“…To test this hypothesis, the differences in values are compared. The same holds for the atomic scattering factors (Wang et al, 1996), which are with the growing value of sin = more related to the electron density of core electrons. 0:553 1=2 À 0:538 1=2 ), whereas the difference in ð10%Þ owing to the X-ray constraint is 0.0185.…”
The extension of the X-ray constrained (XC) wavefunction approach to open-shell systems using the unrestricted Hartree-Fock formalism is reported. The XC method is also extended to include relativistic effects using the scalar second-order Douglas-Kroll-Hess approach. The relativistic effects on the charge and spin density on two model compounds containing the copper and iron atom are reported. The size of the relativistic effects is investigated in real and reciprocal space; in addition, picture-change effects are investigated and discussed for the isolated Cu atom. It is found that the relativistic terms lead to changes in the densities that are much smaller than those from the X-ray constraint. Nevertheless, the use of the relativistic corrections in the ab initio model always leads to an improvement in the agreement statistics. An interesting result of the unrestricted XC technique is the possibility of obtaining experimentally derived spin densities from X-ray data.
“…To test this hypothesis, the differences in values are compared. The same holds for the atomic scattering factors (Wang et al, 1996), which are with the growing value of sin = more related to the electron density of core electrons. 0:553 1=2 À 0:538 1=2 ), whereas the difference in ð10%Þ owing to the X-ray constraint is 0.0185.…”
The extension of the X-ray constrained (XC) wavefunction approach to open-shell systems using the unrestricted Hartree-Fock formalism is reported. The XC method is also extended to include relativistic effects using the scalar second-order Douglas-Kroll-Hess approach. The relativistic effects on the charge and spin density on two model compounds containing the copper and iron atom are reported. The size of the relativistic effects is investigated in real and reciprocal space; in addition, picture-change effects are investigated and discussed for the isolated Cu atom. It is found that the relativistic terms lead to changes in the densities that are much smaller than those from the X-ray constraint. Nevertheless, the use of the relativistic corrections in the ab initio model always leads to an improvement in the agreement statistics. An interesting result of the unrestricted XC technique is the possibility of obtaining experimentally derived spin densities from X-ray data.
“…The 13 C{ 1 H} CP/MAS NMR of C3S hydrated with tartaric acid reveals that calcium tartrate is formed (Figure ). No silicon tartrate species are seen 2 3 …”
Section: Resultsmentioning
confidence: 99%
“…No silicon tartrate species are seen. 11 The Ca:Si ratios at the surface of the hydrating minerals were determined by XPS (Table 1). C3S hydrated in the absence of additives has a Ca:Si ratio of 2.6 after 7 h of hydration, which drops to 2.4 after 1 week.…”
The reaction of the cement retarders tartaric acid, sucrose, and lignosulfonate with tricalcium silicate (C3S),
tricalcium aluminate (C3A), and C3A/gypsum have been studied by 27Al and 29Si MAS NMR spectroscopy,
SEM, XRD, and XPS to gain an understanding of the effect on the individual minerals prior to studying a
typical sample of portland cement. Tartaric acid is the most effective at retarding C3A hydration and ettringite
formation, while sucrose and the lignosulfonate accelerate ettringite formation but are more effective at retarding
C3S hydration. We have confirmed that sucrose acts via nucleation poisoning/surface adsorption while
lignosulfonates involve the formation of a semipermeable layer on the cement grains. The formation of calcium
tartrate is clearly the most important step in tartaric acid inhibition; however, tartaric acid only exhibits a
dissolution−precipitation mechanism for C3A. Under conditions of excess calcium, the formation of a calcium
tartrate overlayer does not require the predissolution of the mineral.
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