We are wasting cement at the time of construction: how can we reduce that and how to re use the concrete?
The initial setting time of concrete is 30 min. The process of setting starts immediately after preparation of concrete. Within 10 min. after preparation it can be used. Other wise it become hard and can not take the shape of form work.
Reuse of the hardening cement may be helpful if we fire cement at 750degree cent., for two hours. This will activate the hardened cement materials
Cement sets/rate of hardening beyond green level by 45 minutes and in a paste of concrete can take a minimum of 6 days. Nevertheless, setting of cement depends largely on the quantity of gypsum present. In general: ASTM C 150 specified set times.
Test Method Set Type Time Specification
Vicat Initial ≥ 45 minutes
Final ≤ 375 minutes
Gillmore Initial ≥ 60 minutes
Final ≤ 600 minutes
Note that: At about -3 to -4°C for cold weather-cement, enough of the pore water in cement will freeze so that hydration will completely stop, Please visit:http://www.na.graceconstruction.com/custom/concrete/downloads/TB_0106.pdf for further information
Concrete set usualy after 3-5 hours (initial set) depending on cement type, water content and additives. If you realize that you can not properly place your concrete before setting then you may add a retarder to increase the setting time.
Once you get to the inital set, there is not much you can do and still have good concrete. Concrete, once set may be reused by crushing it and using it as a medium strength aggregate in a new concrete.
In this discussion, Some thories have to be presented before treating the topic
(1) It should be understood that the reaction between water and cement is chemical and not physical and it is not reversible.
(2) The initial and final setting time of cement paste is 45 minutes and 10 hours respectively.
(3) Also it is considered that already used hardened cement is collected and broken down into smaller particles for reuse.
(4) When water is added to fresh cement powder, the whole volume of cement does not react with water completely at a time it is a gradual process. .
Therefore, at any stage of reuse of already used cement it is only a fraction that has not reacted with water initially that will now be useful and contribute to the new strength. The part that has already reacted with water and hardened will only form an inert material like aggregate.
If reuse is carried out before the final setting time, the hydration will continue in the new form but the strength will not be as expected because more water will be added which will lead to reduction in strength.
The initial and final setting time of cement vary and depend on the cement type eg that of ordinary Portland cement is different from rapid hardening Portland cement. Also setting time depends on the temperature and thus for a type of cement it will vary all over the world where temperature varies from place to place.
The normal initial setting time is around 60 minutes and final setting time is around 600 minutes.
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