Dear All,

I want to know when does complete hydration of cement takes place?

Is it completes within moments of pouring of water or takes complete curing period for its completion?

If the first case is true then only hardening takes place during curing?

If the second case is true then  there will be no hydration of interiormost particles since such parts of the cube mould appears dry on crushing even after 28 days curing period?

 

Kind Regards,

Gulam Dastgir

 

Hydration of cement continues for many months and years after it is mixed with water. That is why concrete continues to increase in strength for years and why we measure strength at 1, 3, 7, 28 days, etc.

I would concur with Dr. Clark. Studies have shown that on samples conserved in good conditions (both laboratory and real-life like bridges supports), slow increase in strength can still be measured more than twenty years after concrete production.

Of course, one could argue that after such a long time, hydration is mostly complete and what we observe is a slow reorganization of the hydrated matrix.

Concerning your initial question, the fact that the concrete looks dry does not mean that hydration, that is chemical reaction with water, is not taking place. Any porous material can hold a certain amount of water (even not chemically bonded) and still look dry.

Best regards,

Dear All,

Exactly the same answer i was hoping. But there is one more 

query here. We add water in cement after determining its W/C ratio

by vicat appratus. This ratio gives amount of total water required for 

forming a paste of cement of required consistency. Is this required consistency means the amount of water required for hydration after leaving water loss during paste formation?

If this is so then why curing is needed, when we have already mixed water reqiured for the hydration?

 

Regards,

Gulam Dastgir

The w/c ratio is a mechanical property that depends mainly on the fineness and granulometric distribution. It is not related to the stoechiometric water needed.

Dear Dastgir

During mechanical testing of cement we are determining water for W/C and for normal consistency. For only hydration portland cement requires about 18% water of its mass. But for normal consistency we're usually adding 24-28% water. Part of excess water evaporates and other parts remains in structure of concrete, pores etc.

Regarding to period of hydration of cement I want to add, that curing  of cement takes place long, but we can't see that without special devices. Bellow I bring the table, that show the level of hydration (thickness of hydrated layer in microns) of main clinker minerals.

  Depth of hydration of clinker minerals, mk (By Butt, Sychev, Okorokov, Timashev)

  Minerals   3d.      7d.     28d.   3m.    6m. 

   C3S        3.5      4.7     7.9     14.5    15

   C2S        0.6      0.9     1.0      2.6     2.7

   C3A       10.4    10.7    11.2    13.5    14.5

  C4AF       7.7      8.0      8.4    12.2    13.2

Of caurse you know the avarage sizes of clinker minerals. As you can see there are grains' layers that have to be hydrated (especially C2S). So the table data is additional information for you.

Best regards. Davit Babayan. 

 

Dear Davo,

 

Is there any mathematical relationship between Time V/S strength?

When we plot graph for 1 to 28 days strength, logarithmic relatiosheep seems

best fit, but it shows increase in strength continuously.

Regards,

Gulam Dastgir