The term "energy modified cement" (EMC) was first used in 1992 by Vladimir Ronin, introduced in a paper by Ronin et al. dated 1993 and presented at a formal meeting of the Nordic Concrete Research academic group.[16]​ The process was refined by Ronin and others, including Lennart Elfgren (now professor emeritus of LTU).[17]​.

At the 45th World Exhibition of Invention, Research and Innovation, held in 1996 in Brussels, Belgium, the EMC received a gold medal with the Eureka mention "Eureka (intergovernmental initiative)") from the European intergovernmental organization for research and development, for energy modification of cements.[18]​.

The term "energetically modified" has been used elsewhere, although such use does not indicate that the method used was activation as defined in this article.[19]​.

The reviewed properties of the product include:[4]​[20]​[21]​[22]​.

Unlike Portland cement, the production of EMC does not generate carbon dioxide at all. This makes EMCs "low carbon cements".[7]​.

The first cited claims about EMC's CO reduction capabilities were made in 1999, when global Portland cement production was 1.6 billion tonnes per year.[20]​[23]​ From 2011 to 2019, global Portland cement production increased from 3.6 to 4.1 billion tonnes per year.[24][note 1]​ The potential for energy-modified cement to contribute to a global CO reduction CO has been recognized externally since 2002 and continues.[5]​[6][8]​ Recent recognition has included the Energy Transition Commission report (drafted under Adair Turner and Nicholas Stern) Possible sector focus mission: cement (2019).[25]​ Recognition of the potential for "zero carbon" was set out by McKinsey & Co in their 2020 report "Establishing the bases for zero-carbon cement".[26]​ In 2023, the contribution that EMCs offer to achieving "low-carbon" materials was further recognized within the academic domain of mechanochemistry.[15]​.

No harmful emissions or toxic chemicals during production

EMC activation is a purely mechanical process. As such, it does not involve heating or burning or, indeed, any chemical treatment. This means that no fumes are produced during manufacturing.[20]​.

Usage history

EMCs have been produced for use in various projects since 1992.[4]​ In 2010, the volume of concrete poured containing EMC was approximately 4,500,000 cubic yards (3,440,496 m), primarily on US DOT projects.[4]​ To put this in context, this is a volume greater than that of the entire Hoover Dam construction (including its associated power plants and works). annexes), where a total of 4,360,000 cubic yards (3,333,459 m³) of concrete was poured, equivalent to a standard US highway from San Francisco to New York City.[27]​.

One of the first projects to use EMC from fly ash was the construction of a road bridge in Karungi), Sweden, in 1999, with the Swedish construction company Skanska. The Karungi road bridge has withstood Karungi's harsh winters and strong diurnal temperature swings throughout the year.[20]​.

In the United States, energy-modified cements have been approved for use by several state transportation agencies, including those of Pennsylvania, Texas, and California.[22]​.

In the United States, highway bridges and hundreds of miles of highway paving have been built with concretes made with EMC derived from fly ash.[4]​ These projects include sections of Interstate 10.[4]​ In these projects, the EMC replaced at least 50% of the Portland cement in the poured concrete.[28]​ This is approximately 2.5 times more than the typical amount of fly ash in projects where no cements are used.[28]​ This is approximately 2.5 times more than the typical amount of fly ash in projects where no cements are used. energy modified.[29]​ Independent test data showed that 28-day strength development requirements were exceeded in all projects.[28]​.

Another project was the expansion of the passenger terminals at the port of Houston, Texas, where the ability of energy modified cement to produce concrete with high resistance to the ionic permeability of chlorides and sulfates (i.e., greater resistance to seawater) was a determining factor.[4]​.

Customized design according to its final use

The behavior of mortars and concretes made from EMC can be custom designed. For example, EMC concretes can range from general applications (depending on strength and durability), to the production of rapid and ultra-rapid hardening cements for high-strength concretes (e.g., more than 70 MPa/10,150 psi in 24 hours and more than 200 MPa/29,000 psi in 28 days).[21]​ This allows energy-modified cements to be used in concretes of high performance.[21]​.

Durability of concrete and EMC mortars

Any cementitious material that undergoes EMC activation is likely to have greater durability, including Portland cement treated with EMC activation.[21]​ Regarding pozzolanic EMCs, concretes made with pozzolanic EMC are more durable than concretes made with Portland cement.[31]​.

Treatment of Portland cement with EMC activation produces high-performance concretes, including high strength, high durability, and faster strength development than concretes produced with untreated Portland cement.[21]​ Treatment of Portland cement with the EMC activation process can increase its strength development by almost 50%, and also significantly improve its durability, measured according to generally accepted methods.[21]​[30]​.

Greater resistance to salt water attack

Concrete made from ordinary Portland cement without additives has a relatively low resistance to brackish water.[30]​ In contrast, EMCs show a high resistance to ionic attack by chlorides and sulfates, together with a low alkali-silica reactivity").[28]​ For example, durability tests have been carried out according to the "Bache method" (see diagram). Concrete samples with respective compressive strengths of 180.3 and 128.4 MPa (26,150 and 18,622 psi) after 28 days of curing, were tested using the Bache method. The samples were made of (a) EMC (comprising Portland cement and silica fume, both subjected to EMC activation) and (b) Portland cement. The resulting mass loss was plotted for durability.

