13C chemical shift anisotropies for carbonate ions in cement minerals and the use of 13C, 27Al and 29Si MAS NMR in studies of Portland cement including limestone additions

¹³C isotropic chemical shifts and chemical shift anisotropy parameters have been determined for a number of inorganic carbonates relevant in cement chemistry from slow-speed ¹³C MAS or ¹³C{¹H} CP/MAS NMR spectra (9.4 T or 14.1 T) for ¹³C in natural abundance. The variation in the ¹³C chemical shift parameters is relatively small, raising some doubts that different carbonate species in Portland cement-based materials may not be sufficiently resolved in ¹³C MAS NMR spectra. However, it is shown that by combining ¹³C MAS and ¹³C{¹H} CP/MAS NMR carbonate anions in anhydrous and hydrated phases can be distinguished, thereby providing valuable information about the reactivity of limestone in cement blends. This is illustrated for three cement pastes prepared from an ordinary Portland cement, including 0, 16, and 25 wt.% limestone, and following the hydration for up to one year. For these blends ²⁹Si MAS NMR reveals that the limestone filler accelerates the hydration for alite and also results in a smaller fraction of tetrahedrally coordinated Al incorporated in the C-S-H phase. The latter result is more clearly observed in ²⁷Al MAS NMR spectra of the cement–limestone blends and suggests that dissolved aluminate species in the cement–limestone blends readily react with carbonate ions from the limestone filler, forming calcium monocarboaluminate hydrate.     

A new aluminium-hydrate species in hydrated Portland cements characterized by Al and Si MAS NMR spectroscopy

Recent ²⁷Al MAS NMR studies of hydrated Portland cements and calcium-silicate-hydrate (C-S-H) phases have shown a resonance from Al in octahedral coordination, which cannot be assigned to the well-known aluminate species in hydrated Portland cements. This resonance, which exhibits the isotropic chemical shift δ_iso = 5.0 ppm and the quadrupole product parameter P_Q = 1.2 MHz, has been characterized in detail by ²⁷Al MAS and ²⁷Al{¹H} CP/MAS NMR for different hydrated white Portland cements and C-S-H phases. These experiments demonstrate that the resonance originates from an amorphous or disordered aluminate hydrate which contains Al(OH)₆³⁻ or O_xAl(OH)_6-x^(3+x)− units. The formation of the new aluminate hydrate is related to the formation of C-S-H at ambient temperatures, however, it decomposes by thermal treatment at temperatures of 70-90 °C. From the experiments in this work it is proposed that the new aluminate hydrate is either an amorphous/disordered aluminate hydroxide or a calcium aluminate hydrate, produced as a separate phase or as a nanostructured surface precipitate on the C-S-H phase. Finally, the possibilities of Al³⁺ for Ca²⁺ substitution in the principal layers and interlayers of the C-S-H structure are discussed.     

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

Effect of calcium to silica ratio on the synthesis of calcium silicate hydrate in high alkaline desilication solution

High alkaline desilication solution (DSS), a high volume byproduct from the pretreatment of high-alumina fly ash, was used as low-cost mother liquor for the synthesis of calcium silicate hydrate (C-S-H). Through the combined analysis of X-ray diffraction, thermogravimetric analysis, X-ray fluorescence, ²⁹Si MAS NMR, and Brunauer-Emmett-Teller, the relationship between chemical composition and structure of C-S-H synthesized under Ca/Si of 0.83:1 to 2.0:1 was investigated. Silicon conversion and yield of product have a positive correlation with Ca/Si. Sodium uptake in C-S-H is inhibited as Ca/Si increases. The formation of sodium in C-S-H transfers from “bound Na” to “mobile Na” and aluminum from tetrahedrally coordinated Al (IV) to octahedrally coordinated Al (VI). The increase of Ca/Si leads to shortening of silicate chain and formation of more dimers, which causes more water bound in C-S-H. The mechanism of calcium addition on silicate chain obtained from DFT calculation primarily results from more interlayer calcium occurrence to affect bridging tetrahedron and cationic bounding states reorganization. Reasonable control for Ca/Si momentously contributes to the adjustment for composition and structure of C-S-H synthesized in DSS.     

