Tobermorite

Tobermorite is a calcium silicate hydrate mineral with chemical formula: Ca5Si6O16(OH)2·4H2O or Ca5Si6(O,OH)18·5H2O.

Not to be confused with Torbernite.
Tobermorite
Crystalline mass of tobermorite
General
CategorySilicate mineral,
Calcium silicate hydrate
Formula
(repeating unit)		Ca5Si6O16(OH)2·4H2O, or;
Ca5Si6(O,OH)18·5H2O
Strunz classification9.DG.10
Crystal systemOrthorhombic
Crystal classDisphenoidal (222)
H-M symbol: (2 2 2)
Space groupC2221 (no. 20)
Unit cella = 11.17 Å, b = 7.38 Å
c = 22.94 Å; β = 90°; Z = 4
Identification
Formula mass702.36 g/mol
ColorPale pinkish white, white, brown
Crystal habitAs minute laths; fibrous bundles, rosettes or sheaves, radiating or plumose, fine granular, massive.
Cleavage{001} Perfect, {100} Imperfect
Mohs scale hardness2.5
LusterVitreous, silky in fibrous aggregates
StreakWhite
DiaphaneityTranslucent to translucent
Specific gravity2.423 - 2.458
Optical propertiesBiaxial (+)
Refractive indexnα = 1.570 nβ = 1.571 nγ = 1.575
Birefringenceδ = 0.005
Ultraviolet fluorescenceFluorescent, Short UV:weak white to yellow, Long UV:weak white to yellow
References[1][2][3]

Two structural varieties are distinguished: tobermorite-11 Å and tobermorite-14 Å. Tobermorite occurs in hydrated cement paste and can be found in nature as alteration mineral in metamorphosed limestone and in skarn. It has been reported from the Maqarin Area of north Jordan and in the Crestmore Quarry near Crestmore Heights, Riverside County, California.

Tobermorite was first described in 1880 for an occurrence in Scotland, on the Isle of Mull, around the locality of Tobermory.[2]

Use in Roman concrete

Aluminum substituted tobermorite is understood to be a key ingredient in the longevity of ancient undersea Roman concrete. The volcanic ash that Romans used for construction of sea walls contained phillipsite, and that an interaction with sea water actually caused the crystalline structures in the mortar to expand and strengthen, making that material substantially more durable than modern concrete when exposed to sea water. [4] [5][6]

Crystal structure of tobermorite: elementary unit cell.

Cement chemistry

Tobermorite is often used in thermodynamical calculations to represent the pole of the most evolved calcium silicate hydrate (C-S-H). The value of its Ca/Si or CaO/SiO2 (C/S) ratio is 0.83 (5/6). Jennite represents the less evolved pole with a C/S ratio of 1.5 (9/6).

See also
  • Other calcium silicate hydrate (C-S-H) minerals:
    • Afwillite
    • Gyrolite – Rare phyllosilicate mineral crystallizing in spherules
    • Jennite
    • Thaumasite – Unusual calcium silicate mineral with carbonate, sulfate and hexacoordinated hydroxysilicate anions. Responsible of a harmful concrete sulfate attack

References
  1. Handbook of Mineralogy
  2. Mindat.org
  3. Webmineral data
  4. Ancient Romans made world’s ‘most durable’ concrete. We might use it to stop rising seas, Washington Post, Ben Guarino, July 4, 2017. Retrieved July 5, 2017.
  5. Ancient lessons: Roman concrete durable, green, Jim Destefani, ed., Ceramic Tech Today, The American Ceramic Society, June 7, 2013
  6. Jackson, Marie D.; Mulcahy, Sean R.; Chen, Heng; Li, Yao; Li, Qinfei; Cappelletti, Piergiulio; Wenk, Hans-Rudolf (2017). "Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete". American Mineralogist. 102 (7): 1435–1450. Bibcode:2017AmMin.102.1435J. doi:10.2138/am-2017-5993CCBY. ISSN 0003-004X.
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