Pozzolanic reaction
Encyclopedia
The Pozzolanic reaction is the chemical reaction
that occurs in hydraulic cement, a mixture of slaked lime (calcium hydroxide) with amorphous siliceous materials (namely, pozzolan
or pozzolana
, a finely divided volcanic ash, rich in obsidian
, a mineral glass commonly found in lava
), forming non-water-soluble calcium silicate hydrate
s. It is the main reaction involved in the Roman concrete
invented in Ancient Rome
and used to build, for example, the Pantheon
.
At the basis of the Pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide
, also known as Portlandite
, or (Ca(OH)2), and silicic acid
(H4SiO4, or Si(OH)4). Simply, this reaction can be schematically represented as follows:
or summarized in abbreviated notation of cement chemists:
The product of general formula (CaH2SiO4 · 2 H2O ) formed is a calcium silicate hydrate
, also abbreviated as CSH
in cement chemist notation
. The ratio Ca/Si, or C/S, and the number of water molecules can vary and the hereabove mentioned stoichiometry
may differ.
As the density
of CSH is lower than that of portlandite and pure silica, a consequence of this reaction is a swelling of the reaction products. This reaction may also occur with time in concrete
between alkaline cement porewater and poorly-crystalline silica aggregate
s. This delayed process is also known as alkali silica reaction
, or alkali-aggregate reaction
, and may seriously damage concrete structures because the resulting volumetric expansion is also responsible for spalling and decrease of the concrete strength.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
that occurs in hydraulic cement, a mixture of slaked lime (calcium hydroxide) with amorphous siliceous materials (namely, pozzolan
Pozzolan
A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...
or pozzolana
Pozzolana
Pozzolana, also known as pozzolanic ash , is a fine, sandy volcanic ash. Pozzolanic ash was first discovered and dug in Italy, at Pozzuoli. It was later discovered at a number of other sites as well...
, a finely divided volcanic ash, rich in obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
, a mineral glass commonly found in lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
), forming non-water-soluble calcium silicate hydrate
Calcium silicate hydrate
Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
s. It is the main reaction involved in the Roman concrete
Roman concrete
Roman concrete was a material used in construction during the late Roman Republic through the whole history of the Roman Empire. Roman concrete was based on a hydraulic-setting cement with many material qualities similar to modern Portland cement...
invented in Ancient Rome
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....
and used to build, for example, the Pantheon
Pantheon, Rome
The Pantheon ,Rarely Pantheum. This appears in Pliny's Natural History in describing this edifice: Agrippae Pantheum decoravit Diogenes Atheniensis; in columnis templi eius Caryatides probantur inter pauca operum, sicut in fastigio posita signa, sed propter altitudinem loci minus celebrata.from ,...
.
At the basis of the Pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
, also known as Portlandite
Portlandite
Portlandite is a rare oxide mineral, the naturally occurring form of calcium hydroxide . It is the calcium analogue of brucite .-Occurrence:...
, or (Ca(OH)2), and silicic acid
Silicic acid
Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...
(H4SiO4, or Si(OH)4). Simply, this reaction can be schematically represented as follows:
- Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42- + 2 H2O → CaH2SiO4 · 2 H2O
or summarized in abbreviated notation of cement chemists:
- CH + SH → CSH
The product of general formula (CaH2SiO4 · 2 H2O ) formed is a calcium silicate hydrate
Calcium silicate hydrate
Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
, also abbreviated as CSH
CSH
CSH is a three-letter acronym with multiple meanings:* Caesium hydride, a crystalline solid with the molecular formula CsH* Calcium silicate hydrate, or calcium silicate hydrogel, the main component of cement paste...
in cement chemist notation
Cement chemist notation
Cement chemist notation was developed to simplify the formulas cement chemists use on a daily basis. It is a "short hand" way of writing the chemical formula of oxides of calcium, silicon, and various metals.-Abbreviations of oxides:...
. The ratio Ca/Si, or C/S, and the number of water molecules can vary and the hereabove mentioned stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
may differ.
As the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of CSH is lower than that of portlandite and pure silica, a consequence of this reaction is a swelling of the reaction products. This reaction may also occur with time in concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
between alkaline cement porewater and poorly-crystalline silica aggregate
Construction Aggregate
Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
s. This delayed process is also known as alkali silica reaction
Alkali Silica Reaction
The alkali–silica reaction is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline silica, which is found in many common aggregates....
, or alkali-aggregate reaction
Alkali-aggregate reaction
Alkali–aggregate reaction is a term mainly referring to a reaction which occurs over time in concrete between the highly alkaline cement paste and non-crystalline silicon dioxide, which is found in many common aggregates...
, and may seriously damage concrete structures because the resulting volumetric expansion is also responsible for spalling and decrease of the concrete strength.
See also
- Aerated autoclaved concreteAerated autoclaved concreteAutoclaved aerated concrete , also known as autoclaved cellular concrete or autoclaved lightweight concrete , was invented in the mid-1920s by the Swedish architect and inventor Johan Axel Eriksson. It is a lightweight, precast building material that simultaneously provides structure, insulation,...
- Alkali Silica ReactionAlkali Silica ReactionThe alkali–silica reaction is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline silica, which is found in many common aggregates....
- Alkali-aggregate reactionAlkali-aggregate reactionAlkali–aggregate reaction is a term mainly referring to a reaction which occurs over time in concrete between the highly alkaline cement paste and non-crystalline silicon dioxide, which is found in many common aggregates...
- Alkali-carbonate reactionAlkali-carbonate reactionThe alkali–carbonate reaction is a process suspected for the degradation of concrete containing dolomite aggregate.Alkali from the cement might react with the dolomite crystals present in the aggregate inducing the production of brucite, 2, and calcite...
- Calcium silicate hydrateCalcium silicate hydrateCalcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
(CSH) - CementCementIn the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
- Cement chemist notationCement chemist notationCement chemist notation was developed to simplify the formulas cement chemists use on a daily basis. It is a "short hand" way of writing the chemical formula of oxides of calcium, silicon, and various metals.-Abbreviations of oxides:...
- CenosphereCenosphereA cenosphere is a lightweight, inert, hollow sphere filled with inert air or gas, typically produced as a byproduct of coal combustion at thermal power plants. The color of cenospheres varies from gray to almost white and their density is about 0.4–0.8 g/cm³, which gives them a great buoyancy...
s - ConcreteConcreteConcrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
- Fly ashFly ashFly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
- Portland cementPortland cementPortland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
- PozzolanPozzolanA pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...
- PozzolanaPozzolanaPozzolana, also known as pozzolanic ash , is a fine, sandy volcanic ash. Pozzolanic ash was first discovered and dug in Italy, at Pozzuoli. It was later discovered at a number of other sites as well...
- Rice husk ash
- Roman concreteRoman concreteRoman concrete was a material used in construction during the late Roman Republic through the whole history of the Roman Empire. Roman concrete was based on a hydraulic-setting cement with many material qualities similar to modern Portland cement...
- Silica fumeSilica fumeSilica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...
- Sodium silicateSodium silicateSodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...