Composition, morphology and nanostructure of C-S-H in white Portland cement pastes hydrated at 55 °C

The C-S-H present in water- and alkali-activated hardened pastes of white Portland cement hydrated at 55 °C has been characterized. The mean length of the aluminosilicate anions in the C-S-H was similar in both systems and increased with age. Inner product C-S-H generally had a fine scale, homogeneous morphology. Outer product C-S-H was generally fibrillar with water, and foil- or lath-like with alkali. There were some regions of C-S-H with coarse morphology. It was not possible to determine the chemical composition of C-S-H using the SEM; TEM-EDX was necessary. The C-S-H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water, which was offset by a larger quantity of calcium hydroxide. The potassium in the KOH-activated paste was present either within the C-S-H structure charge balancing the substitution of Al³⁺ for Si⁴⁺, or adsorbed on the C-S-H charge balancing sulfate ions.     

Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate

In this work, the relationship between the composition of pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, has been studied. Pore solution was extracted from hardened AAS pastes. The analysis of the liquids was performed through different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ionic chromatography (IC) and Si by colorimetry; pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state ²⁹Si and ²⁷Al NMR and BSE/EDX.The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass take place between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. Results of ²⁹Si MAS NMR and FTIR confirm that the activation process takes place with more intensity after 3 h (although at this age, Q² units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to this of pastes activated with waterglass. The decrease of Na and Si contents progresses with time.The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q³ units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass.     

Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, β-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume

The purpose of this article is to discuss the applicability of the tobermorite-jennite (T/J) and tobermorite-‘solid-solution’ calcium hydroxide (T/CH) viewpoints for the nanostructure of C-S-H present in real cement pastes. The discussion is facilitated by a consideration of the author's 1992 model, which includes formulations for both structural viewpoints; its relationship to other recent models is outlined. The structural details of the model are clearly illustrated with a number of schematic diagrams. Experimental observations on the nature of C-S-H present in a diverse range of cementitious systems are considered. In some systems, the data can only be accounted for on the T/CH structural viewpoint, whilst in others, both the T/CH and T/J viewpoints could apply. New data from transmission electron microscopy (TEM) are presented. The ‘inner product’ (Ip) C-S-H in relatively large grains of C₃S or alite appears to consist of small globular particles, which are ≈4-8 nm in size in pastes hydrated at 20 °C but smaller at elevated temperatures, ≈3-4 nm. Fibrils of ‘outer product’ (Op) C-S-H in C₃S or β-C₂S pastes appear to consist of aggregations of long thin particles that are about 3 nm in their smallest dimension and of variable length, ranging from a few nanometers to many tens of nanometers. The small size of these particles of C-S-H is likely to result in significant edge effects, which would seem to offer a reasonable explanation for the persistence of Q⁰(H) species. This would also explain why there is more Q⁰(H) at elevated temperatures, where the particles seem to be smaller, and apparently less in KOH-activated pastes, where the C-S-H has foil-like morphology. In blended cements, a reduction in the mean Ca/Si ratio of the C-S-H results in a change from fibrillar to a crumpled-foil morphology, which suggests strongly that as the Ca/Si ratio is reduced, a transition occurs from essentially one-dimensional growth of the C-S-H particles to two-dimensional; i.e., long thin particles to foils. Foil-like morphology is associated with T-based structure. The C-S-H present in small fully hydrated alite grains, which has high Ca/Si ratio, contains a less dense product with substantial porosity; its morphology is quite similar to the fine foil-like Op C-S-H that forms in water-activated neat slag pastes, which has a low Ca/Si ratio. It is thus plausible that the C-S-H in small alite grains is essentially T-based (and largely dimeric). Since entirely T-based C-S-H is likely to have different properties to C-S-H consisting largely of J-based structure, it is possible that the C-S-H in small fully reacted grains will have different properties to the C-S-H formed elsewhere in a paste; this could have important implications.     

