The pozzolanic activity is a measure for the degree of reaction over time or the

reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per u ...

between a pozzolan and Ca²⁺ or

calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed with water. Annually, approxim ...

(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 Specific surface area (SSA) is a property of solids defined as the total surface area (SA) of a material per unit mass, (with units of m2/kg or m2/g). Alternatively, it may be defined as SA per solid or bulk volume (units of m2/m3 or m−1). I ...

, 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.

Reaction

The pozzolanic reaction is the

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

that occurs in

portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...

upon the addition of pozzolans. It is the main reaction involved in the Roman concrete invented in

Ancient Rome In modern historiography, ancient Rome is the Roman people, Roman civilisation from the founding of Rome, founding of the Italian city of Rome in the 8th century BC to the Fall of the Western Roman Empire, collapse of the Western Roman Em ...

and used to build, for example, the Pantheon. The pozzolanic reaction converts a silica-rich precursor with no cementing properties, to a calcium silicate, with good cementing properties. In chemical terms, the pozzolanic reaction occurs between

, also known as portlandite (Ca(OH)₂), and

silicic acid In chemistry, a silicic acid () is any chemical compound containing the element silicon attached to oxide () and hydroxyl () groups, with the general formula or, equivalently, . Orthosilicic acid is a representative example. Silicic acids are ra ...

(written as H₄SiO₄, or Si(OH)₄, in the geochemical notation): :Ca(OH)₂ + H₄SiO₄ → CaH₂SiO₄·2 H₂O or summarized in abbreviated

cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main ...

: :CH + SH → C-S-H The pozzolanic reaction can also be written in an ancient industrial silicate notations as: : + → or even directly: : + → Both notations still coexist in the literature, depending on the research field considered. However, the more recent geochemical notation in which the Si atom is tetracoordinated by four

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

groups (, also commonly noted ) is more correct than the ancient industrial silicate notation for which

() was represented in the same way as

carbonic acid Carbonic acid is a chemical compound with the chemical formula . The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion ...

() whose geometrical configuration is trigonal planar. When only considering mass balance, they are equivalent and both are used. The product CaH₂SiO₄·2 H₂O is a

calcium silicate hydrate Calcium silicate hydrates (CSH or C-S-H) are the main products of the hydration of Portland cement and are primarily responsible for the strength of cement-based materials. They are the main binding phase (the "glue") in most concrete. Only well de ...

, also abbreviated as C-S-H in

, the hyphenation denotes the variable stoichiometry. The atomic (or molar) ratio Ca/Si, CaO/SiO₂, or C/S, and the number of water molecules can vary and the above-mentioned

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

may differ. Many pozzolans may also contain

aluminate In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminium atom. Aluminate oxyanions ...

, or Al(OH)₄⁻, that will react with

and water to form calcium aluminate hydrates such as C₄AH₁₃, C₃AH₆ or hydrogarnet, or in combination with

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

C₂ASH₈ or strätlingite (

). In the presence of anionic groups such as sulfate, carbonate or chloride, AFm phases and AFt or ettringite phases can form. Pozzolanic reaction is a long term reaction, which involves dissolved silicic acid, water and CaO or Ca(OH)₂ or other pozzolans to form a strong cementation matrix. This process is often irreversible. Sufficient amount of free calcium ion and a high pH of 12 and above is needed to initiate and maintain the pozzolanic reaction. This is because at a pH of around 12, the solubility of silicon and aluminium ions is high enough to support the pozzolanic reaction.

Activity determining parameters

Particle properties

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 Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, can become reactive once ground below a certain critical particle diameter.

Composition

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 Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a meta ...

s are more reactive than

sands or detrital clay minerals. In this respect, the thermodynamic driving force behind 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.

Reaction conditions

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 tests

Mechanical evaluation of the pozzolanic activity is based upon a comparison of the

compressive strength In mechanics, compressive strength (or compression strength) is the capacity of a material or Structural system, structure to withstand Structural load, loads tending to reduce size (Compression (physics), compression). It is opposed to ''tensil ...

of mortar bars containing pozzolans as a partial replacement for

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...

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 In mechanics, compressive strength (or compression strength) is the capacity of a material or Structural system, structure to withstand Structural load, loads tending to reduce size (Compression (physics), compression). It is opposed to ''tensil ...

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 A technical standard is an established norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteristics for products or related processes and production methods, ...

or norms include variations of this methodology.

Chemical tests

A pozzolanic material is by definition capable of binding

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

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 Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used – these are distinguished from one another by the property which is measured: * D ...

or X-ray powder diffraction techniques are commonly used to determine remaining

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 A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

image analysis Image analysis or imagery analysis is the extraction of meaningful information from images; mainly from digital images by means of digital image processing techniques. Image analysis tasks can be as simple as reading barcode, bar coded tags or a ...

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

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 Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

, chemical shrinkage of the pastes or the heat evolution by heat flow calorimetry reside in the latter category.