page_title: Energetically modified cement
section_title: Effect on Pozzolanic reactions
hierarchical_section_title: Energetically modified cement / EMCs using Pozzolans such as volcanic materials / Effect on Pozzolanic reactions
caption_attribution_description: English: Pozzolan (volcanic ash) deposits in Southern California
context_page_description: Energetically modified cements are a class of cements made from pozzolans, silica sand, blast furnace slag, or Portland cement. The term "energetically modified" arises by virtue of the mechanochemistry process applied to the raw material, more accurately classified as "high energy ball milling". This causes, amongst others, a thermodynamic transformation in the material to increase its chemical reactivity. For EMCs, the HEBM process used is a unique form of specialised vibratory milling discovered in Sweden and applied only to cementitious materials, here called "EMC Activation".
  Energetically modified cements have a wide range of uses. For example, EMCs have been used in concretes for large infrastructure projects in the United States, meeting U.S. concrete standards.
context_section_description: EMC Activation is a process that increases a pozzolan's chemical affinity for pozzolanic reactions. This leads to faster and greater strength development of the resulting concrete, at higher replacement ratios, than untreated pozzolans. 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".
  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. 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:
  Standard notation:  
  Balanced:  
  Cement chemist notation (the hyphenation denotes the variable stoichiometry):  C₃S  +  H   →   C-S-H  +  CH
  The underlying hydration reaction forms two products:
  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.
  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). It is also sectile, with flexible cleavage flakes. Portlandite is soluble in water, to yield an alkaline solution which can compromise a concrete's resistance to acidic attack.
  Portlandite makes up about 25% of concrete made with Portland cement without pozzolanic cementitious materials. In this type of concrete, carbon dioxide is slowly absorbed to convert the Portlandite into insoluble calcium carbonate (CaCO₃), in a process called carbonatation:
  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. This results in a denser, less permeable and more durable concrete. This reaction is an acid-base reaction between Portlandite and silicic acid (H₄SiO₄) that may be represented as follows:
   
  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);  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.
  Pozzolanic cement chemistry (along with high-aluminate cement chemistry) 
caption: Volcanic ash deposits situated in Southern California, USA.
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