That could happen due to the difference between different batches of concrete. Small variation in the main concrete components might result in color variations.

Sometimes colour variation is found due to release of free lime after the hydration of cement. The leachate can be seen near the edges of slab in the form of white patches. Because of leaching of free lime from concrete colour difference may be observed.

Consider also different formwork removal times and different drying rates for various surfaces as a factor that may result in discolouration. But if you could share more details on the structure and mix you could receive more insights. Thanks

Dear .. Hothifa Rojob ... Thanks ... You are right.. that could be a good reason .. and with little being careful.. could handle it and get same finishing color...

Thanks again...

Dear ... Om Prakash Chhangani ... Great note...

Please tell us how could treat such a case, ... and how can we preventing it in the future...???

Thanks so much for your answer....

Dear .. Fragkoulis Kanavaris .... Thanks for your answer...

Different state of drying and curing effect of the final color of the topping concrete, .. actually I'm working on research about handling and treating the main reasons effecting the uneven color of concrete...

Thanks again for your answer...

@Hussam Mamuree

There are few ways to prevent leaching of free lime.

1. Use cement of low grade so that release of lime is less.

2. Optimize the cement quantity i.e. the minimum required to achieve strength.

3. Use blended cement like fly ash so that free lime released is consumed by reaction with fly ash which will further strengthened concrete because of C-S-H gel formation and improvement in pore refinement.

4. Instead of full water curing use curing compounds and cover your concrete by polythene sheets to prevent moisture loss and thereby prevent leachetes withdrawal from concrete.

causes may be due to the different temperatures at the time of concreting which causes different rates of setting in concrete at the time.

the film of mould oil applied may lead to different colour,

the differences on moisture content levels especially on timber formwork

Dear.... Om Prakash Chhangani .... Thanks so much for your great answer...

Please explain the following ... need to now more from your answer... :

1. If we using low grade of cement, .. will the effect the finial strength of concrete...?

2. Also, the reducing cement quantity will effect the strength and durability... any other suggestion... ?

2. covering the concrete with polythene sheets might not being possible for large area over 500sq.m, that will cost a lot...

Thanks so much again for your interesting answer...

The problem with high grade cement is it's fineness and components which hydrates early during the early water curing leading to possibility of leachetes and therefore if not required then high grade cement may not be used. If necessary then alternate way of curing must be explored.

The mix must be designed in such a way that for desired strength minimum cement is used. If high grade cement is used then suplasticizers may be used so that by lesser w/c ratio you get strength.

One can use curing compounds and other simple methods then water ponding to avoid leachetes.

Dear ... Seth Ouya Onyango ... Thanks so much for your answer...

Its totally right that when large area want requested to cast, ... that's mean long time, ... maybe take full day, .. with temperature change during the day (morning-noon - maybe night)... color change could be happened...

film of oil will effect the side of the concrete, not the face.. and even for the side. its not my research object..I'm looking for concrete defect...

Thanks again for answering... feed us with more info about our subject if you can.

Dear.... Om Prakash Chhangani ... Thanks for your professional answer...

I will write an article about that reason... special one....

Do you know any references might be useful... ?

Hope the attached file will be of some help to you

Dear... Om Prakash Chhangani ... Thanks for your assistance..

I will check this reference and make a feedback...

Thanks again....

Dear... Om Prakash Chhangani ... Thanks for your assistance..

I will check this reference and make a feedback...

Thanks again....