Cement: Difference between revisions
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Altilunium (talk | contribs) (Created page with "Portland cement consists of the following major compounds : tricalcium alumniate, tricalcium silicate (alite), dicalcium silicate (belite), tetracalcium aluminoferrite and gypsum. == Reactions == When water is added to cement, the following series of reactions occur * The '''tricalcium aluminate''' reacts with the '''gypsum''' in the presence of '''water''' to produce '''ettringite''' and heat (▲H = 207 cal/g). Ettringite consists of long crystals that are only stab...") |
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== Reactions ==
[[File:Cement reaction.png|thumb]]
When water is added to cement, the following series of reactions occur
* The '''tricalcium aluminate''' reacts with the '''gypsum''' in the presence of '''water''' to produce '''ettringite''' and heat (▲H = 207 cal/g). Ettringite consists of long crystals that are only stable in a solution with gypsum.
* The '''alite''' is hydrated to produce '''calcium silicate hydrates''' (CSH), '''lime''' and heat (▲H = 120 cal/g). The CSH has a short-networked fiber structure which contributes greatly to the initial strength of the cement glue.
* Once all the gypsum is used up in the first reaction, the '''ettringite''' becomes unstable and reacts with any remaining '''tricalcium aluminate''' to form '''monosulfate aluminate hydrate''' crystals.
** The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of sulfates, the crystals resort back into ettringite, whose crystals are bigger than the monosulfates (two-and-a-half-times). It is this increase in size that causes cracking when cement is subjected to sulfate attack.
* The '''belite''' also hydrates to form '''calcium silicate hydrates''' and heat (▲H = 62 cal/g).
** Like the second reaction, the calcium silicate hydrates contribute to the strength of the cement paste. This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of belite to the strength of the cement paste will be slow initially. This compound is however responsible for the long term
* The '''ferrite''' undergoes two progressive reactions with the '''gypsum'''. In the first of the reactions, the ferrite reacts with the gypsum and water to form '''ettringite''', '''lime''' and '''alumina hydroxides''' (i.e. ferric aluminum hydroxide). The ferrite further reacts with the ettringite formed above to produce '''garnets'''.
** The garnets only take up space and do not in any way contribute to the strength of the cement paste.
In the end, the hardened cement paste consist of the following :
# Ettringite : 15 to 20%
# Calcium silicate hydrates, CSH : 50 to 60%
# Calcium hydroxide (lime) : 20 to 25%
# Voids : 5 to 6% (in the form of capillary voids and entrapped and entrained air)
== References ==
# The Pennsylvania State University Department of Civil and Environmental Engineering and Instructional Systems "[https://www.engr.psu.edu/ce/courses/ce584/concrete/ Concrete Clinic] - [https://www.engr.psu.edu/ce/courses/ce584/concrete/library/construction/curing/Hydration.htm Hydration of Portland Cement]"
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