Calcium pozzolanic reaction

Reaction with silica and alumina. Hydrated lime reacts with pozzolans (materials containing reactive silica and alumina) in the presence of water to produce hydrated calcium silicates and aluminates. The reactions may take months to proceed to completion at ambient temperatures, as in mortars (section 26.6) and lime treated soil (section 26.3), but proceed within hours at elevated temperatures and water vapour pressures (e.g., in steam at 180 °C and a pressure of 10 bar — see sections 26.10,26.11 and 26.12). This pozzolanic reaction is the basis of the strength generated by hydraulic quicklimes (section 26.9). 

Precipitates impermeable of C-S-H and calcium aluminates (pozzolanic reaction)... 

However, Fierens [9] has shown, that the hydroxylation of the slag glass surface layer, as a result of water molecules chemisorption, which is enhanced by the surface electron defects (trapped electrons), can be considered as a topochemical process. The topochemical processes became important also at later hydration stage, when the pozzolanic reaction of slag glass network, impoverishes of the majority of alkaline elements, with calcium ions is occtrrring, and calcium ions are chemisorbed on the active sites of solid phase. 

It has been suggested that sulfobelitic blended cements could be produced by combining sulfobelitic clinkers with granulated blast furnace slag or fly ash, in addition to calcium sulfate (Sudoh et al, 1980 Beretka et al, 1992, 1993). Such an approach makes little sense, however, because calcium hydroxide is virtually absent among the hydrates formed, and the pH of the pore solution is relatively low. Under these conditions the added fly ash cannot undergo a pozzolanic reaction, and the hydraulic activation of the slag is insufficient. 

The main product of the pozzolanic reaction is an amorphous or nearly amorphous calciiun sihcate/aluminate hydrate phase similar to that formed in the hydration of calcium silicates constituting Portland cement. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

Under these conditions only a small fraction of the ash present participates in the hardening process, as the amount of calcium hydroxide liberated in the hydration of the clinker phases and needed for a pozzolanic reaction is very limited. The products of the pozzolanic reaction concentrate preferentially in the vicinity of the residual ash particles (Zhang, 1995). The ash present in the fresh concrete mix affects its rheology positively, and acts as a filler in the hardened concrete. 

As in the pozzolanic reaction free calcium hydroxide is consumed, and is replaced by phases of extremely low water solubility lime mortars combined with natural or artificial pozzolanas attain a high degree of durability and water resistance if allowed to be precured for a sufficiently long time. Thus, not surprisingly, many stractures built in ancient times—especially by the Romans—using these binders have been preserved until the present day, even when constracted to be used as aqueducts. 

The amount of free calcium hydroxide in Portland cement-microsilica mixes increases initially, as its formation in the hydration of tricalcium silicate is faster than its consumption in the pozzolanic reaction with microsilica. Later on, however, the amount of free calcium hydroxide may start to decline, when the amount of it consumed in the pozzolanic reaction exceeds the rate by which it is formed in the hydration of tiicalcium and dicalcium silicate (Papadakis, 1999). This crossover point— that is, the time at which the rate of Ca(OH)2 consumption exceeds the rate of its formation— will depend on the amount and reactivity of the microsilica present, as well as on the reactivity of the clinker, and can occur after several hours or a few days of hydration, or not at all (especially at low microsilica additions). The amount of residual free calcium hydroxide in mature paste will generally decline with increasing amounts of nucrosilica in the original mix. It will also decline with decreasing watei/solid ratio, as under these eonditions the C/S ratio of the formed C-S-H phase tends to increase. 

The texture of cement pastes in high-performance concretes is characterized by high density and very low porosity. Owing to the pozzolanic reaction between the added microsilica and calcium hydroxide, the amount of ciystalline portlandite in the hardened paste is reduced, and the formed C-S-H phase has a lower C/S ratio (Cheong et al, 1997). The mature cement paste contains a significant fraction of non-hydrated cement. 

Pozzolanicity—capability of a material to enter, with water and calcium hydroxide, into a pozzolanic reaction. 

Pozzolanic materials such as natural poz-zolans (volcanic origin), fly ash (product of combustion of carbon in thermoelectric power stations) or silica fume (very fine powder obtained as waste in the metallurgy of silicon or iron-silicon alloys) do not contain calcium oxide and thus cannot react with water. Instead, these pozzolanic materials react with the free lime (produced by the OPC clinker) according to the pozzolanic reaction... 

Hydration of fly ash cement differs from pure cement in terms of the hydration rates of the clinker phases, amount of calcium hydroxide formed, composition of the clinker hydration products, and additional hydration products from the reaction of the fly ash.I l Lower amounts of lime are formed in the presence of fly ash because ofthe pozzolanic reaction between the fly ash and lime formed in cement hydration. Fly ash generally retards the reaction of alite in the early stages and accelerates the middle stage reaction. The accelerated reaction is attributed to the existence of nucleation sites on fly ash particles. The aluminate and ferrite phases hydrate more rapidly in the presence of fly ash, and also there is a significant difference in the hydration rate of the belite phase up to 28 days. Table 1 gives the relative hydration rates of cement compounds in the presence of fly ash as derived from conduction calorimetry. [" 1 It can be seen that the earlier rates of hydration are generally retarded, and the later stage hydration rates are accelerated. 

The addition of fly ash to cement results in the formation of decreased amounts of calcium hydroxide in the hydration product. This is attributable to the dilution effect and to the consumption of calciiun hydroxide by the pozzolanic reaction with the fly ash. In Fig. 1, the amount of calcium hydroxide formed at different times of hydration in cement containing fly ash is given. The amount of Ca(OH)2 estimated by TG was found to be lower in samples containing fly ash. With the increase in the amount of fly ash, less calcium hydroxide was formed because of the pozzolanic reaction and dilution effect. Even at 60% fly ash, some lime was present in the mortar, and the pH was found to be 13.5. At this pH value, the passivity of steel is assured. It can also be observed that there is more lime at 60% fly ash than at 75% slag addition. 

Calcium phosphate-based systems have wide applications in biomedical areas. Brown has outlined the similarities between the hydration of calcium silicates and calcium phosphates. The hydration products in both systems have high surface areas, variable composition, and poor crystallinity. Pozzolanic reactions and Hadley-like grains form in both systems. The primary cement-water reactions for C3S and tetracalcium phosphate are as follows ... 

In the pozzolanic reaction analog, tetracalcium phosphate is the source of Ca(OH)2. A variety of acidic calcium phosphates can be considered as the Si02 analog. The analogy is extended by the following reaction ... 

The byproduct removed from a lime spray dryei/particulate control system is a dry, flow-able powder containing calcium sulfite, calcium sulfate, fly ash, and unreacted absoibent. It is usually conveyed pneumatically to a silo Ah storage priw to disposal and is typically disposed of in a landfill. Water is often added for dust control This causes pozzolanic reactions to occur resulting in a final byproduct of low penneabili and desirable landfill characteristics (Liegois, 1983). Table 7-17 gives important properties of spray dryer byproduct. 

Two test landfills containing byproduct from Edgewater have been constructed and instrumented. No problems were encountered with rapid set-up of the wetted byproducts or with dusting of unwetted byproducts. A pteliminaiy chemical characterization of the byproduct indicates that the material is similar to other calcium-injecdon byproducts with unreacted lime initiating pozzolanic reactions in the wetted byproduct cementing it into a coherent mass. Solidification decreased the permeability of the byproduct markedly (Holcombe et al., 1990). 

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

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