Calcium silicate hydrate

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Calcium silicate hydrates (CSH or C-S-H) are the main products of the hydration of Portland cement and are primarily responsible for the strength of cement-based materials. [1] They are the main binding phase (the "glue") in most concrete. Only well defined and rare natural crystalline minerals can be abbreviated as CSH while extremely variable and poorly ordered phases without well defined stoichiometry, as it is commonly observed in hardened cement paste (HCP), are denoted C-S-H.

Contents

Preparation

When water is added to cement, each of the compounds undergoes hydration and contributes to the final state of the concrete. [2] Only calcium silicates contribute to the strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). [3] Dicalcium silicate, which reacts more slowly, only contributes to late strength. Calcium silicate hydrate (also shown as C-S-H) is a result of the reaction between the silicate phases of Portland cement and water. This reaction typically is expressed as:

2 Ca3SiO5 + 7 H2O → 3 CaO · 2 SiO2 · 4 H2O + 3 Ca(OH)2 + 173.6 kJ

also written in cement chemist notation, (CCN) as:

2 C
3S + 7 H → C
3S
2H
4 + 3 CH + heat

or, tricalcium silicate + water → calcium silicate hydrate + calcium hydroxide + heat

The stoichiometry of C-S-H in cement paste is variable and the state of chemically and physically bound water in its structure is not transparent, which is why "-" is used between C, S, and H. [4]

Synthetic C-S-H can be prepared from the reaction of CaO and SiO2 in water or through the double precipitation method using various salts. These methods provide the flexibility of producing C-S-H at specific C/S (Ca/Si, or CaO/SiO2) ratios. The C-S-H from cement phases can also be treated with an ammonium nitrate solution in order to induce calcium leaching, and so to achieve a given C/S ratio.

Properties

C-S-H is a nano sized material [5] [6] with some degree of crystallinity as observed by X-ray diffraction techniques. [7] The underlying atomic structure of C-S-H is similar to the naturally occurring mineral tobermorite. [8] It has a layered geometry with calcium silicate sheet structure separated by an interlayer space. The silicates in C-S-H exist as dimers, pentamers and 3n-1 chain units [9] [10] (where n is an integer greater than 0) and calcium ions are found to connect these chains making the three dimensional nano structure as observed by dynamic nuclear polarisation surface-enhanced nuclear magnetic resonance. [11] The exact nature of the interlayer remains unknown. One of the greatest difficulties in characterising C-S-H is due to its variable stoichiometry.[ citation needed ]

The scanning electron microscope micrographs of C-S-H does not show any specific crystalline form. They usually manifest as foils or needle/oriented foils.

Synthetic C-S-H can be divided in two categories separated at the Ca/Si ratio of about 1.1. There are several indications that the chemical, physical and mechanical characteristics of C-S-H varies noticeably between these two categories. [12] [13]

See also

Other C-S-H minerals:

Other calcium aluminium silicate hydrate, (C-A-S-H) minerals:

Mechanisms of formation of C-S-H phases:

Related Research Articles

<span class="mw-page-title-main">Silicate</span> Any polyatomic anion containing silicon and oxygen

A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula [SiO(4-2x)−
4−x]
n, where 0 ≤ x < 2. The family includes orthosilicate SiO4−4, metasilicate SiO2−3, and pyrosilicate Si2O6−7. The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate. The name "silicate" is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as hexafluorosilicate [SiF6]2−. Most commonly, silicates are encountered as silicate minerals.

Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.

In inorganic chemistry, mineral hydration is a reaction which adds water to the crystal structure of a mineral, usually creating a new mineral, commonly called a hydrate.

<span class="mw-page-title-main">Ettringite</span> Hydrous calcium sulfo-aluminate

Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.

Alite is an impure form of tricalcium silicate, Ca3SiO5, sometimes formulated as 3CaO·SiO2, typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Törnebohm in 1897 to a crystal identified in microscopic investigation of Portland cement. Hatrurite is the name of a mineral that is substituted C3S.

Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 (C2S in cement chemist notation).

