Calcium silicate hydrates, formed

Table 6. Calcium Silicate Hydrates Formed at Elevated Temperatures ...

The calcium silicate hydrate formed in both cases appears to be a very poorly crystallized version of the natural mineral tobermorite. The use of the name tobermorite for the calcium silicate hydrate obtained in the hydration of Ca8Si05 and Ca2Si04 is not fully justified nevertheless, because in all publications from this laboratory that name was used, the usage is retained in this paper. 

The calcium silicate hydrate formed on paste hydration of C3S or P-CjS is a particular variety of C-S-H, which is a generic name for any amorphous... 

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]. 

Figure 36. Differential thermograms of calcium silicate hydrates formed under autoclaving conditions.

AfwiUite can also be formed, and appears to be the thermodynamically stable calcium silicate hydrate in pure systems, at room temperature. 

When anhydrous cement mix is added to water, the silicates react, forming hydrates and calcium hydroxide. Hardened Portland cement contains about 70% cross-linked calcium silicate hydrate and 20% crystalline calcium hydroxide. 

The C3A phase is subjected to competitive reaction between water and gypsum to form a small amount of the initial C3A hydrates covered by a protective coating of ettringite, and also a proportion of the C3S hydrates to form a calcium silicate hydrate of low calcium content in the region of 100 nm thick. 

During this phase the initial self-retarding calcium silicate hydrate layer appears to break down, allowing further hydration products from the C3S and C2S phases to be formed. These appear to be produced in three... 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

Sasaki K., Masuda T., IshidaH., MitsudaT. Synthesis of calcium silicate hydrate with Ca/Si=2 by mechanochemical treatment. J. Am. Ceram. Soc. 1996 80 472-76. KosovaN.V., Avvakumov E.G., Malakhov V.V. et al. About nature ofphases formed in soft mechanochemical synthesis of calcium titanate. Doklady Akad. Nauk, 1997 356 350-53. 

The cement-based process consists of the mixing of the waste with cement and, possibility, other additives. The liquid waste or fluid sludge is used as the mix water for hydration of the cement. Cementation of the mixture occurs when the waste is added. First, a calcium-silicate-hydrate gel forms followed by the hardening of the material as thin, densely-packed, silicate fibrils grow and interface. Heavy metals in the waste are converted to insoluble hydroxides in the alkaline environment of the cement paste and trapped within the pores of the cement paste matrix. It is also possible that some of the metals may be physically... 

The hydration of tricalcium silicate C3S and dicalcium silicate C2S (for abbreviations see below Table 5.3-6) are responsible for the further. solidification of Portland cement. This reaction only begins in earnest after ca. 4 hours. Initially long needles of calcium silicate hydrate are formed, which bond the cement particles together. Later, smaller needles of calcium silicate hydrate fill the gaps left. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate. 

Portland cement is typically composed of about 25% P-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)... 

Figure 19. Cement hydration process. Calcium silicate hydrates to form C-S-H, a quasi-amorphous gel of composition close to C3S2H3. The excessive rate of hydration of the aluminate phase is controlled by gypsum through the formation of a calcium trisidfoaluminate hydrate, ettringite. (Reproduced with permission from reference 5. Copyright 1991 Elsevier.)...

Experimental studies of the processes taking place with mixed residues from lignite coal incineration indicate favorable effects of incorporation of both chloride and heavy metals in newly formed minerals. Ettringite in particular, can act as a storage mineral for chloride and metal ions. The former may be incorporated at up to 4 kg CaCl2 per m of the mineral mixture. Calcium-silicate-hydrate phases may be formed in a subsequent process, and by filling further pore space these minerals can... 

This chemical equation is not balanced because, in practice, x varies between the approximate limits of 1.8-2.2, y varies around a mean value of 1.0 and aq means that water is also combined in the material in an indeterminate amount. The idealised composition of the calcium silicate hydrate phase is Ca2Si04 aq. The reaction is rapid and continues for up to approximately 20 days. Considerable heat is evolved - in the order of 500 J per gram of powder -and care must be taken to remove this heat when forming large masses of concrete into structures. The reaction gives a high-strength product. 