In other words, treating Portland cement with the EMC activation process can increase strength development by almost 50% and also significantly improve its durability, measured according to generally accepted methods.[21]​.

Low leachability of concrete with EMC

Luleå University of Technology conducted leaching tests in 2001 in Sweden, on behalf of a Swedish energy production company, on concrete made with an EMC made from fly ash. These tests confirmed that the concrete "showed low specific surface leachability" with respect to "all environmentally relevant metals".[32]​[33]​.

Self-healing properties of pozzolanic EMCs

Natural pozzolanic reactions can cause mortars and concretes containing these materials to "self-heal".[35]​[36][37]​ The EMC activation process can increase the likelihood of these pozzolanic reactions occurring.[38]​[39]​ The same trend has been observed and studied in the various supporting structures of the Hagia Sophia built in the time of the Byzantine Emperor Justinian I. (currently in Istanbul, Turkey).[40]​ There, as in the case of most Roman cements, mortars containing high amounts of pozzolanic ash were used, to obtain greater resistance to the stresses generated in buildings by earthquakes.[41]​.

EMCs made from pozzolanic materials exhibit "biomimetic" self-healing capabilities that can be photographed as they develop (see attached images).[34]​.

EMCs using California pozzolans

Concretes manufactured by replacing at least 50% of Portland cement with EMC have given consistent practical results in high volume applications.[28]​ This is also the case for EMC produced from natural pozzolans (such as volcanic ash).[42]​.

Southern California volcanic ash deposits were independently tested. By substituting 50% of Portland cement in the dosage, the resulting concretes exceeded the requirements of the corresponding ASTM standard.[43] At 28 days, its compressive strength was 4180 psi / 28.8 megapascals "Pascal (unit)") (N "Newton (unit)")/mm²). The 56-day strength exceeded the requirements for 4,500 psi (31.1 MPa) concrete, even taking into account the safety margin recommended by the American Concrete Institute.[44] The concrete made in this manner was workable and sufficiently strong, exceeding the standard of 75% pozzolanic activity at both 7 days and 28 days.[43] It also increased the surface smoothness of the pozzolans in the concrete.[43]​.

Effect on pozzolanic reactions

EMC activation is a process that increases the chemical affinity of a pozzolan.[38]​[39]​ This leads to greater and more rapid strength development of the resulting concrete, at higher replacement ratios than untreated pozzolans.[28]​[42]​ These transformed (highly reactive) pozzolans demonstrate additional benefits by utilizing known pozzolanic reaction forms that generally have as their ultimate target a range of hydrated products. A nuclear magnetic resonance study on EMC samples concluded that its activation caused "the formation of thin layers of SiO (Silicon(IV) oxide") around the C3S crystals")", which in turn "accelerates the pozzolanic reaction and promotes the growth of more extensive networks of hydrated substances".[45]

In simple terms, by using pozzolans in concrete, porous (reactive) portlandite can be transformed into hard, impermeable (relatively non-reactive) compounds, instead of the relatively reactive, porous and soft calcium carbonate generated with ordinary cement.[46] Many of the end products of pozzolanic chemistry exhibit a hardness greater than 7.0. "Self-healing" capabilities can also help improve durability in applications where tensile stresses must be sustained.

In greater detail, the benefits of pozzolanic concrete begin with the understanding that in conventional concretes, Portland cement combines with water to produce a stone-like material through a complex series of chemical reactions, the mechanisms of which are not yet fully understood. The process, called mineral hydration, forms two cementitious compounds in concrete: hydrated calcium silicate (C-S-H) and calcium hydroxide (Ca(OH)). This reaction can occur in three ways:[47]​.

The underlying hydration reaction forms two products:

Portlandite makes up about 25% of concrete made with Portland cement without pozzolanic cementitious materials.[46]​ In this type of concrete, carbon dioxide is slowly absorbed to convert portlandite into insoluble calcium carbonate (CaCO), in a process called carbonation:[46]​.

In mineral form, calcium carbonate can exhibit a wide range of hardness depending on how it is formed. In its softest form, it can form in concrete as chalk "Crete (rock)") (hardness 1.0 on the Mohs scale). Like portlandite, calcium carbonate in mineral form can also be porous, permeable and with little resistance to acid attack, causing it to release carbon dioxide.

However, pozzolanic concretes, including EMCs, continue to consume the soft, porous portlandite as the hydration process continues, converting it to additional hardened concrete as calcium silicate hydrate (C-S-H) instead of calcium carbonate.[46]​ This results in a denser, less permeable and more durable concrete.[46]​ This is an acid-base reaction between portlandite. and silicic acid (HSiO) which can be represented as follows:[51]​.

Additionally, many pozzolans contain aluminate (Al(OH)) which will react with Portlandite and water to form:.

The chemistry of pozzolanic cement (along with the chemistry of high aluminate cement) is complex and is not necessarily restricted to the above pathways. For example, strätlingite can be formed in several ways, including the following equation that can increase the strength of a concrete:[54]​[55]​.

