Setting time water-reducing admixtures

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

The setting of concrete as defined by the stiffening times in BS 5075 Part 1 for concrete containing these admixtures must be within 1 hour of the equivalent plain concrete mix. In practical terms normal water-reducing admixtures can be regarded as not affecting the setting characteristics of the concrete. 

Admixtures are substances that are added during the mixing process in small quantities related to the mass of cement, in order to improve the properties of fresh or hardened concrete [7,8]. The most utilised admixtures are water reducers and superplasticizers that may be added to improve the workability of concrete or reduce the amount of mix water accelerators that are used to increase the rate of development of early strength of concrete set-retarders that reduce the setting time of concrete air-entraining agents that increase the freeze-thaw resistance of concrete (Section 3.1). Recently, corrosion inhibitors have been developed in order to increase the corrosion protection of embedded steel these will be treated in Chapter 13. 

The water-reducing-retarding admixtures are used for avoiding cold joints and facilitating large pours. They are used particularly in hot weathering operations. These admixtures should increase the initial set time by at least one hour, with a maximum by 3 A hours. The compressive strength should be at least 110% of the control at 3, 7, and 28 days. 

Water-reducing and retarding admixtures are required to extend the setting time from 1 to 3.25 hours with respect to the reference concrete. Water-reducing and accelerating admixtures accelerate the setting time between 1 and 3.5 hours. 

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