (cluster analysis and graph basis analysis) revealed that m-C3S can be fully or partly hydrated to C-S-H as the ubiquitous phase. Furthermore, CH could be identified as macroscopic flakes of different size as well as intimately mixed with the C-S-H phase on a sub-micrometer scale. In addition, good quality Raman spectra of non-synthetic C-S-H, containing rarely reported lattice vibrations at ~130cm−1 and other characteristic Raman bands, are reported over a broad spectral range from around 100 up to 3700cm−1. Hence, this study introduces CRM as a promising novel technique for microstructural characterization of hydrating cementitious pastes.Cement chemistry notation: C: CaO, S: SiO2, H: H2O

Formation of C-A-S-H phases from the interaction between concrete or cement and bentonite

AbstractConcrete and bentonite are being considered as engineered barriers for the deep geological disposal of high-level radioactive waste in argillaceous rocks. Three hydrothermal laboratory experiments of different scalable complexity were performed to improve our knowledge of the formation of calcium aluminate silicate hydrates (C-A-S-H) at the interface between the two materials: concretebentonite transport columns, lime mortar-bentonite transport columns and a portlandite- (bentonite and montmorillonite) batch experiment. Precipitation of C-A-S-H was observed in all experiments. Acicular and fibrous morphologies with certain laminar characteristics were observed which had smaller Ca/Si and larger Al/Si ratios with increasing temperature and lack of accessory minerals. The compositional fields of these C-A-S-H phases formed in the experiments are consistent with Al/(Si+Al) ratios of 0.2– 0.3 described in the literature. The most representative calcium silicate hydrate (C-S-H) phase from the montmorillonite–cement interface is Al-tobermorite. Structural analyses revealed a potential intercalation or association of montmorillonite and C-A-S-H phases at the pore scale.

Thermodynamic of incongruent solubility of C–S–H

In most situations, the pore solution of cement-based materials is in thermodynamic equilibrium with the calcium silicate hydrates resulting from the hydration of (tri–di)calcium silicates. This equilibrium is, however, difficult to characterise, the solution phase being incongruent with respect to the calcium silicate hydrate phase. A thermodynamic description of this equilibrium is proposed and some new relationships are derived involving the saturation indexes of dissolved pure substances and the calcium/silicon ratio of the calcium silicate hydrate. These relationships can be understood as a generalisation of the law of mass action for incongruent chemical reactions. The validity of these relationships was checked using measurements reported in the literature.

Thermodynamic of incongruent solubility of C–S–H

In most situations, the pore solution of cement-based materials is in thermodynamic equilibrium with the calcium silicate hydrates resulting from the hydration of (tri–di)calcium silicates. This equilibrium is, however, difficult to characterise, the solution phase being incongruent with respect to the calcium silicate hydrate phase. A thermodynamic description of this equilibrium is proposed and some new relationships are derived involving the saturation indexes of dissolved pure substances and the calcium/silicon ratio of the calcium silicate hydrate. These relationships can be understood as a generalisation of the law of mass action for incongruent chemical reactions. The validity of these relationships was checked using measurements reported in the literature.

