Energetically modified cement

EMC Activation is purely a mechanical process. As such, it does not involve heating or burning or indeed any chemical treatments. This means no fumes at all are produced during an EMC's manufacture.[19]

EMCs have been produced for project usage since 1992 for a wide range of uses.[4] By 2010, the volume of concrete poured containing EMCs was about 4,500,000 cu yd (3,440,496 m³), largely on US DOT projects.[4] To place this into context, that is more than the entire construction of the Hoover Dam, its associated power plants and appurtenant works, where a total of 4,360,000 cu·yds (3,333,459 m³) of concrete was poured—equivalent to a U.S. standard highway from San Francisco to New York City.[25]

An early project using EMC made from fly ash was the construction of a road bridge in Karungi, Sweden, in 1999, with Swedish construction firm Skanska. The Karungi road bridge has withstood Karungi's harsh subarctic climate and divergent annual and diurnal temperature ranges.[19]

In the United States, energetically modified cements have been approved for usage by a number of state transportation agencies, including PennDOT, TxDOT and CalTrans.[21]

In the United States, highway bridges and hundreds of miles of highway paving have been constructed using concretes made from EMC derived from fly ash.[4] These projects include sections of Interstate 10.[4] In these projects, EMC replaced at least 50% of the Portland cement in the concrete poured.[26] This is about 2.5 times more than the typical amount of fly ash in projects where energetic modification is not used.[27] Independent test data showed 28-day strength-development requirements were exceeded in all projects.[26]

Another project was the extension of the passenger terminals at the Port of Houston, Texas, where energetically modified cement's ability to yield concretes that exhibit high resistances to chloride– and sulphate–ion permeability (i.e., increased resistance to sea waters) was a factor.[4]

The performance of mortars and concretes made from EMCs can be custom-designed. For example, EMC concretes can range from general application (for strength and durability) through to the production of rapid and ultra-rapid hardening high-strength concretes (for example, over 70 MPa / 10,150 psi in 24 hours and over 200 MPa / 29,000 psi in 28 days).[20] This allows energetically modified cements to yield High Performance Concretes.

Any cementitious material undergoing EMC Activation will likely marshal improved durability—including Portland cement treated with EMC Activation.[20] As regards pozzolanic EMCs, concretes made from pozzolanic EMCs are more durable than concretes made from Portland cement.[29]

Treating Portland cement with EMC activation will yield high-performance concretes (HPCs). These HPCs will be high strength, highly durable, and exhibiting greater strength-development in contrast to HPCs made from untreated Portland cement.[20] Treating Portland cement with the EMC Activation process may increase the strength development by nearly 50% and also significantly improve the durability, as measured according to generally accepted methods.[20][28]

Concrete made from ordinary Portland cement without additives has a relatively impaired resistance to salt waters.[28] In contrast, EMCs exhibit high resistances to chloride and sulphate ion attack, together with low alkali-silica reactivities (ASR).[26] For example, durability tests have been performed according to the "Bache method" (see diagram). Samples made of HPC having respective compressive strengths of 180.3 and 128.4 MPa (26,150 and 18,622 psi) after 28 days of curing, were then tested using the Bache method. The samples were made of (a) EMC (comprising Portland cement and silica fume both having undergone EMC Activation) and (b) Portland cement. The resulting mass-loss was plotted in order to determine durability. As a comparison, the test results showed:

• Whereas, the reference Portland cement concrete had "total destruction after about 16 Bache method cycles, in line with Bache's own observations for high-strength concrete";[20][28]
• EMC high performance concrete showed a "consistent high-level durability" throughout the entire testing-period of 80 Bache cycles, with for example, "practically no scaling of the concrete has been observed".[20]

In other words, treating Portland cement with the EMC Activation process, may increase the strength-development by nearly 50% and also significantly improve the durability, as measured according to generally accepted methods.[20]

Leachability tests were performed by LTU in 2001 in Sweden, on behalf of a Swedish power production company, on concrete made from an EMC made from fly ash.  These tests confirmed that the cast concrete "showed a low surface specific leachability" with respect to "all environmentally relevant metals." [30][31]  