The work of Powers and Brownyard revisited: Part 2

Powers and Brownyard [T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened Portland cement paste, Bull. 22, Res. Lab. of Portland Cement Association, Skokie, IL, U.S., 1948 reprinted from J. Am. Concr. Inst. (Proc.), 43, 1947, pp. 101-132, pp. 249-336, pp. 469-505, pp. 549-602, pp. 669-712, pp. 845-880, pp. 933-992. [1]] were the first to systematically investigate the reaction of cement and water and the composition of cement paste. In Part I to this paper, their work was recapitulated (Brouwers [H.J.H. Brouwers, The work of Powers and Brownyard revisited: Part 1, accepted for publication in Cem. Concr. Res. 34 (2004) 1697-1716 [2]]). Here, it will be demonstrated that their water retention data also enables the study of the molar reactions of the aluminate (C₃A and C₄AF) and sulphate phases. It follows that the C₄AF most likely reacts with the C₃S and/or C₂S to form a Si containing hydrogarnet and portlandite. The remaining calcium silicates react to C-S-H (C_1.7SH_3.2 when saturated) and CH, as proposed in Part I [H.J.H. Brouwers, The work of Powers and Brownyard revisited: Part 1, accepted for publication in Cem. Concr. Res. 34 (2004) 1697-1716 [2]].The CS¯ seems to react exclusively with the C₃A. In case of carbonation, both phases react to hemi-carbonate, mono-sulphate, ettringite and tetra calcium aluminate hydrate. The concept “degree of carbonation” is introduced to quantify the fraction of mono-sulphate that is carbonated. This enables the quantification of all four hydration products, which represents a principal innovation. Subsequently, using the molar reactions and known specific volumes of the crystalline hydration products, the specific volumes of non-evaporable water (ν_n) and gel water (ν_g) are determined. These values are in line with the values suggested by Powers and Brownyard [T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened Portland cement paste, Bull. 22, Res. Lab. of Portland Cement Association, Skokie, IL, U.S., reprinted from J. Am. Concrete Inst. (Proc.), 43, 1947, pp. 101-132, pp. 249-336, pp. 469-505, pp. 549-602, pp. 669-712, pp. 845-880, pp. 933-992. [1]], which were based on their shrinkage data, implying a successful coupling of the molar reactions and their original paste model.     

Analytical Electron Microscopy of Cement Pastes: III, Pastes Hydrated for Long Times

Analyses of 131 particles of C-S-H in several mature tricalcium silicate pastes hydrated for 1 to 30 years gave a mean Ca/Si ratio of 1.46 with a range of 1.2 to 1.8. Similar analyses of 152 particles in mature cement pastes hydrated for 2 to 3 years gave a mean Ca/ Si ratio of 1.53 with a range of 1.0 to 2.8. Taken together with similar, previously published data for younger pastes, these findings indicate that the mean Ca/Si ratio does not change significantly after 1 day in tricalcium silicate pastes but decreases significantly with time in cement pastes; after several years, the mean Ca/Si ratios are similar in the two cases, despite the presence of other components in the C-S-H formed from cement. For the C-S-H of cement pastes, the mean Al/(Ca plus; Mg) and Fe/(Caplus;Mg) ratios increase with time but the mean S/(Caplus;Mg) ratio decreases. The composition of the AFm phase does not change greatly after 28 days.     

The Effect of Alkali Ions on the Incorporation of Aluminum in the Calcium Silicate Hydrate (C–S–H) Phase Resulting from Portland Cement Hydration Studied by 29Si MAS NMR