The tobermorite supergroup: a new nomenclature

AbstractThe name 'tobermorites' includes a number of calcium silicate hydrate (C-S-H) phases differing in their hydration state and sub-cell symmetry. Based on their basal spacing, closely related to the degree of hydration, 14, 11 and 9 Å compounds have been described. In this paper a new nomenclature scheme for these mineral species is reported. The tobermorite supergroup is defined. It is formed by the tobermorite group and the unclassified minerals plombièrite, clinotobermorite and riversideite. Plombièrite ('14 Å tobermorite') is redefined as a crystalline mineral having chemical composition Ca5Si6O16(OH)2·7H2O. Its type locality is Crestmore, Riverside County, California, USA. The tobermorite group consists of species having a basal spacing of ∼11 Å and an orthorhombic sub-cell symmetry. Its general formula is Ca4+x(AlySi6–y)O15+2x–y·5H2O. Its endmember compositions correspond to tobermorite Ca5Si6O17·5H2O (x= 1 andy= 0) and the new species kenotobermorite, Ca4Si6O15(OH)2·5H2O (x= 0 andy= 0). The type locality of kenotobermorite is the N'Chwaning II mine, Kalahari Manganese Field, South Africa. Within the tobermorite group, tobermorite and kenotobermorite form a complete solid solution. Al-rich samples do not warrant a new name, because Al can only achieve a maximum content of 1/6 of the tetrahedral sites (y= 1). Clinotobermorite, Ca5Si6O17·5H2O, is a dimorph of tobermorite having a monoclinic sub-cell symmetry. Finally, the compound with a ∼9 Å basal spacing is known as riversideite. Its natural occurrence is not demonstrated unequivocally and its status should be considered as “questionable”. The chemical composition of its synthetic counterpart, obtained through partial dehydration of tobermorite, is Ca5Si6O16(OH)2. All these mineral species present an order-disorder character and several polytypes are known. This report has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification.

A luminescence line-narrowing spectroscopic study of the uranium(VI) interaction with cementitious materials and titanium dioxide.

Non-selective luminescence spectroscopy and luminescence line-narrowing spectroscopy were used to study the retention of UO2(2+) on titanium dioxide (TiO2), synthetic calcium silicate hydrate (C-S-H) phases and hardened cement paste (HCP). Non-selective luminescence spectra showed strong inhomogeneous line broadening resulting from a strongly disordered UO2(2+) bonding environment. This problem was largely overcome by using luminescence line-narrowing spectroscopy. This technique allowed unambiguous identification of three different types of UO2(2+) sorbed species on C-S-H phases and HCP. Comparison with spectra of UO2(2+) sorbed onto TiO2 further allowed these species to be assigned to a surface complex, an incorporated species and an uranate-like surface precipitate. This information provides the basis for mechanistic models describing the UO2(2+) sorption onto C-S-H phases and HCP and the assessment of the mobility of this radionuclide in a deep geological repository for low and intermediate level radioactive waste (L/ILW) as this kind of waste is often solidified with cement prior to storage.

Investigation of damaged interior walls using synchrotron-based XPS and XANES.

Evidence of internal sulfate attack in field exposure was demonstrated by the damaged interior wall of a three-year-old house situated in Nakhon Ratchasima Province, Thailand. Partial distension of the mortar was clearly observed together with an expansion of a black substance. Removal of the black substance revealed a dense black layer. This layer was only found in the vicinity of the damaged area, suggesting that this black material is possibly involved in the wall cracking. By employing synchrotron-based X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) techniques, the unknown sample was chemically identified. The S 2p and O 1s XPS results mutually indicated the existence of sulfate species in the materials collected from the damaged area. The XANES results indicated the presence of ferrous (II) sulfate, confirming sulfate-induced expansion and cracking. The sulfate attack in the present case appeared to physically affect the structure whereas the chemical integrity at the molecular level of the calcium silicate hydrate phase was retained since there was a lack of spectroscopic evidence for calcium sulfate. It was speculated that internal sulfate probably originated from the contaminated aggregates used during the construction. The current findings would be beneficial for understanding the sulfate-attack mechanism as well as for future prevention against sulfate attack during construction.

Microstructural characterisation of lightweight granules made from masonry rubble

The pore structure of lightweight granules made from masonry rubble was studied in order to better understand their engineering properties. Thermally and hydrothermally hardened granules were tested. Analysis by ESEM, mercury porosimetry and sorption methods yield important insight into their microstructure. The thermal granules are characterised by partly melted vitreous areas and large internal macropores that are connected via narrow throats. They show a marginal specific surface area along with a hydrophobic behaviour. In contrast, the hydrothermal granules have an accessible mesoporous system containing plate-like and ink-bottle pores. The shape of their water isotherms depending on the granules’ CaO content is sensitive to the morphology of calcium silicate hydrate phases (CSH). The hysteresis changes from a narrow loop that closes at low pressures, which can be attributed to coarser more crystalline CSH, to a large triangular-shaped loop along with a low pressure hysteresis, which is characteristic for fine fibre-like CSH with ink-bottle and plate-like pore morphologies. Granules with fibre-like CSH have the higher specific surfaces areas but those with more crystalline CSH show stronger physisorption of water molecules.