Natural pozzolanic reactions can cause mortars and concretes containing these materials to "self-heal".[33][34][35] The EMC Activation process can increase the likelihood of the occurrence of these pozzolanic reactions.[36][37] The same tendency been noted and studied in the various supporting structures of Hagia Sophia built for the Byzantine emperor Justinian (now, Istanbul, Turkey).[38] There, in common with most Roman cements, mortars comprising high amounts of pozzolana were used — in order to give what was thought to be an increased resistance to the stress-effects caused by earthquakes.[39]

EMCs made from pozzolanic materials exhibit "biomimetic" self-healing capabilities that can be photographed as they develop (see picture insert).[32]

Concretes made by replacing at least 50% of the Portland cement with EMCs have yielded consistent field results in high-volume applications.[26] This is also the case for EMC made from natural pozzolans (e.g., volcanic ash).[40]

Volcanic ash deposits from Southern California were independently tested; at 50% Portland cement replacement, the resulting concretes exceeded the requirements of the relevant US standard.[41] At 28 days, the compressive strength was 4,180 psi / 28.8 MPa (N/mm²). The 56-day strength exceeded the requirements for 4,500 psi (31.1 MPa) concrete, even taking into account the safety margin as recommended by the American Concrete Institute.[42] The concrete made in this way was workable and sufficiently strong, exceeding the 75% standard of pozzolanic activity at both 7 days and 28 days.[41] The surface smoothness of pozzolans in the concrete was also increased.[41]

Volcanic ash deposits situated in Southern California, USA.

EMC Activation is a process that increases a pozzolan's chemical affinity for pozzolanic reactions.[36][37] This leads to faster and greater strength development of the resulting concrete, at higher replacement ratios, than untreated pozzolans.[26][40] These transformed (now highly reactive pozzolans) demonstrate further benefits using known pozzolanic reaction-pathways that typically see as their end-goal a range of hydrated products. An NMR study on EMCs concluded that EMC Activation caused "the formation of thin SiO₂ layers around C3S crystals", which in turn, "accelerates the pozzolanic reaction and promotes growing of more extensive nets of the hydrated products".[43]

In simple terms, by using pozzolans in concrete, porous (reactive) Portlandite can be transformed into hard and impermeable (relatively non-reactive) compounds, rather than the porous and soft relatively reactive calcium carbonate produced using ordinary cement.[44] Many of the end products of pozzolanic chemistry exhibit a hardness greater than 7.0 on the Mohs scale."Self healing" capabilities may also contribute to enhanced field-application durabilities where mechanical stresses may be present.

In greater detail, the benefits of pozzolanic concrete, starts with an understanding that in concrete (including concretes with EMCs), Portland cement combines with water to produce a stone-like material through a complex series of chemical reactions, whose mechanisms are still not fully understood. That chemical process, called mineral hydration, forms two cementing compounds in the concrete: calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)₂). This reaction can be noted in three ways, as follows:[45]

• Standard notation:  ${\displaystyle {\ce {Ca3SiO5 + H2O -> (CaO) * (SiO2) * (H2O) + Ca(OH)2}}}$

• Balanced:  ${\displaystyle {\ce {2Ca3SiO5 + 7H2O -> 3CaO * 2SiO2 * 4H2O + 3Ca(OH)2}}}$

• Cement chemist notation (the hyphenation denotes the variable stoichiometry):  C₃S  +  H   →   C-S-H  +  CH

The underlying hydration reaction forms two products:

1. Calcium silicate hydrate (C-S-H), which gives concrete its strength and dimensional stability. The crystal structure of C-S-H in cement paste has not been fully resolved yet and there is still ongoing debate over its nanostructure.[46]
2. Calcium hydroxide (Ca(OH)₂), which in concrete chemistry is known also as Portlandite. In comparison to calcium silicate hydrate, Portlandite is relatively porous, permeable and soft (2 to 3, on Mohs scale).[47] It is also sectile, with flexible cleavage flakes.[48] Portlandite is soluble in water, to yield an alkaline solution which can compromise a concrete's resistance to acidic attack.[29]