The role of pozzolans in concrete chemistry is not fully understood. For example, strätlingite is metastable, which in an environment of high temperature and water content (which can be generated during the early stages of concrete curing) can itself produce stable calcium-aluminum garnet particles (see the first point above).[58] This process can be represented by the following equation:

According to the first point, although the inclusion of calcium and aluminum garnet in itself is not problematic, if instead it is produced by the previous route, then microcracks and loss of strength in the concrete may occur.[60]​ However, the addition of highly reactive pozzolans to the concrete mixture prevents such a conversion reaction.[61]​.

In summary, while pozzolans provide several chemical possibilities for forming hardened materials, "high reactivity" pozzolans such as blast furnace slag") (GGBFS) can also stabilize some of these processes. In this context, EMCs manufactured from fly ash have been shown to produce concretes that meet the same characteristics as concretes containing "slag 120" (i.e. GGBFS) according to the American ASTM standard. C989.[28][62]​.

Portlandite, when exposed to low temperatures, humid conditions and condensation, can react with sulfate ions, producing efflorescence. On the contrary, pozzolanic chemistry reduces the amount of portlandite available to reduce the generation of efflorescence.[63]​.

Contenido

El propósito de la activación de EMC es fundamentalmente causar la destrucción de la estructura cristalina del material procesado, para convertirlo en un material amorfo.[38]​ Aunque este cambio aumenta la reactividad química del material procesado, no se produce ninguna reacción química "durante" el proceso de activación de EMC.

En su forma más simple, la mecanoquímica puede definirse como "un campo que estudia las reacciones químicas iniciadas o aceleradas por la absorción directa de energía mecánica".[15]​ Más técnicamente, se puede definir como una rama de la química que se ocupa de la "transformación química y fisicoquímica de sustancias en todos los estados de agregación producida por el efecto de la energía mecánica".[65]​ La IUPAC no contiene una definición estándar del término "mecanoquímica", sino que define una ""reacción" mecanoquímica" como una reacción química "inducida por la absorción directa de energía mecánica", al tiempo que señala "corte, estiramiento y molienda son métodos típicos para la generación mecano-química de propiedades reactivas".[66]​[67]​.

Más estrictamente, activación mecánica fue un término definido por primera vez en 1942 como un proceso que implica un aumento en la capacidad de reacción de una sustancia que permanece químicamente sin cambios.[68]​ Aún más estrictamente, la activación de EMC es una forma especializada de activación mecánica limitada a la aplicación de molienda de bolas de alta energía (HEBM) a materiales cementosos. De manera más restringida, la activación de EMC implica el uso de pulverización vibratoria, y usando exclusivamente sus propios medios de molienda.[38]​ Como se indica en un libro de texto académico de 2023 limitado a la mecanoquímica, la activación de EMC ha "demostrado de manera impresionante" sus efectos al provocar un cambio en la reactividad del material de cemento alternativo y las características físicas resultantes del hormigón.[15]​.

Thermodynamic justification

More specifically, the HEBM process can be described as increasing the chemical reactivity of a material by increasing its chemical "potential" energy. In EMC activation, the transferred mechanical energy is stored in the material as defects caused by the destruction of the crystalline structure of the material. Therefore, the process transforms the state of solid substances into other thermodynamically and structurally more "unstable" states, which allows an explanation of this greater reactivity as an increase in their Gibbs energy:[69]​.

In its simplest form, the HEBM process causes the destruction of crystalline bonds to increase the reactivity of a material.[70]​ From the thermodynamic perspective, any subsequent chemical reaction can decrease the level of excess energy in the activated material (i.e., as a reactant) to produce new components comprising a lower chemical energy and a more stable physical structure. On the contrary, to convert the preprocessed material into a more reactive physical state, the disorder generated during the HEBM process can be justified as equivalent to a “decrystallization” (and therefore an increase in entropy) that in part produces an increase in volume (due to the decrease in apparent density). A reverse process, sometimes called "relaxation", can be almost immediate (10 to 10 seconds) or take much longer (e.g. 10 seconds).[71] Ultimately, any general "retained" thermodynamic effect can be justified on the basis that any such "reverse" process is incapable of reaching an ideal thermodynamic end state on its own. As a result, in the course of mechanical activation of minerals, reverse "relaxation" processes cannot completely decrease the Gibbs free energy that has been created. Therefore, energy remains in the material, which is stored in the "defects" created in the crystal lattice.[72]​[73]​.

In general, the HEBM process generates a net thermodynamic effect:[74]​[75]​[76]​.

Where disorder in crystals is low, it is very small (if not negligible). On the contrary, in highly deformed and disordered crystals, the values ​​of can have a significant impact on the Gibbs free energy generated.

Leaving aside the heat generated during the process due to effects caused during the activation process such as friction, the excess Gibbs free energy retained in the activated material can be justified due to two changes, namely, an increase in the specific surface area (), and of the defects of the crystal structure ().[77]​[76]​ In successful HEBM processes such as EMC activation:[78]​[79]​.

The relatively low value of () compared to the high value of () serves to further distinguish the HEBM process from general grinding or "milling" (where, instead, the only objective is to increase the surface area of the processed materials), allowing an explanation of the change in the entropy of the pulverized material in the form of elastic energy (stored in defects of the crystal lattice that can take years to "relax"), which is the "source of excess energy and Gibbs enthalpy".[77]​ Regarding enthalpy, four descriptors can be outlined to give an overview of the total change during said activation process:[78]​[80][81]​.

