Hydration reaction, concrete

Hydration at Ordinary Temperatures. Pordand cement is generally used at temperatures ordinarily encountered in constmction, ie, from 5 to 40°C. Temperature extremes have to be avoided. The exothermic heat of the hydration reactions can play an important part in maintaining adequate temperatures in cold environments, and must be considered in massive concrete stmctures to prevent excessive temperature rise and cracking during subsequent cooling. 

The ions must not interfere with or modify the hydration reaction so that the concrete properties are not adversely affected. 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

The admixture may remain in a free state as a solid, in solution, interact at the surface, or chemically combine with the hydrates. Physicochemical and mechanical properties of the concrete may be influenced by the type and extent of the interaction. Thus, the early hydration reactions of cement may be affected in diverse ways and it is possible that more than one effect occurs at the same time. These are sununarized below [125]. 

Many things that are required to build a house rely on chemical reactions. Concrete, for example, is a mixture of cement, sand, and gravel or crushed stone. When water is added to this mixture, a chemical reaction called hydration takes place. During hydration, the compounds in the cement form chemical connections with the surrounding water molecules. Concrete is a popular building material because it is resistant to wind, water, rodents, and insects. It is also non-combustible, which means it will not catch fire. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

The hydration reactions of alinite cements do not appear to have been reported, but it may be surmised that they are similar to those of Portland cements, with the probable addition to the products of CjA CaCK-lOHjO and possibly also of Ca2Cl2(0H)2 H20. The Cl is, however, not wholly and permanently combined in these or other hydration products. In tests on reinforced concrete, serious corrosion had occurred within 5 years, showing the cement to be unsuitable for such use (L51). 

The workability of any fresh concrete decreases with time after mixing, but this effect, which is called slump loss, is more marked if a water reducer is used. The slump nevertheless remains higher than if the latter was absent. Slump loss is caused by the slow commencement of the hydration reactions, and its increased magnitude in concrete containing water reducers is probably due to the gradual absorption of the admixture by the hydration products. Delay in adding the admixture until a few minutes after mixing minimizes it. 

Hydration of the cement takes place in the presence of moisture at temperatures above 50°F. It is necessary to maintain such a condition in order that the chemical hydration reaction may take place. If drying is too rapid, surface cracking takes place. This would result in reduction of concrete strength due to cracking as well as the failure to attain full chemical hydration. 

The cement paste formed by the hydration reactions always contains interconnected pores of different sizes, as shown in Figure 1.2. The pores can be divided into macropores, capillary pores and gel pores. The interlayer spacing within C-S-H gel pores) have a volume equal to about 28 % of the gel and dimensions ranging from a few fractions of a nm to several nm. These do not affect the durability of concrete and its protection of the reinforcement because they are too small to allow significant transport of aggressive species. The capillary pores are the voids not filled by the solid products of hydration of hardened cement paste. They have dimensions of 10 to 50 nm if the cement paste is well hydrated and produced... 

Abstract. Polyvinylalcohol (PVA) is a polymer soluble in hot water, it has the property of film formation and it can improve the concrete performance. The effects of PVA modified with nano clay on the cement hydration reaction have been investigated by means of semiadiabatic calorimeter, FTIR spectroscopy and SEM. FTIR spectroscopy was employed to monitor chemical transformation of cement. The morphology of the different samples was compared by means of SEM micrographs. With the semiadiabatic calorimeter the hydration kinetic was measured to compare the heat rate of the admixtures materials. Fixing the water-cement ratio, w/c, in 0,45, the ratio of polymer to cement (p/c) was 2 wt% and the ratio of clay to polymer was 4 wt% (0.8wt.% related to cement). The polymer and modified polymer admixtures produced a retardation effect on the kinetic of cement hydration, but the clay acts as nucleating agent. The increase of the temperature with time was measured and a new model with four parameters was employed and the kinetic parameters were determined for each sample. 

