American Society of Civil Engineers

Concurrently to research conducted on ordinary Portland cement (PC), new types of binders were developed during the last decades. These are formed by alkali-activation of metakaolin or ground-granulated blast furnace slag (GGBFS) and are named, respectively, geopolymers (GP) or alkali-activated slag (AAS). Four different cementitious materials were synthesised: PC, AAS, GP, and a mix GP-AAS and fully compared about their compositions and (micro)-structures. X-ray diffraction has revealed the presence of semi-crystalline C S H gel binding phase in PC while AAS, GP and GP-AAS are nearly amorphous. Progressive structural changes have been observed between the different samples by means of infrared spectroscopy, 29 Si and 27 Al magic-angle-spinning nuclear magnetic resonance spectroscopy: there is a polymerisation extent of the (alumino)-silicate framework from PC [SiQ 1 and SiQ 2 units] to AAS [SiQ 2 and SiQ 2 (1Al) units] and finally to GP [SiQ 4 (2Al) and SiQ 4 (3Al) units]. Scanning electron microscopy has shown that GP is a homogeneous matrix while the other materials are composites containing GGBFS grains surrounded by a binding matrix. Energy dispersive X-ray EDX analyses (line scans) have shown the absence of formation of any specific phase at the matrix-grains interfaces.

(Micro)-structural comparison between geopolymers, alkali-activated slag cement and Portland cement

Structural models for the primary strength and durability-giving reaction product in modern cements, a calcium (alumino)silicate hydrate gel, have previously been based solely on non-cross-linked tobermorite structures. However, recent experimental studies of laboratory-synthesized and alkali-activated slag (AAS) binders have indicated that the calcium-sodium aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems can be significantly cross-linked. Here, we propose a model that describes the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM), which can more appropriately describe the spectroscopic and density information available for this material. Analysis of the phase assemblage and Al coordination environments of AAS binders shows that it is not possible to fully account for the chemistry of AAS by use of the assumption that all of the tetrahedral Al is present in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints of the gel. Application of the CSTM can for the first time reconcile this information, indicating the presence of an additional activation product that contains highly connected four-coordinated silicate and aluminate species. The CSTM therefore provides a more advanced description of the chemistry and structure of calcium-sodium aluminosilicate gel structures than that previously established in the literature.

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Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

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.

The role of Al in C-S-H: NMR, XRD, and compositional results for precipitated samples

27 Al magic-angle spinning (MAS) NMR spectroscopy of the central and satellite transitions is shown to be a powerful technique for the characterization of calcium aluminates and aluminate hydrates, which are of importance in the chemistry of high alumina and Portland cements. Synthetic samples of CaO.2Al 2 O 3 , CaO.Al 2 O 3 , 12CaO.7Al 2 O 3 , 3CaO.Al 2 O 3 , CaO.Al 2 O 3 .10H 2 O, γ-Al(OH) 3 , 6CaO.Al 2 O 3 .3SO 3 .32H 2 O (ettringite), 4CaO.Al 2 O 3 .13H 2 O, 4CaO.Al 2 O 3 .SO 3 .12H 2 O (monosulfate) and hydrated samples of high alumina and Portland cements are investigated. For the synthetic samples 27 Al quadrupole coupling constants (C Q ), asymmetry parameters (η) of the electric field gradient (EFG) tensors, and isotropic chemical shifts (γ( 27 Al)) are determined with high accuracies from computer simulations of the complete manifolds of spinning sidebands (ssb's) for the satellite transitions or of the central transition (centerband and ssb's). Simulations of the ssb manifolds show that some of the samples cannot be characterized by a single set of C Q and η values, but rather by a distribution of these values. The ranges of the distribution in C Q and η are estimated by summation of simulated ssb patterns for the satellite transitions employing sets of slightly different C Q and η values. The quadrupole coupling parameters are shown to be an important supplement to γ( 27 Al) in the characterization of structurally different Al sites and for identification of the individual aluminates in 27 Al MAS NMR spectra of hydrated cement samples. Linear correlations between C Q and the mean bond angle deviations from ideal tetrahedry symmetry, as well as between C Q and a calculated estimate for the geometrical dependency of the EEG tensor are reported, for the anhydrous calcium aluminates. These correlations allow assignment of the C Q values for CaO.2Al 2 O 3 , 12CaO.7Al 2 O 3 , and 3CaO.Al 2 O 3 , all of which contain two nonequivalent AlO 4 tetrahedra in the asymmetric unit

Eric Henderson

Papers

Characterization of calcium aluminate phases in cements by aluminum-27 MAS NMR spectroscopy