Because most of the energy required during the EMC activation process is absorbed by (), it is trivial. Therefore, the main functions for the enthalpy change are approximated by:.

In EMC activation, the above terms and are considered particularly noteworthy due to the nature of the observed changes in physical structure.[38]​ Therefore, the change in enthalpy caused during EMC activation can be approximated by:[80][81]​.

From the above thermodynamic construction, EMC activation results in a highly "amorphous" phase that can be justified as a large increase in and also a large increase in .[38]​[80]​[81]​ The benefits of EMC activation are large in , meaning that its reactivity is less dependent on temperature. In terms of the thermodynamic momentum of any reaction, the overall enthalpy of a reactant is not dependent on temperature, meaning that a material that has been subjected to HEBM with a corresponding elevation of can react at a lower temperature (since the "activated" reactant is less dependent on temperature for its progression). Furthermore, an EMC reaction can exhibit physical mechanisms at extremely small scales "with the formation of thin layers of SiO (Silicon(IV) oxide")", with the suggestion that EMC activation increases the proportion of favorable reaction zones.[45] Other studies have determined that the HEBM process can significantly reduce the temperature required for a subsequent reaction to occur (up to a three-fold reduction), so that a major component of the overall reaction dynamics is initiated in a "phase nanocrystalline or amorphous" to exhibit "unusually low or even negative values of the apparent activation energy" necessary to cause a chemical reaction.[82]​.

In general, EMCs are likely to be less dependent on temperature for the progression of a chemical pathway (see previous section on pozzolanic reactions), which may explain why EMCs provide “self-healing” benefits even at the low temperatures recorded in the Arctic.[83][84]​.

Physical justification (amorphization)

The large changes in , more particularly in the resulting values ​​of and , provide insight into the effectiveness of EMC activation. Amorphization of crystalline material under high pressure conditions "is a rather unusual phenomenon" for the simple reason that "most materials actually undergo the reverse transformation from amorphous to crystalline under high pressure conditions."[85]​ Amorphization represents a highly distorted "periodicity" of the lattice element of a material, comprising a relatively high Gibbs free energy.[72][74]​ In fact, amorphization can be compared to a state "almost-melted".[73]​[75]​.

In all, like other HEBM processes, EMC activation causes crystal destruction due to extremely violent and disruptive factors occurring at the nanoscale of the material being processed.[86] Although short-lived and highly focal, the processes repeat at high frequency: therefore, those factors are believed to mimic the pressures and temperatures found deep within the Earth to cause the required phase change.[2] For example, Peter Thiessen") developed the "magma-plasma model" which assumes that localized temperatures (greater than 10 Kelvin) can be generated at the various impact points to induce a momentary excited plasma state in the material, characterized by the ejection of electrons and photons along with the formation of excited fragments (see diagram above).[87] Experimental data collected since 1975 on the generation of localized cracks, itself itself an important component of EMC activation, have confirmed elevated temperatures in this process.[88]​.

Vibratory ball mills (MVB)

For EMC activation, the HEBM method used is a vibrating ball mill (MVB).[38] An MVB uses a vertical eccentric drive mechanism to vibrate a closed chamber up to many hundreds of cycles per minute. The chamber is filled with the material that is processed along with specialized objects called "grinding media." In their simplest format, these media can be simple balls made of special ceramic. In practical terms, EMC activation deploys various grinding media of different sizes, shapes and compounds to achieve the required mechanochemical transformation.[4]​.

It has been suggested that an MVB will grind 20 to 30 times more than a rotary ball mill, reflecting that the mechanism of an MVB is especially efficient.[89]​.

In simple terms, the compressive force acting between two "identical" balls colliding in an MVB can be expressed as:[90]​.

It can be seen that an increase in impact speed increases . The size and mass of the grinding media also contribute to the efficiency of the process. The denominator term incorporates , meaning that the nature of the material used for the grinding media is an important factor (it is ultimately squared in , so its negative value is of no consequence). More fundamentally, due to the rapid vibration, a high acceleration is imparted to the grinding media, whereby continuous, brief and sharp impacts on the cement result in a rapid reduction in particle size.[89]​ Furthermore, high pressures and shear stress facilitate the required phase transition to an amorphous state both at the point of impact and during the transmission of shock waves that can produce pressures even higher than the impact itself.[86]​.

For example, the contact time of a two-ball collision can be as short as 20 μs, generating a pressure of 3.3 GPa "Pascal (unit)") upward and with an associated ambient temperature rise of 20 Kelvin.[86] Due to the short duration of the impact, the rate of change in momentum is significant, generating a shock wave of a duration of only 1-100 μs but with an associated pressure of 10 GPa upward and a focal and very localized temperature (that is, on a nanoscopic scale) up to several thousand degrees Kelvin.[86]​ To put this in context, a pressure of 10 GPa is equivalent to that exerted by about 1,000 kilometers of seawater. As another example, the impact of two identical 2.5 cm diameter steel balls with a speed of 1 m/s will generate a collision with an energy density of more than 10 joules "Joule (unit)")/m; and with alumina balls of the same 2.5 cm diameter and a speed of 1 m/s an even higher energy density is generated.[90]​ Collisions occur on a very short time scale and therefore the "rate of energy release over the relatively small contact area can be very high".[90]​.

Scientific background of EMC activation:.