The incorporation of aluminum in the calcium–silicate–hydrate (C–S–H) phases formed by hydration of three different white Portland cements has been investigated by ²⁹Si MAS NMR. The principal difference between the three cements is their bulk Al₂O₃ contents and quantities of alkali (Na⁺ and K⁺) ions. ²⁹Si MAS NMR allows indirect detection of tetrahedral Al incorporated in the silicate chains of the C–S–H structure by the resonance from Q²(1Al) sites. Analysis of the relative ²⁹Si NMR intensities for this site, following the hydration for the three cements from 0.5 d to 30 weeks, clearly reveals that the alkali ions promote the incorporation of Al in the bridging sites of the dreierketten structure of SiO₄ tetrahedra in the C–S–H phase. The increased incorporation of Al in the C–S–H phase with increasing alkali content in the anhydrous cement is in accord with a proposed substitution mechanism where the charge deficit, obtained by the replacement of Si⁴⁺ by Al³⁺ ions in the bridging sites, is balanced by adsorption/binding of alkali ions in the interlayer region most likely in the near vicinity of the AlO₄ tetrahedra. This result is further supported by similar ²⁹Si MAS NMR experiments performed for the white Portland cements hydrated in 0.30M NaOH and NaAlO₂ solutions.     

Characterization of white Portland cement hydration and the C-S-H structure in the presence of sodium aluminate by Al and Si MAS NMR spectroscopy

The effects of hydrating a white Portland cement (wPc) in 0.30 and 0.50 M solutions of sodium aluminate (NaAlO₂) at 5 and 20 °C are investigated by ²⁷Al and ²⁹Si magic-angle spinning (MAS) NMR spectroscopy. It is demonstrated that NaAlO₂ accelerates the hydration of alite and belite and results in calcium-silicate-hydrate (C-S-H) phases with longer average chain lengths of SiO₄/AlO₄ tetrahedra. The C-S-H phases are investigated in detail and it is shown that the Al/Si ratio for the chains of tetrahedra is quite constant during the time studied for the hydration (6 h to 2 years) but increases for higher concentration of the NaAlO₂ solution. The average chain lengths of “pure” silicate and SiO₄/AlO₄ tetrahedra demonstrate that Al acts as a linker for the silicate chains, thereby producing aluminosilicate chains with longer average chain lengths. Finally, it is shown that NaAlO₂ reduces the quantity of ettringite and results in larger quantities of monosulfate and a calcium aluminate hydrate phase.     

The composition of the C-S-H phases in portland cement pastes

The composition of the calcium silicate hydrate (C-S-H) present in Portland cement pastes from eight days to five years old, and in concrete samples ten years old, has been determined by electron probe micro-analysis. The dependence of C-S-H composition on water/cement ratios has also been studied. When the cement is only partially hydrated inner and outer hydrate layers are seen around alite grains. These hydrates have different chemical compositions. With progressive hydration it becomes impossible to distinguish the inner from outer hydrates without a reference alite grain. At this stage two distinct C-S-H compositions can still be found by microanalysis. After further hydration a single CSH composition predominates. The calcium content of C-S-H varies significantly with water/cement ratio and there is new evidence to suggest that in the C-S-H structure Mg replaces Ca, and that Al, S and Fe replace Si.     

Composition and microstructure of 20-year-old ordinary Portland cement-ground granulated blast-furnace slag blends containing 0 to 100% slag

The morphology of outer-product (Op) C-S-H in 20-year-old slag-cement pastes appeared in most blends to be finer than at younger ages. The Ca/Si and Ca/(Si + Al) ratios of the Op C-S-H decreased with increasing slag content, and the Al/Si ratio increased. The Ca/Si ratio of C-S-H in the slag-containing pastes was lower at 20 years than at 14 months and the amount of Ca(OH)₂ was reduced indicating that additional slag must have reacted. The mean aluminosilicate chain length of the C-S-H was very long in all the samples and would be expected to have increased with age. The TEM-EDX and NMR data are consistent with nanostructural models for C-S-H. The Mg/Al ratio of the Mg-Al layered double hydroxide phase (LDH) was lower at 20 years than at 14 months in all cases except for the neat slag paste; aluminium hydroxide-based structure might be interstratified with those of the Mg-Al LDH.     

