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The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or Ca(OH)2 in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition and the active phase content.
Physical surface adsorption is not considered as being part of the pozzolanic activity, because no irreversible molecular bonds are formed in the process.
Activity determining parameters
Prolonged grinding results in an increased pozzolanic activity by creating a larger specific surface area available for reaction. Moreover, grinding also creates crystallographic defects at and below the particle surface. The dissolution rate of the strained or partially disconnected silicate moieties is strongly enhanced. Even materials which are commonly not regarded to behave as a pozzolan, such as quartz, can become reactive once ground below a certain critical particle diameter.
The overall chemical composition of a pozzolan is considered as one of the parameters governing long-term performance (e.g. compressive strength) of the blended cement binder, ASTM C618 prescribes that a pozzolan should contain SiO2 + Al2O3 + Fe2O3 ≥ 70 wt.%. In case of a (quasi) one phase material such as blast-furnace slags the overall chemical composition can be considered as meaningful parameter, for multi-phase materials only a correlation between the pozzolanic activity and the chemistry of the active phases can be sought.
Many pozzolans consist of a heterogeneous mixture of phases of different pozzolanic activity. Obviously, the content in reactive phases is an important property determining the overall reactivity. In general, the pozzolanic activity of phases thermodynamically stable at ambient conditions is low when compared to on an equal specific surface basis to less thermodynamically stable phase assemblages. Volcanic ash deposits containing large amounts of volcanic glass or zeolites are more reactive than quartz sands or detrital clay minerals. In this respect, the thermodynamic driving force behnind the pozzolanic reaction serves as a rough indicator of the potential reactivity of a (alumino-)silicate material. Similarly, materials showing structural disorder such as glasses show higher pozzolanic activities than crystalline ordered compounds.
The rate of the pozzolanic reaction can also be controlled by external factors such as the mix proportions, the amount of water or space available for the formation and growth of hydration products and the temperature of reaction. Therefore, typical blended cement mix design properties such as the replacement ratio of pozzolan for Portland cement, the water to binder ratio and the curing conditions strongly affect the reactivity of the added pozzolan.
Pozzolanic activity tests
Mechanical evaluation of the pozzolanic activity is based upon a comparison of the compressive strength of mortar bars containing pozzolans as a partial replacement for Portland cement to reference mortar bars containing only Portland cement as binder. The mortar bars are prepared, cast, cured and tested following a detailed set of prescriptions. Compressive strength testing is carried out at fixed moments, typically 3, 7, and 28 days after mortar preparation. A material is considered pozzolanically active when it contributes to the compressive strength, taking into account the effect of dilution. Most national and international technical standards or norms include variations of this methodology.
A pozzolanic material is by definition capable of binding calcium hydroxide in the presence of water. Therefore, the chemical measurement of this pozzolanic activity represents a way of evaluating pozzolanic materials. This can be done by directly measuring the amount of calcium hydroxide a pozzolan consumes over time. At high water to binder ratio (suspended solutions), this can be measured by titrimety or by spectroscopic techniques. At lower water to binder ratios (pastes), thermal analysis or X-ray powder diffraction techniques are commonly used to determine remaining calcium hydroxide contents. Other direct methods have been developed that aim to directly measure the degree of reaction of the pozzolan itself. Here, selective dissolutions, X-ray powder diffraction or scanning electron microscopy image analysis methods have been used.
Indirect methods comprise on the one hand methods that investigate which material properties are responsible for the pozzolan’s reactivity with portlandite. Material properties of interest are the (re)active silica and alumina content, the specific surface area and/or the reactive mineral and amorphous phases of the pozzolanic material. Other methods indirectly determine the extent of the pozzolanic activity by measuring an indicative physical property of the reacting system. Measurements of the electrical conductivity, chemical shrinkage of the pastes or the heat evolution by heat flow calorimetry reside in the latter category.
- Pozzolanic reaction
- Energetically modified cement (EMC)
- Fly ash
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