Calcium silicate hydrate
{{Short description|Main product of the hydration of Portland cement}}
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.{{cite journal | last1 = Richardson | first1 = I.G. | title = The calcium silicate hydrates | journal = Cement and Concrete Research | date = February 2008 | volume = 38 | issue = 2 | pages = 137–158 | issn = 0008-8846 | doi = 10.1016/j.cemconres.2007.11.005 | pmid = | url = }} 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.
Preparation
When water is added to cement, each of the compounds undergoes hydration and contributes to the final state of the concrete.{{Cite web|title=Hydration of Portland Cement|url=https://www.engr.psu.edu/ce/courses/ce584/concrete/library/construction/curing/Hydration.htm|accessdate=2022-11-29|work=engr.psu.edu}} Only calcium silicates contribute to the strength. Tricalcium silicate is responsible for most of the early strength (first 7 days).{{Cite web|title=Building Materials|url=https://www.indiabix.com/civil-engineering/building-materials/discussion-86|accessdate=2022-11-29|work=indiabix.com}} 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:
: {{chem2|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 {{chem|C|3|S}} + 7 H → {{chem|C|3|S|2|H|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.{{Cite web |url=http://people.ce.gatech.edu/~kk92/hyd07.pdf |title=Portland Cement Hydration |access-date=2013-02-21 |archive-url=https://web.archive.org/web/20170215064216/http://people.ce.gatech.edu/~kk92/hyd07.pdf |archive-date=2017-02-15 |url-status=dead }}
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{{cite journal |last1=Allen |first1=Andrew J. |last2=Thomas |first2=Jeffrey J. |last3=Jennings |first3=Hamlin M. |title=Composition and density of nanoscale calcium–silicate–hydrate in cement |journal= Nature Materials |date=25 March 2007 |volume=6 |issue=4 |pages=311–316 |doi=10.1038/nmat1871|pmid=17384634 |bibcode=2007NatMa...6..311A }}{{Cite journal|last1=Andalibi|first1=M. Reza|last2=Kumar|first2=Abhishek|last3=Srinivasan|first3=Bhuvanesh|last4=Bowen|first4=Paul|last5=Scrivener|first5=Karen|last6=Ludwig|first6=Christian|last7=Testino|first7=Andrea|s2cid=103781671|date=2018|title=On the mesoscale mechanism of synthetic calcium–silicate–hydrate precipitation: a population balance modeling approach|journal=Journal of Materials Chemistry A|language=en|volume=6|issue=2|pages=363–373|doi=10.1039/C7TA08784E|issn=2050-7488}} with some degree of crystallinity as observed by X-ray diffraction techniques.{{cite journal |last1=Renaudin |first1=Guillaume |last2=Russias |first2=Julie |last3=Leroux |first3=Fabrice |last4=Frizon |first4=Fabien |last5=Cau-dit-Coumes |first5=Céline |title=Structural characterization of C–S–H and C–A–S–H samples—Part I: Long-range order investigated by Rietveld analyses |journal=Journal of Solid State Chemistry |date=December 2009 |volume=182 |issue=12 |pages=3312–3319 |doi=10.1016/j.jssc.2009.09.026|bibcode=2009JSSCh.182.3312R }} The underlying atomic structure of C-S-H is similar to the naturally occurring mineral tobermorite.{{cite journal |last1=Taylor |first1=Harry F.W. |title=Proposed Structure for Calcium Silicate Hydrate Gel |journal=Journal of the American Ceramic Society |date=June 1986 |volume=69 |issue=6 |pages=464–467 |doi=10.1111/j.1151-2916.1986.tb07446.x}} 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 {{cite journal |last1=Cong |first1=Xiandong |last2=Kirkpatrick |first2=R. James |title=29Si and 17O NMR investigation of the structure of some crystalline calcium silicate hydrates |journal=Advanced Cement Based Materials |date=April 1996 |volume=3 |issue=3–4 |pages=133–143 |doi=10.1016/S1065-7355(96)90045-0}}{{cite journal |last1=Brunet |first1=F. |last2=Bertani |first2=Ph. |last3=Charpentier |first3=Th. |last4=Nonat |first4=A. |last5=Virlet |first5=J. |title=Application of Si Homonuclear and H−Si Heteronuclear NMR Correlation to Structural Studies of Calcium Silicate Hydrates |journal=The Journal of Physical Chemistry B |date=October 2004 |volume=108 |issue=40 |pages=15494–15502 |doi=10.1021/jp031174g}} (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.{{cite journal |last1=Kumar |first1=Abhishek |last2=Walder |first2=Brennan J. |last3=Kunhi Mohamed |first3=Aslam |last4=Hofstetter |first4=Albert |last5=Srinivasan |first5=Bhuvanesh |last6=Rossini |first6=Aaron J. |last7=Scrivener |first7=Karen |last8=Emsley |first8=Lyndon |last9=Bowen |first9=Paul |title=The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate |journal=The Journal of Physical Chemistry C |date=7 July 2017 |volume=121 |issue=32 |pages=17188–17196 |doi=10.1021/acs.jpcc.7b02439|url=http://infoscience.epfl.ch/record/231655 }} The exact nature of the interlayer remains unknown. One of the greatest difficulties in characterising C-S-H is due to its variable stoichiometry.{{cn|date=September 2023}}
The scanning electron microscope micrographs of C-S-H do not show any specific crystalline form. They usually manifest as foils or needle/oriented foils.
Synthetic C-S-H can be schematically divided into two categories, depending on whether their Ca/Si molar ratio is below or above a threshold value of 1.1. There are several indications that the chemical, physical and mechanical characteristics of C-S-H vary noticeably between these two categories.{{Cite web |url=http://www.mdpi.com/1996-1944/3/2/918/pdf |title=Archived copy |access-date=2013-06-01 |archive-date=2015-03-15 |archive-url=https://web.archive.org/web/20150315015437/http://www.mdpi.com/1996-1944/3/2/918/pdf |url-status=dead }}{{Cite web |url=http://ww2.mackblackwell.org/web/research/ALL_RESEARCH_PROJECTS/2000s/2095/MBTC%202095-3004.pdf |title=Potential Application of Nanotechnology on Cement Based Materials |access-date=2013-02-21 |archive-url=https://web.archive.org/web/20120217154034/http://ww2.mackblackwell.org/web/research/ALL_RESEARCH_PROJECTS/2000s/2095/MBTC%202095-3004.pdf |archive-date=2012-02-17 |url-status=dead }}
See also
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Other C-S-H minerals:
- {{annotated link|Afwillite}}
- {{annotated link|Gyrolite}} (a rare mineral from hydrothermal alteration, or an ageing product of alkali-silica reaction)
- {{annotated link|Jennite}}
- {{annotated link|Thaumasite}}
- {{annotated link|Tobermorite}}
- {{annotated link|Xonotlite}}
Other calcium aluminium silicate hydrate, (C-A-S-H) minerals:
- {{annotated link|Hydrogarnet}}
- {{annotated link|Hydrotalcite}}
- {{annotated link|Tacharanite}} ({{chem2|Ca12Al2Si18O33(OH)36}}, and also {{chem2|Ca12Al2Si18O51(OH)2 · 18 H2O}})
Mechanisms of formation of C-S-H phases:
- {{annotated link|Alkali–silica reaction}}
- {{annotated link|Alkali–aggregate reaction}}
- {{annotated link|Energetically modified cement}}
- {{annotated link|Pozzolanic reaction}}
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