In summer, concrete production is often mixed with retarder or water reducing retarder to improve its fluidity. However, sometimes improper use of admixtures will occur in slow-setting or even super slow-setting phenomena, resulting in concrete can not be timely released or not set for several days. The problem of slow-setting accidents in summer should not be ignored. Especially in the poor adaptation of admixtures and cement, when encountering a sudden drop in temperature is more likely to make the concrete set slowly.
(A) Cement and concrete setting
(1) Cement setting
The cement reacts with water to produce hydration products. The hydration products make cement particles lap and solidify the condensation. The setting of cement is related to the activity of cement, water-cement ratio, temperature, and other factors. When the water-cement ratio is large, the distance between cement particles is large, requiring more hydration products to fill, and the setting time is relatively long. When the cement activity is high, the hydration speed is fast, and the generation of hydration products is also fast, resulting in a shorter setting time. Therefore, the factors affecting the hydration rate of cement will affect the setting time of cement.
(2) Concrete setting
The setting of concrete is caused by the hydration reaction between cement and water, and the setting of concrete is closely related to the setting of cement. In the case of the same ambient temperature, the setting time of cement can be used to roughly infer the setting time of concrete. For example, the setting time of cement (standard consistency with 27% water): 185min initial setting time, 260min final setting time, the concrete water-cement ratio at 0.54. It can be inferred that the setting time of concrete (without considering the admixture factor) is about two times the setting time of cement. This inference is derived from the same or similar test conditions, it is not suitable for the casual application.
(B) Reasons for the phenomenon of slow setting
Why does this phenomenon of super slow condensation occur? It’s believed that there are two main reasons for the following.
(1) Cement setting time is too long
Concrete condensation is mainly caused by the condensation of cement. Therefore, the setting time of the cement determines the concrete setting time. If the cement setting time is long, the concrete setting time is relatively slow. Especially in the case of large concrete water-cement ratio or retarder (or retarded water reducing agent) in large quantities, it is easy to occur a long setting time or super-long phenomenon.
(2) Retarder or water reducing retarder is too large
An excessive amount of retarder or water reducing retarder is the main reason for the long setting time of concrete.
The main retarders or water reducing retarders used in concrete are
1. sugars, such as calcium sugars, etc.
2. lignosulfonates, such as calcium lignosulfonate, sodium lignosulfonate, etc.
3. hydroxy carboxylic acids and salts, such as citric acid, potassium tartrate, etc.
4. inorganic salts, such as zinc salts, borates, phosphates, etc.
5. Others, such as amine salts and their derivatives.
Retarders are different from gypsum in the setting of cement. Gypsum dihydrate as a retarding agent, with the amount of increased cement setting time is almost no longer delayed. For example, after the SO3 content up to 2.5%, and then increase the SO3, the setting time does not change much. But the concrete retarder is different. In the case of concrete retarder amount is small, it can produce a strong retarding effect. And as the amount increases, the effect is a linear growth or exponential curve growth. In short, the amount of retarder is too large, not only will make the concrete set too long, but also may make the early strength development slow.
(3) Other factors
Low ambient temperature, low activity and large amount of mixed materials (or mineral admixture) and coarse cement will also result in cement setting time delayed. If the setting time relative value of the ambient temperature of 15 ℃ is 1.0, then the initial setting time at 10 ℃ is about 1.2, the final setting time of 1.4; when the ambient temperature drops to 5 ℃, the initial setting time relative value is about 1.3, and the final setting time is about 2.6. The lower ambient temperature will make the cement setting time longer, especially the final setting time is more extended. Therefore, when the temperature is low, it should be mixed with less or no retarder to avoid the phenomenon of super slow setting.
(C) Preventive measures for slow setting of ready-mixed concrete
Except in the hot summer and long transport distance, the phenomenon of super slow setting is generally unfavorable and should be avoided as far as possible. To this end, the following measures should be taken.
(1) Correctly select the variety of retarder and mixing amount
Retarder or water reducing retarder should be selected depending on the specific circumstances:
1. For the cement with long setting time, it is appropriate to use retarder or water reducing retarder with slow setting effect, such as lignosulfonates, especially lignosulfonates containing less reducing sugar and the amount should be small. When used separately with a mass fraction of 0.25% is appropriate, not more than 0.3%. Excessive amount may cause a long period of non-setting resulting in a decrease in strength.
2. Hydroxy carboxylic acid and its salts have a strong retarding effect. This type of retarder and water reducing retarder containing such retarder amount (by mass of cement) should only be 0.03% to 0.1%. Such retarders should not be used in concrete with low cement amount and large water-cement ratio alone
3. Sugar compounds are mixed with 0.1% to 0.3%. Excessive admixture of sugar will instead play a role in promoting coagulation.
4. Calcium sugar water reducer and lignosulfonates water reducer will reduce the solubility of hard gypsum and fluorine gypsum, so that the amount of SO42- dissolved in the cement slurry is reduced, so that C3A can be rapidly hydrated and lead to rapid setting. Even if it does not reach the degree of rapid setting, it will greatly reduce the fluidity of the concrete mix, resulting in slump loss. In addition, this type of retarder or water reducing retarder effect is very strong. Excessive amount will cause slow setting or even a long time not set. Therefore, the amount of mixing must be accurate. Low ambient temperature should be less or not mixed with such retarder.
(2) Choose cement with suitable setting time
Cement setting time too long is another important reason for the super slow setting of concrete. Generally speaking, the production of ready-mixed concrete should choose silicate cement and ordinary silicate cement. In addition to strength and homogeneity, the final setting time of cement is controlled within 300min, to avoid the lowering of temperature or increase the amount of admixtures causing the concrete setting time extended.
(3) Control the quality, amount and water quantity of mineral admixture
When using mineral admixture, you should pay attention to control the amount of admixture, especially when the mineral admixture activity is low and the specific surface area is small. In addition, when using ordinary silicate cement, you should pay attention to the quantity of admixture in the cement. Nowadays, many cements have excessive admixture of mixed materials. Operators or transport truck drivers should not add water arbitrarily to ready-mixed concrete based on their subjective judgment.
(4) Measures to deal with the phenomenon of slow-setting concrete
Due to the excessive use of retarder or water reducing retarder, rather than the quality of cement, or cement and mineral admixture misuse causing the phenomenon of concrete retardation, strengthen the maintenance and extend the demoulding time can ensure that the quality of concrete is not reduced. If the concrete can be set in 48-72h, the late strength will not be significantly affected. But if the concrete can not be set after more than 72 hours, its late strength may be difficult to ensure.