Portlandite makes up about 25% of concrete made with Portland cement without pozzolanic cementitious materials.[44] In this type of concrete, carbon dioxide is slowly absorbed to convert the Portlandite into insoluble calcium carbonate (CaCO₃), in a process called carbonatation:[44]

${\displaystyle {\ce {Ca(OH)2 + CO2 -> CaCO3 + H2O}}}$

In mineral form, calcium carbonate can exhibit a wide range of hardness depending on how it is formed. At its softest, calcium carbonate can form in concrete as chalk (of hardness 1.0 on Mohs scale). Like Portlandite, calcium carbonate in mineral form can also be porous, permeable and with a poor resistance to acid attack, which causes it to release carbon dioxide.

Pozzolanic concretes, including EMCs, however, continue to consume the soft and porous Portlandite as the hydration process continues, turning it into additional hardened concrete as calcium silicate hydrate (C-S-H) rather than calcium carbonate.[44] This results in a denser, less permeable and more durable concrete.[44] This reaction is an acid-base reaction between Portlandite and silicic acid (H₄SiO₄) that may be represented as follows:[49]

${\displaystyle {\ce {Ca(OH)2 + H4SiO4 -> Ca^2+ + H2SiO4^2- + 2H2O -> CaH2SiO4 * 2H2O}}}$ [Note 4]

Further, many pozzolans contain aluminate (Al(OH)₄⁻) that will react with Portlandite and water to form:

• calcium aluminate hydrates, such as calcium aluminium garnet (hydrogrossular: C₄AH₁₃ or C₃AH₆ in cement chemist notation, hardness 7.0 to 7.5 on Mohs scale);[50]  or
• in combination with silica, to form strätlingite (Ca₂Al₂SiO₇·8H₂O or C₂ASH₈ in cement chemist notation), which geologically can form as xenoliths in basalt as metamorphosed limestone.[51]

Pozzolanic cement chemistry (along with high-aluminate cement chemistry) is complex and per se is not constrained by the foregoing pathways. For example, strätlingite can be formed in a number of ways, including per the following equation which can add to a concrete's strength:[52]

C₂AH₈  +  2CSH  +  AH₃  +  3H    →    C₂ASH₈    (cement chemist notation) [53]

The role of pozzolans in a concrete's chemistry is not fully understood. For example, strätlingite is metastable, which in a high temperature and water-content environment (that can be generated during the early curing stages of concrete) may of itself yield stable calcium aluminium garnet (see first bullet point above).[54] This can be represented per the following equation:

3C₂AH₈    →    2C₃AH₆  +  AH₃  +  9H    (cement chemist notation) [55]

Per the first bullet point, although the inclusion of calcium aluminium garnet per se is not problematic, if it is instead produced by foregoing pathway, then micro-cracking and strength-loss can occur in the concrete.[56] However, adding high-reactivity pozzolans into the concrete mix prevents such a conversion reaction.[57] In sum, whereas pozzolans provide a number of chemical pathways to form hardened materials, "high-reactivity" pozzolans such as blast furnace slag (GGBFS) can also stabilise certain pathways. In this context, EMCs made from fly ash have been demonstrated to produce concretes that meet the same characteristics as concretes comprising "120 Slag" (i.e., GGBFS) according to U.S. standard ASTM C989.[26][58]

Portlandite, when exposed to low temperatures, moist conditions and condensation, can react with sulphate ions to cause efflorescence; pozzolanic chemistry reduces the amount of Portlandite available, to reduce efflorescence.[59]

More particularly, HEBM can be described as increasing the chemical reactivity of a material by increasing its chemical potential energy. In EMC Activation, transferred mechanical energy is stored in the material as lattice defects caused by destroying the material's crystalline structure. Hence, the process transforms solid substances into thermodynamically and structurally more unstable states, allowing an explanation for that increased reactivity as an increase in Gibbs energy:[65]

${\displaystyle \Delta G=G_{T}^{*}-G_{T}}$  where, for temperature ${\displaystyle T}$, the terms ${\displaystyle G_{T}^{*}}$ and ${\displaystyle G_{T}}$ are the respective Gibbs values in the processed and unprocessed material.