Rock alteration in alkaline cement waters over 15 years and its relevance to the geological disposal of nuclear waste

The interaction of groundwater with cement in a geological disposal facility (GDF) for intermediate level radioactive waste will produce a high pH leachate plume. Such a plume may alter the physical and chemical properties of the GDF host rock. However, the geochemical and mineralogical processes which may occur in such systems over timescales relevant for geological disposal remain unclear. This study has extended the timescale for laboratory experiments and shown that, after 15years two distinct phases of reaction may occur during alteration of a dolomite-rich rock at high pH. In these experiments the dissolution of primary silicate minerals and the formation of secondary calcium silicate hydrate (C–S–H) phases containing varying amounts of aluminium and potassium (C–(A)–(K)–S–H) during the early stages of reaction (up to 15months) have been superseded as the systems have evolved. After 15years significant dedolomitisation (MgCa(CO3)2+2OH−→Mg(OH)2+CaCO3+CO32−(aq)) has led to the formation of magnesium silicates, such as saponite and talc, containing variable amounts of aluminium and potassium (Mg–(Al)–(K)–silicates), and calcite at the expense of the early-formed C–(A)–(K)–S–H phases. This occured in high pH solutions representative of two different periods of cement leachate evolution with little difference in the alteration processes in either a KOH and NaOH or a Ca(OH)2 dominated solution but a greater extent of alteration in the higher pH KOH/NaOH leachate. The high pH alteration of the rock over 15years also increased the rock’s sorption capacity for U(VI). The results of this study provide a detailed insight into the longer term reactions occurring during the interaction of cement leachate and dolomite-rich rock in the geosphere. These processes have the potential to impact on radionuclide transport from a geodisposal facility and are therefore important in underpinning any safety case for geological disposal.

Evaluation of novel reactive MgO activated slag binder for the immobilisation of lead and zinc

Although Portland cement is the most widely used binder in the stabilisation/solidification (S/S) processes, slag-based binders have gained significant attention recently due to their economic and environmental merits. In the present study, a novel binder, reactive MgO activated slag, is compared with hydrated lime activated slag in the immobilisation of lead and zinc. A series of lead or zinc-doped pastes and mortars were prepared with metal to binder ratio from 0.25% to 1%. The hydration products and microstructure were studied by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. The major hydration products were calcium silicate hydrate and hydrotalcite-like phases. The unconfined compressive strength was measured up to 160d. Findings show that lead had a slight influence on the strength of MgO-slag paste while zinc reduced the strength significantly as its concentration increased. Leachate results using the TCLP tests revealed that the immobilisation degree was dependent on the pH and reactive MgO activated slag showed an increased pH buffering capacity, and thus improved the immobilisation efficiency compared to lime activated slag. It was proposed that zinc was mainly immobilised within the structure of the hydrotalcite-like phases or in the form of calcium zincate, while lead was primarily precipitated as the hydroxide. It is concluded, therefore, that reactive MgO activated slag can serve as clinker-free alternative binder in the S/S process.