Alite reacts with water to form calcium silicate hydrate and calcium hydroxide, which is also known as portlan-dite. The hardened paste has high strength when the reaction is completed, and because alite is the most abundant compound in cement, it also makes the dominant contribution to the mechanical properties of the final product. The hydration reaction proceeds at an appreciable rate a few hours after the addition of water and lasts up to about 20 days. The reaction of alite with water is accelerated by aluminate and gypsum. 

Setting of Biodentine is partly caused by condensation polymerization of the silicate phase to form a chain structure based on SiO tetrahedra joined at two comers (Q2 structures). This is similar to the setting of Portland cement [61], though it occurs faster due to the accelerating effect of calcium chloride. Setting is accompanied first by a contraction in volume, as the initial solidification occurs. This is followed by expansion as secondary hydration reactions take place to form the various calcium silicate hydrates [83]. 

Set Biodentine material consists substantially of hydrated calcium silicate [79], some of which forms more rapidly in Biodentine than in MTA because of the presence of calcium chloride and calcium carbonate. This latter component, which accounts for 15% of the powder by mass, acts as nucleation sites for the deposition of calcium silicate hydrate. Consequently, the precipitation of calcium silicate hydrate occurs more rapidly within this material, which leads to a shorter setting time [79]. 

The setting reactions of the varions tricalcium silicate cements are very similar, as described in Chapter 8. They also resemble the setting of Portland cement. The initial setting involves the hydration of the alite (Ca SiO ) and belite (p-Ca SiO ) phases to form a poorly crystalline gel phase consisting of calcinm hydroxide in calcium silicate hydrate (approximate formula CajSi O ) [102], After the initial hardening, further condensation reactions occur which improve the strength and give rise to short silicate chains within the structure [103],... 

Carbon fiber reinforced concrete can be utilized to achieve three times higher tensile forces compared to steel reinforced concrete. The tensile stress at break is even six times higher. Moreover the fibers are as far as possible insensitive against corrosion. This leads to a higher durability. Mader et al. for example proclaims that a shell like structure out of calcium-silicate-hydrate-phases and calciumhydroxid-phases are formed around the carbon fiber. Hence, the interface and the fiber itself are damaged. This leads to a brittleness and a prematurely failure. 

In Fig. 4.53 the stability ranges of some of calcium silicate hydrates as a function of temperature and pressure are presented [156]. Because of experimerrtal difficulties the Ca0-Si02-H20 system has not been elaborated and the Uqttid phase remaining in equilibrium with the solid phases is not knowa However, the research was undertaken to learn the stable, equilibrium phases, formed at variable C/S molar ratio, as a function of temperature and pressure these problems was studied by Roy [162]. She investigated also the cirrves of saturated water vapom of many calcium silicate hydrates [163]. 

There are also the other reactive aggregates, namely gneiss and mica containing shales [41], In the interfacial transition zone, in the vicinity of aggregate surface— kaolinite and hydromicas, while from cement paste side—gel of sodium-calcium silicate hydrate, respectively are formed. However, in the case of serpentine concrete deterioration is due to the formation of brucite [75]. The clay minerals, such as chlorites, vermiculite, as well as micas and feldspars, are also included to reactive aggregate components. 

The well crystallized, autoclaved calcium silicate hydrates show low caibon-ation rate in spite of the higher lime content, than the amorphous forms of tober-morite, poorer in calcium [298]. However, Roy [80] found that the C-S-H richer in calcium than tobermorite, was readily carbonated and the eqnilibrinm CO2 partial pressure was lower. 

The two most important constituents of Portland cement are alite, a form of tricalcium silicate, and belite, a form of dicalcium silicate. In their hydration both calcium silicates yield—in addition to calcium hydroxide—a nearly amorphous calcium silicate hydrate phase (C-S-H phase), and this hydration product is mainly responsible for the strength and other physico-mechanical properties of the hardened cement paste. 

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