At its simplest, HEBM causes the destruction of crystalline bonds, to increase a material's reactivity.[66] From the thermodynamic perspective, any subsequent chemical reaction can decrease the excess energy level in the activated-material (i.e. as a reactant) to produce new components comprising both a lower chemical energy and a more stable physical structure. Conversely, to render the pre-processed material into a more reactive physical state, the disordering process during the HEBM process can be justified as being equivalent to a decrystallisation (and hence an entropy increase) that in part yields a volume increase (decrease of bulk density). A reverse process, sometimes called "relaxation", can be almost immediate (10⁻⁷ to 10⁻³ seconds) or take much longer (e.g. 10⁶ seconds).[67] Ultimately, any overall retained thermodynamic effect can be justified on the basis that any such reverse process is incapable of reaching an ideal thermodynamic end-state of its own accord. As a result, in the course of the mechanical activation of minerals, reverse "relaxation" processes cannot completely decrease the Gibbs free energy that has been created. Hence, energy remains in the material, which is stored in the crystal-lattice defects created.[68][69]

Overall, HEBM renders a net thermodynamic effect:[70][71][72]

• The structural disordering implies an increase of both entropy and enthalpy and thus stimulates the crystal properties according to the thermodynamic modifications. Only a small fraction (approximately 10%) of the excess enthalpy of the activated product may be accounted-for as surface-area enlargement.
• Instead, the main part of the excess enthalpy and modified properties can mostly be assigned to the development of thermodynamically unstable states in the material's lattice (and not as a reduction of particle size).
• Since the activated system is unstable, the process of activation is reversible—resulting in deactivation, re-crystallization, entropy loss and energy output of system. That reverse ("relaxation") process continues to a thermodynamic equilibrium, but ultimately can never reach an ideal structure (i.e. one free of defects).
• A more complete description of such an "activation" process factors-in enthalpy also, by which according to the Gibbs-Helmholtz equation, the Gibbs free energy between activated and non-activated solid state can be represented:

${\displaystyle \Delta G=\Delta H-T\Delta S}$   where, ${\displaystyle \Delta H}$ is the change in enthalpy and ${\displaystyle \Delta S}$ the change in entropy.

Where the crystal disordering is low, ${\displaystyle \Delta S}$ is very small (if not negligible). In contrast, in highly deformed and disordered crystals, the values of ${\displaystyle \Delta S}$ can have a significant impact on the rendered Gibbs free energy. Leaving aside the heat generated during the process on account of friction etc. occasioned during the activation process, the excess Gibbs free energy retained in the activated material can be justified as being due to two changes, namely an increase in (${\displaystyle \mathrm {I} }$) specific surface area; and (${\displaystyle \mathrm {I} }$${\displaystyle \mathrm {I} }$) defect structure.[73][72] In successful HEBM processes such as EMC Activation:[74][75]

• as to (${\displaystyle \mathrm {I} }$), only about 10% of the excess energy of such an activated product may be accounted-for as a change in surface area.
• as to (${\displaystyle \mathrm {I} }$${\displaystyle \mathrm {I} }$), almost all the imparted energy is contained in the actual structural defects in the material processed.

