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8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Spatially resolved nanostructural analysis of disordered phases in carbonated alkali-activated slag

Alkali-activated slag (AAS) is a promising low-CO 2 alternative cement consisting of several disordered phases of similar composition. Although their local atomic arrangements are known to influence macroscopic behavior, determination of structural changes in response to external stimuli remains a challenge. Here, X-ray diffraction-computed tomography (XRD-CT), pair distribution function-CT (PDF-CT), and nanoprobe X-ray fluorescence (nano-XRF) have been used to uncover how an increase of magnesium in AAS affects the atomic structure and spatial arrangement of phases after aggressive carbonation (100% dry CO 2 ), conditions experienced in applications such as oil and gas wells and geological storage of CO 2 . From PDF-CT it is found that a higher magnesium content decreases the average nanoscale crystallite size of disordered calcium carbonate. At the same time, higher magnesium content is correlated with a less decalcified C-(N)-A-S-H gel, as determined via analysis of Ca-Si atom-atom correlations from PDF-CT and Ca/Si ratios from nano-XRF. Finally, nano-XRF reveals that the disordered (i.e., amorphous) calcium carbonate is stabilized by the presence of silicates.

McCaslin, Eric R. [Princeton Univ., NJ (United Sta↗
DOE OSTI2020

Materials Data on CaSi by Materials Project

CaSi crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. Ca is bonded in a 7-coordinate geometry to seven equivalent Si atoms. There are a spread of Ca–Si bond distances ranging from 3.09–3.18 Å. Si is bonded in a 9-coordinate geometry to seven equivalent Ca and two equivalent Si atoms. Both Si–Si bond lengths are 2.45 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ca2Si by Materials Project

Ca2Si is Cotunnite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. there are two inequivalent Ca2+ sites. In the first Ca2+ site, Ca2+ is bonded to four equivalent Si4- atoms to form a mixture of corner and edge-sharing CaSi4 tetrahedra. There are three shorter (2.99 Å) and one longer (3.07 Å) Ca–Si bond lengths. In the second Ca2+ site, Ca2+ is bonded in a 5-coordinate geometry to three equivalent Si4- atoms. There are one shorter (3.11 Å) and two longer (3.25 Å) Ca–Si bond lengths. Si4- is bonded in a 9-coordinate geometry to seven Ca2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaSi2 by Materials Project

CaSi2 is alpha boron-derived structured and crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Ca is bonded in a 7-coordinate geometry to seven Si atoms. There are a spread of Ca–Si bond distances ranging from 3.01–3.12 Å. There are two inequivalent Si sites. In the first Si site, Si is bonded to three equivalent Ca and three equivalent Si atoms to form distorted edge-sharing SiCa3Si3 octahedra. All Si–Si bond lengths are 2.39 Å. In the second Si site, Si is bonded in a 7-coordinate geometry to four equivalent Ca and three equivalent Si atoms. All Si–Si bond lengths are 2.44 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaSi2 by Materials Project

CaSi2 is hexagonal omega structure structured and crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Ca is bonded to six equivalent Si atoms to form distorted CaSi6 octahedra that share corners with six equivalent SiCa3Si3 octahedra, edges with six equivalent CaSi6 octahedra, and edges with six equivalent SiCa3Si3 octahedra. The corner-sharing octahedral tilt angles are 21°. All Ca–Si bond lengths are 3.04 Å. Si is bonded to three equivalent Ca and three equivalent Si atoms to form distorted SiCa3Si3 octahedra that share corners with three equivalent CaSi6 octahedra, corners with three equivalent SiCa3Si3 octahedra, edges with three equivalent CaSi6 octahedra, and edges with nine equivalent SiCa3Si3 octahedra. The corner-sharing octahedra tilt angles range from 0–21°. All Si–Si bond lengths are 2.40 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaSi2 by Materials Project

CaSi2 is hexagonal omega structure-like structured and crystallizes in the tetragonal I4_1/amd space group. The structure is three-dimensional. Ca is bonded to twelve Si atoms to form a mixture of edge and face-sharing CaSi12 cuboctahedra. There are four shorter (3.11 Å) and eight longer (3.21 Å) Ca–Si bond lengths. There are four inequivalent Si sites. In the first Si site, Si is bonded in a 9-coordinate geometry to six equivalent Ca and three Si atoms. There are one shorter (2.29 Å) and two longer (2.40 Å) Si–Si bond lengths. In the second Si site, Si is bonded in a 9-coordinate geometry to six equivalent Ca and three Si atoms. There are one shorter (2.29 Å) and two longer (2.40 Å) Si–Si bond lengths. In the third Si site, Si is bonded in a 9-coordinate geometry to six equivalent Ca and three Si atoms. In the fourth Si site, Si is bonded in a 9-coordinate geometry to six equivalent Ca and three Si atoms. There are two shorter (3.11 Å) and four longer (3.21 Å) Si–Ca bond lengths. There are one shorter (2.29 Å) and two longer (2.40 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ca5Si3 by Materials Project

Ca5Si3 crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. there are two inequivalent Ca sites. In the first Ca site, Ca is bonded to five Si atoms to form a mixture of distorted edge, face, and corner-sharing CaSi5 trigonal bipyramids. There are a spread of Ca–Si bond distances ranging from 3.05–3.24 Å. In the second Ca site, Ca is bonded in a square co-planar geometry to four equivalent Si atoms. All Ca–Si bond lengths are 3.08 Å. There are two inequivalent Si sites. In the first Si site, Si is bonded in a 9-coordinate geometry to eight Ca and one Si atom. The Si–Si bond length is 2.44 Å. In the second Si site, Si is bonded in a 8-coordinate geometry to eight equivalent Ca atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ca2Si by Materials Project

Ca2Si is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ca2+ is bonded to four equivalent Si4- atoms to form a mixture of edge and corner-sharing CaSi4 tetrahedra. All Ca–Si bond lengths are 3.10 Å. Si4- is bonded in a body-centered cubic geometry to eight equivalent Ca2+ atoms.

36 MATERIALS SCIENCE↗