Tricalcium aluminate Ca3Al2O6, often formulated as 3CaO·Al2O3 to highlight the proportions of the oxides from which it is made, is the most basic of the calcium aluminates. It does not occur in nature, but is an important mineral phase in Portland cement.

An AFm phase is an "alumina, ferric oxide, monosubstituted" phase, or aluminate ferrite monosubstituted, or Al2O3, Fe2O3 mono, in cement chemist notation (CCN). AFm phases are important hydration products in the hydration of Portland cements and hydraulic cements.

<span class="mw-page-title-main">Thaumasite</span> Complex calcium silicate hydrate mineral

Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca3Si(OH)6(CO3)(SO4)·12H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.

<span class="mw-page-title-main">Alkali–silica reaction</span> Chemical reaction damaging concrete

The alkali–silica reaction (ASR), also commonly known as concrete cancer, is a deleterious internal swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive amorphous silica found in many common aggregates, given sufficient moisture.

Friedel's salt is an anion exchanger mineral belonging to the family of the layered double hydroxides (LDHs). It has affinity for anions as chloride and iodide and is capable of retaining them to a certain extent in its crystallographical structure.

<span class="mw-page-title-main">Jennite</span> Inosilicate alteration mineral in metamorphosed limestone and in skarn

Jennite is a calcium silicate hydrate mineral of general chemical formula: Ca9Si6O18(OH)6·8H2O.

<span class="mw-page-title-main">Tobermorite</span> Inosilicate alteration mineral in metamorphosed limestone and in skarn

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

<span class="mw-page-title-main">Concrete degradation</span> Damage to concrete affecting its mechanical strength and its durability

Concrete degradation may have many different causes. Concrete is mostly damaged by the corrosion of reinforcement bars due to the carbonatation of hardened cement paste or chloride attack under wet conditions. Chemical damage is caused by the formation of expansive products produced by chemical reactions, by aggressive chemical species present in groundwater and seawater, or by microorganisms Other damaging processes can also involve calcium leaching by water infiltration, physical phenomena initiating cracks formation and propagation, fire or radiant heat, aggregate expansion, sea water effects, leaching, and erosion by fast-flowing water.

The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or calcium hydroxide (Ca(OH)2) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition and the active phase content.

<span class="mw-page-title-main">Energetically modified cement</span> Class of cements, mechanically processed to transform reactivity

Energetically modified cements (EMCs) are a class of cements made from pozzolans, silica sand, blast furnace slag, or Portland cement. The term "energetically modified" arises by virtue of the mechanochemistry process applied to the raw material, more accurately classified as "high energy ball milling" (HEBM). At its simplest this means a milling method that invokes high kinetics by subjecting "powders to the repeated action of hitting balls" as compared to (say) the low kinetics of rotating ball mills. This causes, amongst others, a thermodynamic transformation in the material to increase its chemical reactivity. For EMCs, the HEBM process used is a unique form of specialised vibratory milling discovered in Sweden and applied only to cementitious materials, here called "EMC Activation".

Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C3S) (alite), and dicalcium silicate (C2S) (belite). Upon hydration, the main reaction products are calcium silicate hydrates (C-S-H) and calcium hydroxide Ca(OH)2, written as CH in the cement chemist notation. C-S-H is the phase playing the role of the glue in the cement hardened paste and responsible of its cohesion. Cement also contains two aluminate phases: C3A and C4AF, respectively the tricalcium aluminate and the tetracalcium aluminoferrite. C3A hydration products are AFm, calcium aluminoferrite monosulfate, and ettringite, a calcium aluminoferrite trisulfate (AFt). C4AF hydrates as hydrogarnet and ferrous ettringite.

<span class="mw-page-title-main">Gyrolite</span> Rare phyllosilicate mineral crystallizing in spherules

Gyrolite, NaCa16(Si23Al)O60(OH)8·14H2O, is a rare silicate mineral (basic sodium calcium silicate hydrate: N-C-S-H, in cement chemist notation) belonging to the class of phyllosilicates. Gyrolite is also often associated with zeolites. It is most commonly found as spherical or radial formations in hydrothermally altered basalt and basaltic tuffs. These formations can be glassy, dull or fibrous in appearance.

