For a mix design of Grade 30 or Grade 40 concrete, whether the admixtures (retarder and superplasticizer) will chemically react with the OPC or BHC in a different manner and results changes in initial and final setting time of concrete?
Retarder and superplasticizer have different role in concrete mix design. Retarder is retards the setting time and superplasticizer is used to reduce the W/C ratio keeping the workability of concrete same. Again the OPC and BHC or composite cement have different effect of Retarder and superplasticizer depending upon chemical composition of OPC and BHC or composite cement selected. Concrete mix trials will give clear picture regarding changes in initial and final setting time of concrete as well as other properties.
The setting time of cement refers to the time it takes for the cement to harden after it has been mixed with water. OPC (Ordinary Portland Cement) and BHC (Blast Furnace Cement) have different chemical compositions that can affect their setting times.
In regards to your question about using the same retarder and superplasticizer in a mix design, it is difficult to provide a definitive answer. The setting time of a cement mix is influenced by many different factors, including the type and amount of cement, water-cement ratio, temperature, and admixtures used (such as retarders and superplasticizers).
That being said, it is generally recommended to perform trial batches with various mix designs to determine the appropriate setting times for a specific application. The use of a retarder and superplasticizer in OPC and BHC or a composite cement mix may result in varying setting times, and it would be best to perform testing to determine the specific setting time changes.
I would agree with the answers above. Setting time depends on a multitude of factors. Each project should determine how long a set time they need and then design the mix to set within the desired time span. The effect of admixtures is hard to predict. I had a case where we took a regular water reducing agent, dosed it at twice the manufacturer's recommended dose and achieved excellent retarding effects. In that case we delayed the set time for several hours (we had to transport concrete from a batch plant to a loading dock, transfer it by crane and bucket to barges fitted out with agitating hoppers, tow the barge to concrete placement site, and then place it with a crane barge and buckets in the piers), but we had rapid strength development after that.
So the practice to follow is to determine your requirements then design your mix.
One comment about superplasticizers: They typically increase the workability for about 30 minutes, then need to be re-dosed. You can add superplasticizers several times as the effect wanes.
Blast furnace cement may have a delayed beginning of the setting and generally shifted and extended setting dynamics due to the reaction between the slag and portlandite derived from the hydration of calcium silicates/aluminates (from Portland clinker). This means that hydration of the clinker components must first occur and only then the slag begins to react. The product is mainly hydrated calcium silicates, i.e. C-S-H.
And this delay results from the presence of slag itself (not admixtures).
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