# Hydration, Setting and Crack-Resistance Properties of Activated HVFACs at Early Stages
## 1 Introduction  
With the development of the economy worldwide and the large production of fly ash (FA), as a by-product of various industries, such as thermal power stations, FA has been concerned as one of the potential alternatives of conventional cementitious materials (i.e., mainly Portland cement (PC)) or a part-replacement material of PC to propel eco-concrete since the 1930s. This is attributed to the fact that a number of researchers have attempted to solve/reduce to a large degree an unavoidable problem caused by the manufacturing of PC: its large amount of carbon dioxide (CO\({}_{2}\)) emission. As a construction material, the application of FA in concretes can improve the greenhouse effect of Earth's atmosphere caused by the large amount of the emission of CO\({}_{2}\) via reducing the utilization of PC in concretes and decrease natural environment pollution by decreasing the floating of FA in the air todecrease the danger to humans and other lives. Besides, it also has been reported that FA can improve the workability and durability properties of concrete and reduce the construction cost of reinforced concrete (RC) structures [1]. Fly ash has been studied sufficiently and used in concrete structures since the 1980s. In practical construction, FA is commonly used in RC structures with a cement replacement ratio of less than 30% of the volume or mass ratio till now. However, in 1986, high volume fly ash concrete (HVFAC) was first developed for mass concrete structures to obtain low heat generation and acceptable early strength [2]. Subsequently, numerous studies [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13] focused on the properties of HVFAC, which was defined as an FAC whose 50% or greater than 50% of cement (volume ratio) was replaced by FA in the American Concrete Institute (ACI)-ACI 332R code [14]. Researchers [15; 16; 17] also suggested using HVFAC in other structural and pavement concretes considering its comprehensive properties and economy. It is well known that mass concrete usually produces a great amount of heat caused mainly by the long-term and large volume hydration reaction of cementitious material. This results in that the internal temperature of concrete rises largely to form a temperature gradient compared to the external one, which then results in the inconsistent deformation/stress between external and internal concretes. The uneven temperature stress further leads to the development of internal and external crack devolvement, which subsequently affects the durability of these concrete structures. A number of studies [18; 19; 20; 21] indicated that the use of FA in concrete is one of the most effective methods to reduce the hydration heat of mass concrete. However, it has been [22] also reported that fly ash cannot retard the hydration reactions of the cement, but can significantly reduce the heat release-peak temperature at early ages under semi-adiabatic conditions. On the other hand, setting time is one of the key technology factors to determine the casting time and construction methods of RC structures. FA and ACs both will affect the initial and final setting times of concrete, in theory. At the same time, finding the ways to resist effectively and limit the crack development of concrete at the early stage is always one of the concerned research topics in concrete industries. It is attributed to the fact that the early unrecoverable plastic shrinkage cracks in concrete have a significant influence on the durability and other properties of concrete at later stages, e.g., they will result in the water moving from the surface to the internal concrete, and the tensile strength of concrete is lower than the concrete shrinkage stress. In addition, various chemical activators (AC) usually have been developed and applied to activate FA in PC concrete at the early age to improve the hydration action and obtain higher strength level.