<span class="mw-page-title-main">Tacharanite</span> Calcium aluminium silicate hydrate mineral

Tacharanite is a calcium aluminium silicate hydrate (C-A-S-H) mineral of general chemical formula Ca12Al2Si18O33(OH)36 with some resemblance to the calcium silicate hydrate (C-S-H) mineral tobermorite. It is often found in mineral assemblage with zeolites and other hydrated calcium silicates.

AFt Phases refer to the calcium Aluminate Ferrite trisubstituted, or calcium aluminate trisubstituted, phases present in hydrated cement paste (HCP) in concrete.

References

  1. ↑ Richardson, I.G. (February 2008). "The calcium silicate hydrates". Cement and Concrete Research. 38 (2): 137–158. doi:10.1016/j.cemconres.2007.11.005. ISSN   0008-8846.
  2. ↑ "Hydration of Portland Cement". engr.psu.edu. Retrieved 2022-11-29.
  3. ↑ "Building Materials". indiabix.com. Retrieved 2022-11-29.
  4. ↑ "Portland Cement Hydration" (PDF). Archived from the original (PDF) on 2017-02-15. Retrieved 2013-02-21.
  5. ↑ Allen, Andrew J.; Thomas, Jeffrey J.; Jennings, Hamlin M. (25 March 2007). "Composition and density of nanoscale calcium–silicate–hydrate in cement". Materials. 6 (4): 311–316. Bibcode:2007NatMa...6..311A. doi:10.1038/nmat1871. PMID   17384634.
  6. ↑ Andalibi, M. Reza; Kumar, Abhishek; Srinivasan, Bhuvanesh; Bowen, Paul; Scrivener, Karen; Ludwig, Christian; Testino, Andrea (2018). "On the mesoscale mechanism of synthetic calcium–silicate–hydrate precipitation: a population balance modeling approach". Journal of Materials Chemistry A. 6 (2): 363–373. doi:10.1039/C7TA08784E. ISSN   2050-7488. S2CID   103781671.
  7. ↑ Renaudin, Guillaume; Russias, Julie; Leroux, Fabrice; Frizon, Fabien; Cau-dit-Coumes, Céline (December 2009). "Structural characterization of C–S–H and C–A–S–H samples—Part I: Long-range order investigated by Rietveld analyses". Journal of Solid State Chemistry. 182 (12): 3312–3319. Bibcode:2009JSSCh.182.3312R. doi:10.1016/j.jssc.2009.09.026.
  8. ↑ Taylor, Harry F.W. (June 1986). "Proposed Structure for Calcium Silicate Hydrate Gel". Journal of the American Ceramic Society. 69 (6): 464–467. doi:10.1111/j.1151-2916.1986.tb07446.x.
  9. ↑ Cong, Xiandong; Kirkpatrick, R.James (April 1996). "29Si and 17O NMR investigation of the structure of some crystalline calcium silicate hydrates". Advanced Cement Based Materials. 3 (3–4): 133–143. doi:10.1016/S1065-7355(96)90045-0.
  10. ↑ Brunet, F.; Bertani, Ph.; Charpentier, Th.; Nonat, A.; Virlet, J. (October 2004). "Application of Si Homonuclear and H− Si Heteronuclear NMR Correlation to Structural Studies of Calcium Silicate Hydrates". The Journal of Physical Chemistry B. 108 (40): 15494–15502. doi:10.1021/jp031174g.
  11. ↑ Kumar, Abhishek; Walder, Brennan J.; Kunhi Mohamed, Aslam; Hofstetter, Albert; Srinivasan, Bhuvanesh; Rossini, Aaron J.; Scrivener, Karen; Emsley, Lyndon; Bowen, Paul (7 July 2017). "The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate". The Journal of Physical Chemistry C. 121 (32): 17188–17196. doi:10.1021/acs.jpcc.7b02439.
  12. ↑ "Archived copy". Archived from the original on 2015-03-15. Retrieved 2013-06-01.{{cite web}}: CS1 maint: archived copy as title (link)
  13. ↑ "Potential Application of Nanotechnology on Cement Based Materials" (PDF). Archived from the original (PDF) on 2012-02-17. Retrieved 2013-02-21.
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