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The crystallography has always been interesting to cement and concrete researchers. In hydrated
  cement, hydrated products consist of various chemical compounds depending on the hydration time. The shape and morphology of these crystals, which is a means of their identification using SEM, can be of hexagonal and prism form for example. Here are some nice examples showing how beautiful the cement can be at micro and nano levels!

SEM image of Hydrated Portland CementHydrated Cement Paste

Depending on the time of hydration and the Portland cement composition, several crystalline pictures can be observed in hydrated cement paste. A typical one contains C-S-H, calcium hydroxide and ettringite as shown in this picture (photo courtesy of Mr. Jim Margeson, NRC-IRC). 


SEM picture of Calcium HydroxideCalcium Hydroxide

Calcium hydroxide has a distinguishing hexagonal shape.
The dimensions, of course, depends on the concentration in aqueous solution and the age of the crystal. This nice crystalline shape has been produced on a piece of glass
fibre in saturated Ca(OH)2 solution.



SEM picture of Calcium Hydroxide




Depending on the concentration of the solution, other shapes of the crystalline calcium hydroxide can be formed.

Calcium Hydroxide crystals on the C3S grain after 2.5 hours of hydration

 

 

Small 100nm hexagonal shape Ca(OH)2 crystals on the surface of the C3S after 2.5 hours of hydration at room temperature (W/C=0.8). The hydration was stopped by isopropanol and subsequent vacuum drying.


calcium carbonate crystals on the calcium-silicate-hydrate compactCalcium Carbonate

Strange crystals appeared on a compact of C-S-H after unexpected long exposure to the atmosphere. Their "alien" and "snow flake" shapes seem to be similar to the calcium carbonate crystals formed as a result of the C-S-H carbonation due to the carbon dioxide in the air.

 


Wollastonite and Calcium CarbonateCalcite rhombs mixed with the Wollastonite needles. Wollastonite is a form of calcium silicates (CaSiO3) that is not usually carbonated unless through a specific treatment.








calcium carbonate aragonite calcium hydroxide etch pitThis is a colorized SEM picture of aragonite on the surface of calcium hydroxide. The sample was collected from the unusually large transparent crytals found on the surface of a 45 year old cement paste specimen (W/C=0.5). The crystals were shown to be calcium hydroxide (colored blue). The surface of CH contained etch pits (colored gray) and aragonite (a polymorph of calcium carbonate - colored red) at various spots. (The scale bar indicates 2 micro meters)


Calcium Silicate Hydrate (C-S-H) formed in the hydration of C3SCalcium Silicate Hydrate (C-S-H)

The flaky C-S-H crystals grown after 2 weeks of hydration of C3S (W/C=0.8). The stoichiometric C/S ratio of C-S-H and morphology of its crystals in conventional cement systems depend on the curing condition. 


C-S-H and Calcium Hydroxide crystals in the hydration of C3S after 2 weeksThe growth of C-S-H and calcium hydroxide crystals beside each other. The dominating phase in forming the shape in the contact surface is C-S-H probably due to its significantly higher strength.
 

 


Calcium Silicate Hydrate (C-S-H) formed in the hydration of C3S The formation of needle-like C-S-H stacks grown after 5 hoursof of hydration of triclinic C3S (W/C=0.4). The green substrate is the C3S etch-pitted as a result of dissolution of Si, Ca and O atoms.

© 2007-2011 Rouhollah Alizadeh, all rights reserved