The term "energy modified cement" (EMC) was first used in 1992 by Vladimir Ronin, introduced in a paper by Ronin et al. dated 1993 and presented at a formal meeting of the Nordic Concrete Research academic group.[16]​ The process was refined by Ronin and others, including Lennart Elfgren (now professor emeritus of LTU).[17]​.

At the 45th World Exhibition of Invention, Research and Innovation, held in 1996 in Brussels, Belgium, the EMC received a gold medal with the Eureka mention "Eureka (intergovernmental initiative)") from the European intergovernmental organization for research and development, for energy modification of cements.[18]​.

The term "energetically modified" has been used elsewhere, although such use does not indicate that the method used was activation as defined in this article.[19]​.

The reviewed properties of the product include:[4]​[20]​[21]​[22]​.

Unlike Portland cement, the production of EMC does not generate carbon dioxide at all. This makes EMCs "low carbon cements".[7]​.

The first cited claims about EMC's CO reduction capabilities were made in 1999, when global Portland cement production was 1.6 billion tonnes per year.[20]​[23]​ From 2011 to 2019, global Portland cement production increased from 3.6 to 4.1 billion tonnes per year.[24][note 1]​ The potential for energy-modified cement to contribute to a global CO reduction CO has been recognized externally since 2002 and continues.[5]​[6][8]​ Recent recognition has included the Energy Transition Commission report (drafted under Adair Turner and Nicholas Stern) Possible sector focus mission: cement (2019).[25]​ Recognition of the potential for "zero carbon" was set out by McKinsey & Co in their 2020 report "Establishing the bases for zero-carbon cement".[26]​ In 2023, the contribution that EMCs offer to achieving "low-carbon" materials was further recognized within the academic domain of mechanochemistry.[15]​.

No harmful emissions or toxic chemicals during production

EMC activation is a purely mechanical process. As such, it does not involve heating or burning or, indeed, any chemical treatment. This means that no fumes are produced during manufacturing.[20]​.

Usage history

EMCs have been produced for use in various projects since 1992.[4]​ In 2010, the volume of concrete poured containing EMC was approximately 4,500,000 cubic yards (3,440,496 m), primarily on US DOT projects.[4]​ To put this in context, this is a volume greater than that of the entire Hoover Dam construction (including its associated power plants and works). annexes), where a total of 4,360,000 cubic yards (3,333,459 m³) of concrete was poured, equivalent to a standard US highway from San Francisco to New York City.[27]​.

One of the first projects to use EMC from fly ash was the construction of a road bridge in Karungi), Sweden, in 1999, with the Swedish construction company Skanska. The Karungi road bridge has withstood Karungi's harsh winters and strong diurnal temperature swings throughout the year.[20]​.

In the United States, energy-modified cements have been approved for use by several state transportation agencies, including those of Pennsylvania, Texas, and California.[22]​.

In the United States, highway bridges and hundreds of miles of highway paving have been built with concretes made with EMC derived from fly ash.[4]​ These projects include sections of Interstate 10.[4]​ In these projects, the EMC replaced at least 50% of the Portland cement in the poured concrete.[28]​ This is approximately 2.5 times more than the typical amount of fly ash in projects where no cements are used.[28]​ This is approximately 2.5 times more than the typical amount of fly ash in projects where no cements are used. energy modified.[29]​ Independent test data showed that 28-day strength development requirements were exceeded in all projects.[28]​.

Another project was the expansion of the passenger terminals at the port of Houston, Texas, where the ability of energy modified cement to produce concrete with high resistance to the ionic permeability of chlorides and sulfates (i.e., greater resistance to seawater) was a determining factor.[4]​.

Customized design according to its final use

The behavior of mortars and concretes made from EMC can be custom designed. For example, EMC concretes can range from general applications (depending on strength and durability), to the production of rapid and ultra-rapid hardening cements for high-strength concretes (e.g., more than 70 MPa/10,150 psi in 24 hours and more than 200 MPa/29,000 psi in 28 days).[21]​ This allows energy-modified cements to be used in concretes of high performance.[21]​.

Durability of concrete and EMC mortars

Any cementitious material that undergoes EMC activation is likely to have greater durability, including Portland cement treated with EMC activation.[21]​ Regarding pozzolanic EMCs, concretes made with pozzolanic EMC are more durable than concretes made with Portland cement.[31]​.

Treatment of Portland cement with EMC activation produces high-performance concretes, including high strength, high durability, and faster strength development than concretes produced with untreated Portland cement.[21]​ Treatment of Portland cement with the EMC activation process can increase its strength development by almost 50%, and also significantly improve its durability, measured according to generally accepted methods.[21]​[30]​.

Greater resistance to salt water attack

Concrete made from ordinary Portland cement without additives has a relatively low resistance to brackish water.[30]​ In contrast, EMCs show a high resistance to ionic attack by chlorides and sulfates, together with a low alkali-silica reactivity").[28]​ For example, durability tests have been carried out according to the "Bache method" (see diagram). Concrete samples with respective compressive strengths of 180.3 and 128.4 MPa (26,150 and 18,622 psi) after 28 days of curing, were tested using the Bache method. The samples were made of (a) EMC (comprising Portland cement and silica fume, both subjected to EMC activation) and (b) Portland cement. The resulting mass loss was plotted for durability.

