1 On the setting time of concrete

Initial setting: The initial setting time is required from the cement is mixed with water until the slurry begins to lose its plasticity.

Final Setting Time:  This is the time required for cement, after being mixed with water, to fully lose its plasticity and start to harden and gain strength. Setting time is critical in construction—if the initial setting is too short, the cement may not be workable; if the final setting is too long, the construction process is delayed.

For silicate cement:

Initial Setting Time: This should be no less than 45 minutes.

Final Setting Time: Should not exceed 390 minutes.

For ordinary cement:

Initial Setting Time: This Should also be no less than 45 minutes.

Final Setting Time: Should not exceed 600 minutes.

If the cement’s initial setting time doesn’t meet the requirements, the cement must be scrapped. If the final setting time is not within the required limits, it will be considered unqualified.

The initial setting time of concrete generally depends on the type of cement used. While there’s no uniform time, it typically falls within a range of 2-3 hours. Adding an Early Setting Agent can reduce the initial setting time to half an hour while adding a Retarder can extend it to 5-10 hours.

The exact initial setting time is usually determined through testing, and every batch of cement from each factory should undergo this test. The initial setting time is defined as the period from when the cement is mixed with water until the slurry begins to lose plasticity. The final setting time is when the slurry completely loses plasticity and starts to gain strength.

To ensure that the cement paste remains workable during construction, national standards specify a minimum initial setting time. Additionally, to allow the formed structure to gain strength quickly enough to bear loads, the final setting time must not exceed the specified limits set by these standards.

2 How to determine the setting time of concrete.

2.1 Determination of the time of initial coagulation 

The formation process of cement paste structure reveals that hydration products must grow sufficiently to link particles into a cohesive network, allowing the cement paste to start setting. From a rheological perspective, to make the paste flow, external force must be applied until the resulting shear stress breaks the forming mesh structure. This required shear stress is known as the “yield value.”

After cement is mixed with water, the yield value initially increases with hydration, slows down, and then rises again more rapidly. The first increase is due to the rapid formation of calcium aluminate, with gypsum dihydrate forming if gypsum hemihydrate is present. The second rapid rise in yield value is attributed to the strong hydration of tricalcium silicate, forming C-S-H (calcium silicate hydrate).

The “initial setting time” corresponds to the point when the yield value reaches a specific level just before this second rapid rise. Therefore, initial setting time is influenced not only by the hydration of tricalcium aluminate and iron phases but also closely related to the hydration of tricalcium silicate. The overall setting process, from initial to final setting, is primarily controlled by the hydration of tricalcium silicate.

Cement Testing Conditions:

Temperature: The test room should be maintained at 17–25°C, with relative humidity above 50%.

Curing Box: The temperature should be 20±1°C.

Materials: The temperatures of the cement specimen, standard sand, mixing water, and test molds should match the test room temperature.

Water: Use clean, fresh water for testing.

National Standards:

Initial Setting Time: Should not be earlier than 45 minutes, typically ranging from 1 to 3 hours.

Final Setting Time: Should not exceed 12 hours, generally between 5 to 8 hours.

Test Method:

Add water at the standard consistency to the cement and create a net paste in the test mold.

Initial Setting Time: Measured when the tester’s needle sinks into the paste to a depth of 0.5–1.0 mm from the bottom.

Final Setting Time: Measured when the needle sinks no more than 1.0 mm into the paste.

For concrete, the initial setting time is generally 2–4 hours. With a retarder, this can extend to 6–10 hours. However, continuous mixing during transportation and high temperatures in summer can further extend the initial setting time.

Here’s a revised version of the text to improve clarity and logical flow:

Concrete setting time is divided into initial and final setting stages. The initial setting occurs when the concrete starts to lose plasticity, while the final setting is when it completely loses plasticity.

The setting time of concrete is generally related to the setting time of the cement used. For ordinary cement, the initial setting should be no less than 45 minutes, and the final setting should not exceed 10 hours. Concrete typically follows a similar pattern.

However, modern concrete is often mixed with various materials and additives, which can significantly affect the setting time. These additives, such as retarders or accelerators, can either extend or shorten the setting time depending on their specific function.

2.2 Determination of final coagulation time

Concrete setting time, also known as curing time or age in days, follows a curvilinear relationship with strength development. In the first 14 days, the strength increases rapidly. After 14 days, the growth rate slows, and after 28 days, it becomes even more gradual. Generally, concrete continues to gain strength over 2 to 3 years, after which the strength growth ceases.

The strength development of concrete is influenced not only by curing time but also by factors such as the type of cement, curing conditions, and ambient temperature. For example, concrete made with 425 ordinary silicate cement can achieve 60% of its design strength in 7 days at 20°C, reaching 95-100% in 28 days. At 10°C, it only reaches about 45% in 7 days and 80% in 28 days. Under freezing conditions, as long as the concrete reaches more than 30% of its design strength before freezing, it can continue to gain strength, albeit more slowly.

Setting Process:

Initial Set: This occurs within 45 minutes to 1 hour after mixing, where the cement gel begins to solidify.

Final Set: Completes around 12 hours after mixing, but the gel remains in a soft plastic state before gradually hardening into a solid.

During the initial setting phase, the cement paste retains some fluidity, making it suitable for transportation, pouring, and compaction. From the initial set to the final set, the mobility gradually decreases. After 6 to 8 hours (approaching the final set), the paste loses mobility and has no strength, making it vulnerable to damage. During this period, extra care is needed to ensure stable strength development.