Formation of chemical gardens on granitic rock: a new type of alteration for alkaline systems

In order to understand the groundwater flow at near-underground facilities such as waste repositories, we have studied the effects of flowing an alkaline solution leached from cementitious building materials through the fractures of low-porosity granitic rocks under laboratory conditions. The results show that silica released from the dissolution of sodium-rich plagioclase and quartz reacts with the calcium leached from cementitious buildings to form calcium silicate hydrates (C-S-H) phases in the form of hollow tubular structures. These tubular structures form selectively on the surface of plagioclase in a similar way to reverse silica gardens structures. It was found that the rate of precipitation of C-S-H phases is faster than the rate of dissolution of plagioclase. This self-triggered dissolution/precipitation phenomenon may be an important factor controlling groundwater permeation in natural alkaline underground systems.

The role of calcium in blended fly ash geopolymers

In this research effort, the role of calcium in geopolymers was investigated through a series of syntheses where a high-calcium fly ash was blended with a low-calcium fly ash. Increased calcium content led to accelerated set-up times, increased compressive strength, and increased product formation. Powder X-ray diffraction results showed the majority of that product to be geopolymer framework with only minor contributions from calcium silicate phases. Thermal analysis confirmed the absence of a calcium silicate hydrate phase. Analysis of fly ash dissolution showed that calcium aided in aluminosilicate dissolution and therefore the geopolymerization reaction. While aiding in this reaction, calcium became incorporated into the pore structure of the geopolymer as a counter-balancing cation, according to ion exchange experiments. Thus, geopolymer synthesis with increased calcium content through the use of a high-calcium fly ash under these experimental conditions produced a quick-setting, strong, calcium incorporated geopolymer material.

Thermal Activation of a Pure Montmorillonite Clay and Its Reactivity in Cementitious Systems

Clay minerals are potential candidates as raw materials for new supplementary cementitious materials (SCMs) that can partly replace Portland cement and thereby significantly reduce CO2 emissions associated with cement production. We present the characterization of the complex, disordered structure of a pure montmorillonite clay heated at various temperatures (110–1100 °C), by solid-state 27Al and 29Si MAS NMR methods. The SiO4 tetrahedra and AlO6 octahedral sites become progressively more distorted, exhibit a significant degree of disorder upon dehydroxylation (600–800 °C), and do not lead to the formation of any metastable phase. At high temperatures (1000–1100 °C), the layer structure of the clay breaks down, forming stable crystalline phases. The chemical reactivity, measured as the degree of dissolution/precipitation in an alkaline solution, is found to be proportional to the degree of disorder/dehydroxylation. The maximum reactivity as a function of the heating temperature is achieved at 800 °C prior to the formation of inert, condensed Q4-type phases in the material. At maximum reactivity the calcium silicate hydrate (C-S-H) phase contains silicate chains with the highest aluminum incorporation, leading to blended cements containing a C-S-H phase with longer chain lengths. Most importantly, by exploiting the differential spin–lattice relaxation behavior of the 29Si spins, evidence of multiple sites and components in both the naturally occurring and heated montmorillonite is being reported for the first time.

Adhesion properties of soy protein crosslinked with organic calcium silicate hydrate hybrids

ABSTRACTThe objective of this work was to investigate if inorganic calcium silicate hydrate (CSH) hybrids would improve soy protein wet adhesion properties. 3‐aminopropyltriethoxysilane (APTES) was used as a crosslinking agent to make covalent linkage between organic soy protein and inorganic CSH phases. Soy protein–calcium silicate hydrate (MSP‐CSH) composites with different mole ratio of APTES were prepared and the effect of crosslinking reaction on physicochemical properties such as thermal, rheological, FTIR spectroscopic, and morphological and adhesion properties were studied with physical aging effect. Covalent linkage was observed between CSH and soy protein using the FTIR technique. With aging effect, the denaturation temperature (Td) and enthalpies (ΔHd) of each fraction of soy protein increased in DSC thermograms, representing higher thermal stability, and the viscoelasticity of the composites also increased. The roughly coated surface of the MSP‐CSH composite was observed in SEM images. All these changes further confirmed the interaction between CSH and soy protein molecules. Dry and wet adhesion strength of the MSP‐CSH composites was higher than the control MSP alone. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40693.