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 nature of C-S-H in hardened cements

http://www.civil.northwestern.edu/people/thomas/pdf/Chen_CSHSol_CCR_sub.pdf

Solubility and structure of calcium silicate hydrate

Calcium silicate hydrate gel is a very poorly crystalline material that is the main product of reaction of portland cement with water. Evidence is presented for a disordered layer structure, in which most of the layers are structurally imperfect ones of jennite (Ca9Si6O32H22), and others are similarly related to 1.4-nm tobermorite (Ca5Si6O26H18), both structures being modified by omission of many of their silicate tetrahedra. The evidence comprises conditions of formation, silicate anion type, Ca/Si ratio, H2O/Ca ratios and densities for various drying conditions, thermogravimetric curve, X-ray powder and selected-area electron diffraction patterns, and results of analytical electron microfcopy.

Proposed Structure for Calcium Silicate Hydrate Gel

The calcium silicate hydrate (C-S-H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAlO(2) solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by (27)Al and (29)Si MAS NMR spectroscopy. (27)Al MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the (27)Al isotropic chemical shift (delta(iso)) and quadrupolar product parameter (P(Q) = C(Q)) for tetrahedrally coordinated Al incorporated in the C-S-H phase and for a pentacoordinated Al site. The latter site may originate from Al(3+) substituting for Ca(2+) ions situated in the interlayers of the C-S-H structure. The spectral region for octahedrally coordinated Al displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (delta(iso) = 5.0 ppm and P(Q) = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral Al incorporated in the C-S-H phase has been quantitatively determined from (27)Al MAS spectra at 14.09 T and indirectly observed quantitatively in (29)Si MAS NMR spectra by the Q(2)(1Al) resonance at -81.0 ppm. A linear correlation is observed between the (29)Si MAS NMR intensity for the Q(2)(1Al) resonance and the quantity of Al incorporated in the C-S-H phase from (27)Al MAS NMR for the different samples of hydrated wPc. This correlation supports the assignment of the resonance at delta(iso)((29)Si) = -81.0 ppm to a Q(2)(1Al) site in the C-S-H phase and the assignment of the (27)Al resonance at delta(iso)((27)Al) = 74.6 ppm, characterized by P(Q)((27)Al) = 4.5 MHz, to tetrahedrally coordinated Al in the C-S-H. Finally, it is shown that hydration of wPc in a NaAlO(2) solution results in a C-S-H phase with a longer mean chain length of SiO(4) tetrahedra and an increased quantity of Al incorporated in the chain structure as compared to the C-S-H phase resulting from hydration of wPc in water.

Incorporation of aluminum in the calcium silicate hydrate (C-S-H) of hydrated portland cements: a high-field 27Al and 29Si MAS NMR investigation.

Identification of some of the polysilicate components of trimethylsilylated cement paste

A study was made of the yield and distribution of the products formed when Na2H2SiO4,4H2O, Na2H2SiO4,5H2O, and Na2H2SiO4,8H2O were treated by a direct method of trimethylsilylation reported previously. These soluble silicates are particularly prone to side reactions due to polymerisation of silicate ions in solution, yielding a distribution of products many of which are undetected chromatographically.Quantitative yields of the expected orthosilicate derivatives were obtained only when vanishingly small concentrations of silicate were used. Further experiments with the mineral hemimorphite confirmed that, for a unified method of trimethylsilylation applicable to different structures, careful control of the water content in the reaction medium is essential. Improvements in the trimethylsilylation and chromatographic procedures are described.

Trimethylsilyl Derivatives for the Study of Silicate Structures. Part III: Sodium Silicate Hydrates,

A detailed study was made of the yield and products of trimethylsilylation of calcium orthosilicate (Ca2[SiO4]), rankinite (Ca3[Si2O7]), and pseudowollastonite (Ca3[Si3O9]) under various experimental conditions. With hexamethyldisiloxane, chlorotrimethylsilane, and isopropyl alcohol as reagents, in the absence of added water Ca2[SiO4] yielded predominantly SiO4(SiMe3)x(Pri)4–x(x= 1–4) and a lesser amount of Si2O7(SiMe3)x(Pri)6–x(x= 2–6), Ca3[Si2O7] gave predominantly Si2O7(SiMe3)x(Pri)6–x(x= 2–6), and Ca3[Si3O9] gave predominantly Si3O9(SiMe3)x(Pri)6–x(x= 2–6) and Si3O10(SiMe3)x(Pri)8–x(x= 3–8). Mixed trimethylsilyl–isopropyl derivatives were converted to the fully trimethylsilylated derivatives SiO4(SiMe3)4, Si2O7(SiMe3)6, and Si3O10(SiMe3)8 on treatment with Amberlyst 15 ion-exchange resin in the presence of hexamethyldisiloxane. In the trimethylsilylation of Ca2[SiO4] reaction of the mineral itself is complete within ca. 6 min at room temperature, under the conditions employed, and the majority of unwanted side reactions which yield Si2O7 and Si3O10 derivatives occur during the first 3–4 min. Side reactions may be suppressed by increasing the proportion of isopropyl alcohol and decreasing that of Ca2[SiO4] in the reagents. The presence of both isopropyl alcohol and chlorotrimethylsilane is necessary for reaction to occur. Isopropyl alcohol may be replaced by either ethyl alcohol or water, although the yield is reduced. When isopropyl alcohol is replaced by water, only the fully trimethylsilylated derivatives are formed. The mechanism of the reaction is discussed and experimental conditions are described which give highest yields of the desired products.

Trimethylsilyl derivatives for the study of silicate structures. Part 7. Calcium silicate structures

Trimethylsilylation of Ag10Si4O13 by a modified ‘direct method’ yielded the trimethylsilyl (TMS) derivative of the linear chain ion Si4O1310– as the major product; this ion readily undergoes cyclisation in acidic aqueous media to yield the cyclic tetramer Si4O128– and the presence of linear and branched chain isomers of the TMS derivative of Si4O1310– was confirmed in the tri-methylsilylation of lead fluorosilicate glass.

Trimethylsilylation of Ag10Si4O13: cyclisation of the linear chain anion Si4O1310–

The method of direct trimethylsilylation described previously has been used to follow the change in constitution of hemimorphite, Zn4(OH)2(Si2O7)·H2O, on heating at various temperatures up to 1 300 °C. The conversion has also been followed by differential thermal analysis, thermal gravimetric analysis, and X-ray diffraction measurements. An abrupt change in the chromatographic pattern occurs at 600–630 °C, corresponding to the disappearance of the Si2O7 and emergence of the SiO4 derivative as the main product of trimethylsilylation. This coincides with the loss of hydroxide groups and the formation of β-Zn2SiO4. The initial stage of the reaction at 627 °C shows first-order kinetics. The transformation of β-Zn2SiO4 into willemite, α-Zn2SiO4, commences at 835 °C and is unaccompanied by further changes in the yield or chromatographic pattern of the products. The results illustrate the use of the method as an adjunct to conventional procedures for studying the change in anionic constitution of silicates having discrete silicate structures.

Charles R. Masson

Papers

Identification of some of the polysilicate components of trimethylsilylated cement paste

Trimethylsilyl Derivatives for the Study of Silicate Structures. Part III: Sodium Silicate Hydrates,

Trimethylsilyl derivatives for the study of silicate structures. Part 7. Calcium silicate structures

Trimethylsilylation of Ag10Si4O13: cyclisation of the linear chain anion Si4O1310–

Trimethylsilyl derivatives for the study of silicate structures. Part 4. The conversion of hemimorphite into willemite