What Is happening?
The chemistry of concrete systems is complex, and a basic understanding of the reactions occurring in the systems is essential in applying the protocol. Hydraulic cementitious systems stiffen, set, and harden by a process called hydration, which is a series of nonreversible chemical reactions with water.
Two aluminate compounds, C3A and C4AF, are present in portland cement. C4AF does not contribute significantly to system performance; however, C3A reacts rapidly when mixed with water and generates a large amount of heat (Figure 1) unless the reaction is controlled by the presence of sulfate. If the reaction of C3A with water is uncontrolled because there is insufficient sulfate in the solution for the amount of C3A involved, then flash (or permanent) set can occur.
Calcium sulfate is added to cement as gypsum (CSH2) during grinding to control the initial reaction of C3A. During grinding, some of the gypsum is dehydrated to form plaster (CSH1/2). The amount of dehydration is controlled by the manufacturer to provide optimum performance of the cement; however, if the amount of dehydration is incorrect, then false (temporary) set can occur.
Use of a fly ash containing C3A may result in flash set or rapid stiffening because of insufficient sulfate to control its hydration.
Some Type A water-reducing ad-mixtures also may influence the balance between C3A and sulfates because they tend to accelerate C3A hydration. Likewise, increasing temperatures accelerate the chemical reactions and also increase the risk of uncontrolled stiffening if marginally balanced materials are in use. Other contributors to potential risks are very finely ground cements, high alkali content in the system, and very low water-to-cementitious materials ratios.
All of these reactions and changes occur within the first 15 to 30 minutes after mixing, which has implications for concrete paving that uses nonagitating transporters. Even when agitators or truck mixers carry the concrete to the paver, the delivery time may be so short that there may not be an opportunity to work through a false set if it occurs. On longer deliveries for structures or flatwork, early stiffening may be less evident, but it may cause the addition of excessive water to the concrete delivered in a truck mixer.
One of the hydration products of the silicates (C2S and C3S) in cement is calcium silicate hydrate (CSH), which is the primary contributor to concrete strength, durability, and the heat of hydration.The silicates start to react two to four hours after mixing, when calcium reaches supersaturation in the mix solution. These reactions lead to setting and strength gain. If too much calcium has been consumed during earlier, uncontrolled C3A reactions, then setting may be delayed. In addition, the same Type A water-reducing admixtures that accelerate C3A reactions may retard silicate reactions, also potentially adding to the delay. Low temperatures also slow the hydration process.
Both accelerated C3A (uncontrolled stiffening) and delayed silicate reactions (delayed setting) can occur in the same mix. When rapid stiffening occurs in paving, the concrete mixture may be workable when delivered, but will stiffen up in the paving machine, leading to poor consolidation and difficulties with finishing and texturing.
Delayed setting significantly increases the risk of plastic shrinkage cracking, and makes it difficult to get the saw-cutting of joints completed at the right time. For concrete delivered in truck mixers, more water may have to be added before discharge for either paving or structural applications.