The relatively low value of (${\displaystyle \mathrm {I} }$) as against the high value of (${\displaystyle \mathrm {I} }$${\displaystyle \mathrm {I} }$) serves to further distinguish HEBM from general grinding or "milling" (where instead the only aim there is to increase the surface area of the materials processed), thereby accounting for an explanation for the change in entropy ${\displaystyle S}$ of the rendered material in the form of elastic energy (stored in lattice defects that can take years to "relax" ) that is the "source of excess Gibbs energy and enthalpy".[73] As for enthalpy ${\displaystyle H}$, four descriptors can be derived to provide an overview as to the total change during such an activation process:[74][76][77]

${\displaystyle \Delta H_{T}=\Delta H_{d}+\Delta H_{S}+\Delta H_{A}+\Delta H_{p}}$  where:

• ${\displaystyle \Delta H_{d}}$  is a measure of the dislocation density;
• ${\displaystyle \Delta H_{p}}$  is a measure of new phases (polymorphic transformation);
• ${\displaystyle \Delta H_{A}}$  is a measure of the formation of amorphous material;
• ${\displaystyle \Delta H_{S}}$  is a measure of specific surface area.

Because the majority of the work exacted during the EMC Activation process goes to aspect (${\displaystyle \mathrm {I} }$${\displaystyle \mathrm {I} }$) above, ${\displaystyle \Delta H_{S}}$ is trivial. Hence the major functions for the change in enthalpy approximate to:

${\displaystyle \Delta H_{EMC}\approxeq \Delta H_{d}+\Delta H_{A}+\Delta H_{p}}$

In EMC Activation, the foregoing terms ${\displaystyle \Delta H_{d}}$ and ${\displaystyle \Delta H_{A}}$ are seen as being particularly prominent because of the nature of the changes in the physical structure observed.[36] Hence, the change in enthalpy ${\displaystyle H}$ occasioned during EMC Activation can be approximated to:[76][77]

${\displaystyle \Delta H_{EMC}\thickapprox \Delta H_{d}+\Delta H_{A}}$      i.e,   ${\displaystyle \Delta H_{EMC}\thickapprox (\rho M_{V}){\frac {b^{2}\mu _{s}}{4\pi }}\ln \left({\frac {2(\rho )^{1/2}}{b}}\right)+C_{A}E_{A}}$

where:

• ${\displaystyle M_{V}}$, ${\displaystyle b}$, ${\displaystyle \mu _{s}}$ and ${\displaystyle \rho }$ correspond respectively to the molar volume of the material, Burgers vector, shear modulus and dislocation density;[76][77]
• ${\displaystyle C_{A}}$ and ${\displaystyle E_{A}}$ are respectively the concentration of the amorphous phase and molar amorphisation energy.[76][77]

From the above thermodynamic construct, EMC Activation results in a highly amorphous phase that can be justified as a large ${\displaystyle \Delta H_{A}}$ and also a large ${\displaystyle \Delta H_{d}}$ increase.[36][76][77] The benefits of the EMC Activation being large in ${\displaystyle H}$ means that an EMC's reactivity is less temperature dependent. In terms of any reaction's thermodynamic impetus, a reactant's overall ${\displaystyle H}$ is not ${\displaystyle T}$ dependent, meaning that a material having undergone HEBM with a corresponding elevation of ${\displaystyle H}$ can react at a lower temperature (as the "activated" reactant is rendered less reliant on the temperature-dependent function ${\displaystyle T\Delta S}$ for its onward progression). Further, an EMC's reaction can exhibit physical mechanisms at extremely small scales "with the formation of thin SiO₂ layers" to aid a reaction's pathway—with the suggestion that EMC Activation increases the ratio of favourable reaction sites.[43] Studies elsewhere have determined that HEBM can significantly lower the temperature required for a subsequent reaction to proceed (up to a three-fold reduction), whereby a major component of the overall reaction-dynamics is initiated at a "nanocrystalline or amorphous phase" to exhibit "unusually low or even negative values of the apparent activation energy" required to cause a chemical reaction to occur.[78]

Overall, EMCs are likely less temperature dependent for a chemical pathway's onward progression (see section above on Pozzolanic reactions), which may explain why EMCs provide self-healing benefits even at low arctic temperatures.[79][80]

