Heat of Hydration of Cement
Heat of Hydration of Cement:
Cement’s reaction with water is exothermic. The reaction generates a significant amount of heat. This heat is referred to as heat of hydration. This is readily apparent when freshly mixed cement is placed in a vacuum flask and the mass’s temperature is monitored at regular intervals. The investigation and monitoring of hydration heat becomes critical in the building of concrete dams and other large-scale concrete structures.
The temperature within large mass concrete has been shown to be 50°C higher than the initial temperature of the concrete mass at the time of placement, and this elevated temperature has been shown to continue for an extended duration. The pattern of heat release from setting cement and during the early hardening phase is depicted in Figure 1.
As cement and water are combined, a brief period of accelerated heat evolution occurs. This heat transfer is almost certainly caused by the reaction between aluminates and sulphates in solution (ascending peak A). When the solubility of aluminate is decreased by gypsum, this initial heat evolution ends rapidly (descending peak A). The subsequent heat evolution is due to the formation of ettringite and may also be a result of C3S reaction (ascending peak B).
Different compounds hydrate at varying rates and release varying amounts of heat. The amount of hydration of pure compounds is depicted in Figure 1. Due to the addition of retarders to regulate the flash setting properties of C3A, the early heat of hydration is currently contributed primarily by C3S hydration. The fineness of the cement also has an effect on the rate of heat production, but not on the overall amount of heat. The overall amount of heat produced during full hydration is proportional to the relative amounts of the main compounds contained in a cement.
Figure 1: Rate of hydration of pure compoundsVerbec and Foster calculated the heat evolution of four main cement compounds using heat of hydration data from a vast number of cements. The heats of hydration of four compounds are mentioned in Table 1.
Due to the fact that the heat of hydration of cement is an additive property, it can be predicted using a form term.: H = aA + bB + cC + dD
Where H is the heat of hydration, A, B, C, and D are the percentage contents of C3S, C2S, C3A, and C4AF, respectively, and a, b, c, and d are coefficients reflecting the heat of hydration contribution of 1% of the related compound.
Normal cement usually produces 89-90 calories per gramme in seven days and 90-100 calories per gramme in 28 days.
Hydration is not an immediate operation. The initial phase of the reaction is quicker and lasts indefinitely at a declining rate. Complete hydration cannot be achieved in less than a year or longer unless the cement is very thinly ground and reground with excess water at intervals to expose new surfaces. Otherwise, the obtained product exhibits unattacked cores of tricalcium silicate encased in a sheet of hydrated silicate, which is comparatively impervious to water and thus retards more attack. After 28 days of curing, it was discovered that cement grains had hydrated to a depth of just 4µ. Additionally, it has been found that under normal conditions, total hydration is possible only for cement particles smaller than 50µ.
A grain of cement can contain numerous C3S or other crystals. The largest crystals of C3S or C2S are about 40µ. A typical size will range between 15-20µ. It is likely that the C2S crystals on the surface of a cement grain will become hydrated, while a more reactive compound such as C3S in the interior of the grain will remain unhydrated.
In a grain of cement, the hydrated product of the cement compound adheres tightly to the unhydrated nucleus. That is, unhydrated cement remaining in a grain of cement has little effect on the strength of cement mortar or concrete, as long as the hydration products are compacted properly. Abrams achieved strengths of up to 280 MPa with water/cement ratios as low as 0.08. Essentially, he has applied extensive pressure to ensure that such a mixture is well compacted. Due to the poor water/cement ratio, hydration could only occur at the surface of cement grains, leaving a significant number of cement grains unhydrated.
Today’s High Performance concrete is manufactured with a water cement ratio of about 0.25, which means that a significant portion of the cement grain may remain unhydrated in the core. Just surface hydration occurs. The unhydrated center of the cement grain may be considered to act as extremely fine aggregates in the method.
Read Also: ASTM C403: Setting Time of Concrete