Lime-pozzolan reaction

It should be noted that lime reacts with clay particles. This leads to strength increase by pozzolanic and carbonation cementation processes. Cation exchange and pozzolanic reactions result in strength increase. The level of reactivity and hence strength gained in soil-lime mixtures depends on the level of pozzolanic product created. The chemical reaction between soil and lime can be presented as below ... 

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). 

The reaction between pozzolanic materials, lime and water is known as the pozzolanic reaction ... 

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. 

It is uncertain where it was first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture (see also Pozzolanic reaction), but concrete made Irom such mixtures was first used by the Ancient Macedonians and three centuries later on a large scale by Roman engineers. They used both natural pozzolans (trass or pumice) and artificial pozzolans (groimd brick or pottery) in these concretes. Many excellent examples of structures made from these concretes are still standing, notably the huge... 

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... 

During hydration, pozzolanic reactions occur between free lime CaO and silicoalutnina components. 

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. 

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). 

Lime/fly ash pozzolanic processes combine the properties of lime and fly ash to produce low-strength cementation. Kiln dust processes involve the addition of kiln dust to eliminate free liquids and usually form a low-strength solid. Lime-based processes for solidification use reactions of lime with water and pozzolanic (siliceous) materials, such as fly ash or dust from cement kilns, to form concrete, called a pozzolanic concrete. Wastes of desulfurization of gases and other inorganic wastes can be immobilized by this method. 

This describes the specific curing behavior of fly ash, cement dusts, and certain steel works byproducts, is based on the reaction of silicate and aluminous materials with quick lime. Here too, as with the above-mentioned additives, a higher pH causes the precipitation of metal hydroxides and carbonates. The British SEALOSAFE-Process uses fly-ash plus Portland cement, or alkali silicate glass and Fe/Al hydroxides to solidify a broad spectrum of wastes. In the POZ-O-TEC-Process, the wastes from flue gas scrubbers are solidified together with grate ash and fly-ash. The pozzolanic processes have the advantage of excellent longterm stability however, the products solidify rather slowly and are susceptible to acids. 

Both natural and artificial pozzolans can be used to make cements by reaction with lime in the presence of water. 

Thermo-analytical techniques are applied widely to monitor the reactions occurring in the raw clay materials and also to follow the pozzolanic effects of metakaolinite when combined with lime or cement. 

Thermal analytical techniques have been applied to investigate the causes leading to the deterioration of concrete subjected to various environmental factors. A mortar nearly two thousand years old obtained from El Tajan near Mexico City was analyzed by TG and electron microscopy. TG analysis was done on samples taken from different areas and depths. The loss of water below 100°Cwas caused by the adsorbed water from the volcanic tuff, while the endothermal effect at >700°C corresponded to the carbonated lime and carbonated silicates and aluminates derived from the pozzolan. The extent of the reaction of lime with pozzolan was computed to be 6.91%. 

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