In other words, treating Portland cement with the EMC activation process can increase strength development by almost 50% and also significantly improve its durability, measured according to generally accepted methods.[21]​.

Low leachability of concrete with EMC

Luleå University of Technology conducted leaching tests in 2001 in Sweden, on behalf of a Swedish energy production company, on concrete made with an EMC made from fly ash. These tests confirmed that the concrete "showed low specific surface leachability" with respect to "all environmentally relevant metals".[32]​[33]​.

Self-healing properties of pozzolanic EMCs

Natural pozzolanic reactions can cause mortars and concretes containing these materials to "self-heal".[35]​[36][37]​ The EMC activation process can increase the likelihood of these pozzolanic reactions occurring.[38]​[39]​ The same trend has been observed and studied in the various supporting structures of the Hagia Sophia built in the time of the Byzantine Emperor Justinian I. (currently in Istanbul, Turkey).[40]​ There, as in the case of most Roman cements, mortars containing high amounts of pozzolanic ash were used, to obtain greater resistance to the stresses generated in buildings by earthquakes.[41]​.

EMCs made from pozzolanic materials exhibit "biomimetic" self-healing capabilities that can be photographed as they develop (see attached images).[34]​.

EMCs using California pozzolans

Concretes manufactured by replacing at least 50% of Portland cement with EMC have given consistent practical results in high volume applications.[28]​ This is also the case for EMC produced from natural pozzolans (such as volcanic ash).[42]​.

Southern California volcanic ash deposits were independently tested. By substituting 50% of Portland cement in the dosage, the resulting concretes exceeded the requirements of the corresponding ASTM standard.[43] At 28 days, its compressive strength was 4180 psi / 28.8 megapascals "Pascal (unit)") (N "Newton (unit)")/mm²). The 56-day strength exceeded the requirements for 4,500 psi (31.1 MPa) concrete, even taking into account the safety margin recommended by the American Concrete Institute.[44] The concrete made in this manner was workable and sufficiently strong, exceeding the standard of 75% pozzolanic activity at both 7 days and 28 days.[43] It also increased the surface smoothness of the pozzolans in the concrete.[43]​.

Effect on pozzolanic reactions

EMC activation is a process that increases the chemical affinity of a pozzolan.[38]​[39]​ This leads to greater and more rapid strength development of the resulting concrete, at higher replacement ratios than untreated pozzolans.[28]​[42]​ These transformed (highly reactive) pozzolans demonstrate additional benefits by utilizing known pozzolanic reaction forms that generally have as their ultimate target a range of hydrated products. A nuclear magnetic resonance study on EMC samples concluded that its activation caused "the formation of thin layers of SiO (Silicon(IV) oxide") around the C3S crystals")", which in turn "accelerates the pozzolanic reaction and promotes the growth of more extensive networks of hydrated substances".[45]

In simple terms, by using pozzolans in concrete, porous (reactive) portlandite can be transformed into hard, impermeable (relatively non-reactive) compounds, instead of the relatively reactive, porous and soft calcium carbonate generated with ordinary cement.[46] Many of the end products of pozzolanic chemistry exhibit a hardness greater than 7.0. "Self-healing" capabilities can also help improve durability in applications where tensile stresses must be sustained.

In greater detail, the benefits of pozzolanic concrete begin with the understanding that in conventional concretes, Portland cement combines with water to produce a stone-like material through a complex series of chemical reactions, the mechanisms of which are not yet fully understood. The process, called mineral hydration, forms two cementitious compounds in concrete: hydrated calcium silicate (C-S-H) and calcium hydroxide (Ca(OH)). This reaction can occur in three ways:[47]​.

The underlying hydration reaction forms two products:

Portlandite makes up about 25% of concrete made with Portland cement without pozzolanic cementitious materials.[46]​ In this type of concrete, carbon dioxide is slowly absorbed to convert portlandite into insoluble calcium carbonate (CaCO), in a process called carbonation:[46]​.

In mineral form, calcium carbonate can exhibit a wide range of hardness depending on how it is formed. In its softest form, it can form in concrete as chalk "Crete (rock)") (hardness 1.0 on the Mohs scale). Like portlandite, calcium carbonate in mineral form can also be porous, permeable and with little resistance to acid attack, causing it to release carbon dioxide.

However, pozzolanic concretes, including EMCs, continue to consume the soft, porous portlandite as the hydration process continues, converting it to additional hardened concrete as calcium silicate hydrate (C-S-H) instead of calcium carbonate.[46]​ This results in a denser, less permeable and more durable concrete.[46]​ This is an acid-base reaction between portlandite. and silicic acid (HSiO) which can be represented as follows:[51]​.

Additionally, many pozzolans contain aluminate (Al(OH)) which will react with Portlandite and water to form:.

The chemistry of pozzolanic cement (along with the chemistry of high aluminate cement) is complex and is not necessarily restricted to the above pathways. For example, strätlingite can be formed in several ways, including the following equation that can increase the strength of a concrete:[54]​[55]​.