Large changes in ${\displaystyle \Delta G}$, more particularly in the resultant values of ${\displaystyle \Delta H_{A}}$ and ${\displaystyle \Delta H_{d}}$ provide an insight into EMC Activation's efficacy. The amorphisation of crystalline material at high-pressure conditions "is a rather unusual phenomenon" for the simple reason that "most materials actually experience the reverse transformation from amorphous to crystalline at high-pressure conditions".[81] Amorphisation represents a highly distorted "periodicity" of a material's lattice element, comprising a relatively high Gibbs free energy.[68][70] Indeed, amorphisation may be compared to a quasi-molten state.[69][71]

All told, in common with other HEBM processes, EMC Activation causes crystalline destruction because of extremely violent and disruptive factors that are occasioned at the nanoscale of the material being processed.[82] Although over in short duration and highly focal, the processes are repeated at a high frequency: hence those factors are thought to mimic pressures and temperatures found deep inside the Earth to cause the required phase change.[2] For example, Peter Thiessen developed the magma-plasma model that assumes localised temperatures—higher than 10³ 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 together with the formation of excited fragments (see diagram above).[83] Experimental data gathered from localised crack-generation, itself an important component of EMC Activation, has confirmed temperatures in this region as long ago as 1975.[84]

For EMC activation, the HEBM method used is a vibratory ball mill (VBM).[36] A VBM uses a vertical eccentric drive-mechanism to vibrate an enclosed chamber up to many hundreds of cycles per minute. The chamber is filled with the material being processed together with specialised objects called grinding media. In their most simple format, such media can be simple balls made from specialised ceramics. In practical terms, EMC Activation deploys a range of grinding media of different sizes, shapes and composites to achieve the required mechanochemical transformation.[4]

It has been suggested that a VBM will grind at 20 to 30 times the rate of a rotary ball mill, reflecting that a VBM's mechanism is especially rapacious.[85]

In simple terms, the compressive force ${\displaystyle F}$ acting between two identical colliding balls in a VBM can be expressed:[86]

${\displaystyle F=\left[\left({\frac {5m}{8}}\right)^{3/5}\left({\frac {2r}{9\pi ^{2}k^{2}}}\right)^{1/5}\right]v^{6/5}}$     where, ${\displaystyle k={\frac {1-v^{2}}{\pi E}}}$

where, ${\displaystyle m}$ is the mass of both balls, ${\displaystyle r}$ the radius, ${\displaystyle v}$ the absolute velocity of impact and ${\displaystyle E}$ the Young's modulus of the balls' material.[86]

As can be seen, an increase in velocity of impact increases ${\displaystyle F}$. The size and mass of the grinding media also contribute. ${\displaystyle F}$'s denominator term ${\displaystyle k}$ incorporates ${\displaystyle E}$ meaning that the nature of the material used for the grinding media is an important factor (${\displaystyle k}$ is ultimately squared in ${\displaystyle F}$, so its negative value is of no consequence). More fundamentally, due to the rapid vibration a high acceleration is imparted to the grinding media, whereupon the continuous, short, sharp impacts on the load result in rapid particle-size reduction.[85] In addition, high pressures and shear stresses facilitate the required phase transition to an amorphous state both at the point of impact and also during the transmission of shock-waves that can yield even greater pressures than the impact itself.[82]

For example, the contact time of a two-ball collision can be as short as 20μs, generating a pressure of 3.3 GPa upwards and with an associated ambient temperature increase of 20 Kelvin.[82] Because of the short duration of the impact, the rate of change in momentum is significant—generating a shock wave of duration only 1-100μs but with an associated pressure of 10 GPa upwards and a highly localised and focal temperature (i.e., at the nanoscale) up to several thousands of degrees Kelvin.[82] To place this into context, a pressure of 10GPa is equivalent to about 1,000 kilometers of sea water. As a further example, the impact of two identical steel balls of 2.5 cm diameter of velocity 1 m/s will generate a collision energy density of over 10⁹ joules/m², with alumina balls of the same 2.5 cm diameter and velocity of 1 m/s generating an even greater energy density.[86] The collisions occur in a very short timescale and hence the "rate of energy release over the relatively small contact area can be very high".[86]