Sodium silicate-based, alkali-activated slag mortars: Part I. Strength, hydration and microstructure

Alkali activation of ground granulated blast furnace slag (GGBFS) with sodium silicate gave clinker-free binders, with high strength and early strength development, although set times were short and somewhat variable. Isothermal calorimetry detected three heat evolution peaks (wetting, gelation of activator and bulk reaction of slag). X-ray diffraction (XRD) showed no crystalline products. Hydration was investigated by scanning electron microscopy (SEM; with quantitative image analysis) and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR). From early age, a uniform gel filled the initially water-filled space, and gradually densified as reaction proceeded. Microanalysis of outer product (OP) showed an Al-substituted C-S-H gel phase of widely variable (0.5-1.0) Ca/Si ratio. NMR showed long-chain substituted C-S-H with Al/Si ratio rising to 0.19 at 1 year, and also cross-linked material, consistent with a Ca- or Al-modified silica gel. Inner product (IP) regions around slag grains probably also contained hydrotalcite. Activation with KOH gave more rapid reaction of slag than for silicate activation, a less homogeneous microstructure, and lower strengths. The hydrates contained a substituted C-S-H gel of low Ca/Si ratio probably mixed with hydrotalcite, and occasional higher Al regions in the OP regions.     

On the coordination of Al in ill-crystallized C-S-H phases formed by hydration of tricalcium silicate and by precipitation reactions at ambient temperature

High-resolution solid-state ²⁷Al MAS NMR measurements suggest that Al incorporated into C-S-H phases, prepared by precipitation reactions from sodium silicate and calcium chloride solutions and by hydration of tricalciumsilicate (C₃S), may occur tetrahedrally (Al^[4]) as well as octahedrally coordinated (Al^[6]). The amount of which depends on the composition of the C-S-H phase. With increasing CaO/SiO₂-ratio the portion of Al^[6] increases and that of Al^[4] decreases. The products of paste hydration of C₃S contain about 80 % of the Al as Al^[6] and 20 % as Al^[4].     

Influence of polymer on cement hydration in SBR-modified cement pastes

The influence of styrene-butadiene rubber (SBR) latex on cement hydrates Ca(OH)₂, ettringite, C₄AH₁₃ and C-S-H gel and the degree of cement hydration is studied by means of several measure methods. The results of DSC and XRD show that the Ca(OH)₂ content in wet-cured SBR-modified cement pastes increases with polymer-cement ratio (P/C) and reaches a maximum when P/C is 5%, 10% and 10% for the pastes hydrated for 3 d, 7 d and 28 d, respectively. With wet cure, appropriate addition of SBR promotes the hydration of cement, while the effect of SBR on the content of Ca(OH)₂ and the degree of cement hydration is not remarkable in mixed-cured SBR-modified cement pastes. XRD results illustrate that SBR accelerates the reaction of calcium aluminate with gypsum, and thus enhances the formation and stability of the ettringite and inhibits the formation of C₄AH₁₃. The structure of aluminum-oxide and silicon-oxide polyhedron is characterized by ²⁷Al and ²⁹Si solid state NMR spectrum method, which shows that tetrahedron and octahedron are the main forms of aluminum-oxide polyhedrons in SBR-modified cement pastes. There are only [SiO₄]⁴⁻ tetrahedron monomer and dimer in the modified pastes hydrated for 3 d, but there appears three-tetrahedron polymer in the modified pastes hydrated for 28 d. The effect of low SBR dosage on the structure of aluminum-oxide and silicon-oxide polyhedron is slight. However, the combination of Al³⁺ with [SiO₄]⁴⁻ is restrained when P/C is above 15%, and the structure of Al³⁺ is changed obviously. Meantime, the polymerization of the [SiO₄]⁴⁻ tetrahedron in C-S-H gel is controlled.     