The reaction of cement with water causes the general reduction of this mixture volume. It can be calcirlated based on the specific density of substrates and cement hydration products. These volume changes are called the chemical shrinkage or contraction and are linked with the lower water volume in the hydrated phases as compared to its volirme in the liquid phase. On the base of contraction of cement mixture with water the progress of hydration reaction can be determined and even the strength of concrete [108]. 

Yoshida, K., and Okabayashi, S. (1992) Physical properties and early hydration reaction of finely ground cement, in Proceedings 9th ICCC, New Delhi, Vol. 4, pp. 104-110. Yu, Q. et al. (1998) Effect of electron water curing and electron charging curing on concrete strength. Cement and Concrete Research 28, 1201-1208. 

Hanic, F., Kapralik, ., and Gabrisova, A. (1989) Mechanism of hydration reactions in the system C4A3S-CS-Ca0-H20 referred to hydration of sulfoaluminate cements. Cement and Concrete Research 19,671-682... 

Xi, Y., Siemer, D.D., and Scheetz, B.E. (1997) Strength developmenf hydration reaction and pore stracture of autoclaved slag cement with added silica fume. Cement and Concrete Research 27,75-82. 

When the base plate poured with concrete which hydration reaction occurs, large amounts of heat would be released. The heat of hydration accumulate within the concrete caused the temperature of the concrete rising. However, the surface in contact with the air temperature is low. The temperature difference between inside and outside caused the surface of the concrete shoes tensile stress the cooling process will make the concrete surface stress. Statistics show that when the difference between inside and outside temperatures exceeds 25°C or average temperature down more than 6-8°C within 2-4 d, the concrete surface will develop cracks caused by the temperature stress. The monitoring data from... 

Since ettringite forms a diffusion barrier around the C3A grains, the hydration reaction is initially delayed. Only after about 12 h will this barrier be broken by the onset of transformation of ettringite into monosulfate (Figure 5.7). This reaction is reversible, as the monosulfate can retransform to ettringite in the presence of sulfate ions in the groundwater, leading to the stmctural destruction of concrete, as can be observed on many urban concrete structures. In modern sulfate-resistant... 

The normal mechanism of cement hydration is altered in the presence of wood extractives. There are many cases in which wood is used in contact with concrete. For example, cement is mixed in or molded with wooden forms, or cement and wood are mixed together for board production. Cement yields calcium ions when mixed with water and exhibits a pH of 13. The alkaline solution dissolves extractives from wood into the slurry. The resulting substances have a low solubility and they precipitate with CaO. CaO is one of the most important chemical compounds in concrete, and its loss retards the normal hydration reaction (35). These occurrences are dependent not only on the kinds of extractive, but also on whether the exposed wood surface is a transvers or a longitudinal section. 

Concretes prepared with blast furnace slag cement and pozzolanic cement have lower Ca(OH)2 content and pH of the pore solution, thus in principle providing less protection against carbonation or chloride induced corrosion with time. However, the pozzolanic reaction leads to a filling and refining of the pore system and thus to a less or much less permeable concrete. Due to the slow hydration reaction curing (keeping wet the concrete in its early age) is much more important to achieve low porosity. 

Hydration is the reaction of portland cement with water. The most important hydration reactions are those of the calcium silicates, which react with water to form calcium silicate hydrate and calcium hydroxide. Calcium silicate hydrate forms a layer on each cement grain. The amount of water present controls the porosity of the concrete less water results in a denser, stronger matrix, which in turn leads to lower permeability and higher durability and strength [21]. 

The passivation of reinforcement of concrete is provided by the high alkaline environment (pH = 12.8) from lime continuously produced from the hydration reaction of cement. This passive layer may, however, be destroyed due to ingress of chloride and carbon dioxide in certain environments and serious corrosion is caused. Concrete is permeable and allows the ingress of atmosphere. The carbon dioxide reacts with alkalies and forms carbonate and thus reduces the pH values to 9. 

Sufficient time must be allowed to ensure the completion of the hydration reaction and achieve maximum hardening of the concrete. 

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