The role of pozzolans in concrete chemistry is not fully understood. For example, strätlingite is metastable, which in an environment of high temperature and water content (which can be generated during the early stages of concrete curing) can itself produce stable calcium-aluminum garnet particles (see the first point above).[58] This process can be represented by the following equation:

According to the first point, although the inclusion of calcium and aluminum garnet in itself is not problematic, if instead it is produced by the previous route, then microcracks and loss of strength in the concrete may occur.[60]​ However, the addition of highly reactive pozzolans to the concrete mixture prevents such a conversion reaction.[61]​.

In summary, while pozzolans provide several chemical possibilities for forming hardened materials, "high reactivity" pozzolans such as blast furnace slag") (GGBFS) can also stabilize some of these processes. In this context, EMCs manufactured from fly ash have been shown to produce concretes that meet the same characteristics as concretes containing "slag 120" (i.e. GGBFS) according to the American ASTM standard. C989.[28][62]​.

Portlandite, when exposed to low temperatures, humid conditions and condensation, can react with sulfate ions, producing efflorescence. On the contrary, pozzolanic chemistry reduces the amount of portlandite available to reduce the generation of efflorescence.[63]​.

Contenido

El propósito de la activación de EMC es fundamentalmente causar la destrucción de la estructura cristalina del material procesado, para convertirlo en un material amorfo.[38]​ Aunque este cambio aumenta la reactividad química del material procesado, no se produce ninguna reacción química "durante" el proceso de activación de EMC.

En su forma más simple, la mecanoquímica puede definirse como "un campo que estudia las reacciones químicas iniciadas o aceleradas por la absorción directa de energía mecánica".[15]​ Más técnicamente, se puede definir como una rama de la química que se ocupa de la "transformación química y fisicoquímica de sustancias en todos los estados de agregación producida por el efecto de la energía mecánica".[65]​ La IUPAC no contiene una definición estándar del término "mecanoquímica", sino que define una ""reacción" mecanoquímica" como una reacción química "inducida por la absorción directa de energía mecánica", al tiempo que señala "corte, estiramiento y molienda son métodos típicos para la generación mecano-química de propiedades reactivas".[66]​[67]​.

Más estrictamente, activación mecánica fue un término definido por primera vez en 1942 como un proceso que implica un aumento en la capacidad de reacción de una sustancia que permanece químicamente sin cambios.[68]​ Aún más estrictamente, la activación de EMC es una forma especializada de activación mecánica limitada a la aplicación de molienda de bolas de alta energía (HEBM) a materiales cementosos. De manera más restringida, la activación de EMC implica el uso de pulverización vibratoria, y usando exclusivamente sus propios medios de molienda.[38]​ Como se indica en un libro de texto académico de 2023 limitado a la mecanoquímica, la activación de EMC ha "demostrado de manera impresionante" sus efectos al provocar un cambio en la reactividad del material de cemento alternativo y las características físicas resultantes del hormigón.[15]​.

Thermodynamic justification

More specifically, the HEBM process can be described as increasing the chemical reactivity of a material by increasing its chemical "potential" energy. In EMC activation, the transferred mechanical energy is stored in the material as defects caused by the destruction of the crystalline structure of the material. Therefore, the process transforms the state of solid substances into other thermodynamically and structurally more "unstable" states, which allows an explanation of this greater reactivity as an increase in their Gibbs energy:[69]​.

In its simplest form, the HEBM process causes the destruction of crystalline bonds to increase the reactivity of a material.[70]​ From the thermodynamic perspective, any subsequent chemical reaction can decrease the level of excess energy in the activated material (i.e., as a reactant) to produce new components comprising a lower chemical energy and a more stable physical structure. On the contrary, to convert the preprocessed material into a more reactive physical state, the disorder generated during the HEBM process can be justified as equivalent to a “decrystallization” (and therefore an increase in entropy) that in part produces an increase in volume (due to the decrease in apparent density). A reverse process, sometimes called "relaxation", can be almost immediate (10 to 10 seconds) or take much longer (e.g. 10 seconds).[71] Ultimately, any general "retained" thermodynamic effect can be justified on the basis that any such "reverse" process is incapable of reaching an ideal thermodynamic end state on its own. As a result, in the course of mechanical activation of minerals, reverse "relaxation" processes cannot completely decrease the Gibbs free energy that has been created. Therefore, energy remains in the material, which is stored in the "defects" created in the crystal lattice.[72]​[73]​.

In general, the HEBM process generates a net thermodynamic effect:[74]​[75]​[76]​.

Where disorder in crystals is low, it is very small (if not negligible). On the contrary, in highly deformed and disordered crystals, the values ​​of can have a significant impact on the Gibbs free energy generated.

Leaving aside the heat generated during the process due to effects caused during the activation process such as friction, the excess Gibbs free energy retained in the activated material can be justified due to two changes, namely, an increase in the specific surface area (), and of the defects of the crystal structure ().[77]​[76]​ In successful HEBM processes such as EMC activation:[78]​[79]​.

The relatively low value of () compared to the high value of () serves to further distinguish the HEBM process from general grinding or "milling" (where, instead, the only objective is to increase the surface area of the processed materials), allowing an explanation of the change in the entropy of the pulverized material in the form of elastic energy (stored in defects of the crystal lattice that can take years to "relax"), which is the "source of excess energy and Gibbs enthalpy".[77]​ Regarding enthalpy, four descriptors can be outlined to give an overview of the total change during said activation process:[78]​[80][81]​.