Analytical Electron Microscopy of Cement Pastes: II, Pastes of Portland Cements and Clinkers

Three hundred sixty-five particles of C-S-H, Ca(OH)₂, AFm phase, and AFt phase from pastes of normally ground portland cements and of finely ground cements and clinkers were analyzed. All the phases, except the Ca(OH)₂, showed significant variation in composition among paste specimens and among particles within each specimen. The C-S-H contains significant amounts of Al, Fe, and S; for that of a normally ground portland cement paste, cured for 28 days, the median Si:AI, Si:Fe, and Si:S ratios were 11, 43, and 15, respectively, whereas the mean Ca:Si ratio for all the particles analyzed was 2.0. The AFm phase in cement pastes is not pure monosulfate but has a mixture of sulfate, hydroxide, and Al- and Si-bearing ions in its interlayer sites; the AFt phase is not pure ettringite but contains Si and its sulfate is probably partly replaced by hydroxide. The Al and Fe contents in the C-S-H and the Si contents in the AFm and AFt phases are greater when finely ground starting materials are used. This fact, together with the marked variation among particles, emphasizes the difficulty of ionic transport in cement pastes.     

Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement (OPC) and white Portland cement (WPC) pastes were measured during the first 28 days of hydration at room temperature. Saturation indices (SI) with respect to various solid phases known to occur in cement pastes were calculated from a thermodynamic analysis of the elemental concentrations, resulting in good agreement between the two pastes. In agreement with other published work, gypsum was saturated during the first several hours of hydration and then undersaturated thereafter, while portlandite was modestly supersaturated after the first few hours. High levels of supersaturation with respect to ettringite and calcium monosulfoaluminate were calculated, particularly prior to the consumption of gypsum at around 10 h. Results are consistent with published thermodynamic studies that show calcium monosulfoaluminate is metastable with respect to ettringite under normal hydration conditions. Three different ion activity product (IAP) equations for C-S-H were applied to the data. From 10 h onward, each of the IAP values declined gradually over time and the values for the OPC and WPC pastes were in close agreement. The same IAP equations were applied to experimental data from the pure CaO-SiO₂-H₂O system, resulting in good agreement between the cement paste pore solutions and the equilibrium between portlandite and the upper, or metastable, C-S-H solubility curve.     

Thermodynamic modeling of hydrated white Portland cement–metakaolin–limestone blends utilizing hydration kinetics from 29Si MAS NMR spectroscopy

Hydration kinetics for the principal phases of Portland cement blends have been incorporated in thermodynamic modeling (GEMS package), utilizing degrees of hydration from ²⁹Si MAS NMR. An empirical relationship for the reaction of these phases is established which includes three variable parameters that all can be estimated from the degrees of hydration. This approach is compared with thermodynamic equilibrium modeling (full hydration) for white Portland cement–metakaolin (0–30 wt.%) blends and for ternary blends of white Portland cement (65 wt.%)–metakaolin–limestone. The predicted phase assemblages have been compared with the phases identified by XRD, ²⁷Al and ²⁹Si MAS NMR which reveals that the incorporation of hydration kinetics improves the agreement between modeling and experiments. The results show also that the formation of strätlingite depends critically on the quantity of charge-balancing anions in the AFm phases, especially carbonate and sulfate anions, and on the degree of hydration for metakaolin.     

The C-S-H gel of Portland cement mortars: Part I. The interpretation of energy-dispersive X-ray microanalyses from scanning electron microscopy, with some observations on C-S-H, AFm and AFt phase compositions

Scanning electron microscopy (SEM) microanalyses of the calcium-silicate-hydrate (C-S-H) gel in Portland cement pastes rarely represent single phases. Essential experimental requirements are summarised and new procedures for interpreting the data are described. These include, notably, plots of Si/Ca against other atom ratios, 3D plots to allow three such ratios to be correlated and solution of linear simultaneous equations to test and quantify hypotheses regarding the phases contributing to individual microanalyses. Application of these methods to the C-S-H gel of a 1-day-old mortar identified a phase with Al/Ca=0.67 and S/Ca=0.33, which we consider to be a highly substituted ettringite of probable composition C₆A₂S?₂H₃₄ or {Ca₆[Al(OH)₆]₂·24H₂O}(SO₄)₂[Al(OH)₄]₂. If this is true for Portland cements in general, it might explain observed discrepancies between observed and calculated aluminate concentrations in the pore solution. The C-S-H gel of a similar mortar aged 600 days contained unsubstituted ettringite and an AFm phase with S/Ca=0.125.