Because most of the energy required during the EMC activation process is absorbed by (), it is trivial. Therefore, the main functions for the enthalpy change are approximated by:.

In EMC activation, the above terms and are considered particularly noteworthy due to the nature of the observed changes in physical structure.[38]​ Therefore, the change in enthalpy caused during EMC activation can be approximated by:[80][81]​.

From the above thermodynamic construction, EMC activation results in a highly "amorphous" phase that can be justified as a large increase in and also a large increase in .[38]​[80]​[81]​ The benefits of EMC activation are large in , meaning that its reactivity is less dependent on temperature. In terms of the thermodynamic momentum of any reaction, the overall enthalpy of a reactant is not dependent on temperature, meaning that a material that has been subjected to HEBM with a corresponding elevation of can react at a lower temperature (since the "activated" reactant is less dependent on temperature for its progression). Furthermore, an EMC reaction can exhibit physical mechanisms at extremely small scales "with the formation of thin layers of SiO (Silicon(IV) oxide")", with the suggestion that EMC activation increases the proportion of favorable reaction zones.[45] Other studies have determined that the HEBM process can significantly reduce the temperature required for a subsequent reaction to occur (up to a three-fold reduction), so that a major component of the overall reaction dynamics is initiated in a "phase nanocrystalline or amorphous" to exhibit "unusually low or even negative values of the apparent activation energy" necessary to cause a chemical reaction.[82]​.

In general, EMCs are likely to be less dependent on temperature for the progression of a chemical pathway (see previous section on pozzolanic reactions), which may explain why EMCs provide “self-healing” benefits even at the low temperatures recorded in the Arctic.[83][84]​.

Physical justification (amorphization)

The large changes in , more particularly in the resulting values ​​of and , provide insight into the effectiveness of EMC activation. Amorphization of crystalline material under high pressure conditions "is a rather unusual phenomenon" for the simple reason that "most materials actually undergo the reverse transformation from amorphous to crystalline under high pressure conditions."[85]​ Amorphization represents a highly distorted "periodicity" of the lattice element of a material, comprising a relatively high Gibbs free energy.[72][74]​ In fact, amorphization can be compared to a state "almost-melted".[73]​[75]​.

In all, like other HEBM processes, EMC activation causes crystal destruction due to extremely violent and disruptive factors occurring at the nanoscale of the material being processed.[86] Although short-lived and highly focal, the processes repeat at high frequency: therefore, those factors are believed to mimic the pressures and temperatures found deep within the Earth to cause the required phase change.[2] For example, Peter Thiessen") developed the "magma-plasma model" which assumes that localized temperatures (greater than 10 Kelvin) can be generated at the various impact points to induce a momentary excited plasma state in the material, characterized by the ejection of electrons and photons along with the formation of excited fragments (see diagram above).[87] Experimental data collected since 1975 on the generation of localized cracks, itself itself an important component of EMC activation, have confirmed elevated temperatures in this process.[88]​.

Vibratory ball mills (MVB)

For EMC activation, the HEBM method used is a vibrating ball mill (MVB).[38] An MVB uses a vertical eccentric drive mechanism to vibrate a closed chamber up to many hundreds of cycles per minute. The chamber is filled with the material that is processed along with specialized objects called "grinding media." In their simplest format, these media can be simple balls made of special ceramic. In practical terms, EMC activation deploys various grinding media of different sizes, shapes and compounds to achieve the required mechanochemical transformation.[4]​.

It has been suggested that an MVB will grind 20 to 30 times more than a rotary ball mill, reflecting that the mechanism of an MVB is especially efficient.[89]​.

In simple terms, the compressive force acting between two "identical" balls colliding in an MVB can be expressed as:[90]​.

It can be seen that an increase in impact speed increases . The size and mass of the grinding media also contribute to the efficiency of the process. The denominator term incorporates , meaning that the nature of the material used for the grinding media is an important factor (it is ultimately squared in , so its negative value is of no consequence). More fundamentally, due to the rapid vibration, a high acceleration is imparted to the grinding media, whereby continuous, brief and sharp impacts on the cement result in a rapid reduction in particle size.[89]​ Furthermore, high pressures and shear stress facilitate the required phase transition to an amorphous state both at the point of impact and during the transmission of shock waves that can produce pressures even higher than the impact itself.[86]​.

For example, the contact time of a two-ball collision can be as short as 20 μs, generating a pressure of 3.3 GPa "Pascal (unit)") upward and with an associated ambient temperature rise of 20 Kelvin.[86] Due to the short duration of the impact, the rate of change in momentum is significant, generating a shock wave of a duration of only 1-100 μs but with an associated pressure of 10 GPa upward and a focal and very localized temperature (that is, on a nanoscopic scale) up to several thousand degrees Kelvin.[86]​ To put this in context, a pressure of 10 GPa is equivalent to that exerted by about 1,000 kilometers of seawater. As another example, the impact of two identical 2.5 cm diameter steel balls with a speed of 1 m/s will generate a collision with an energy density of more than 10 joules "Joule (unit)")/m; and with alumina balls of the same 2.5 cm diameter and a speed of 1 m/s an even higher energy density is generated.[90]​ Collisions occur on a very short time scale and therefore the "rate of energy release over the relatively small contact area can be very high".[90]​.

Scientific background of EMC activation:.