Energetically modified cement
Energetically modified cements are a class of cementitious materials made from pozzolans, silica sand, blast furnace slag, or Portland cement.
Classification and field-usage potential
The term "energetically modified cement" refers to a class of cementitious materials which have been produced using a special activation process. This process is entirely mechanical, as opposed to thermal, and is carried out by finely grinding the materials to increase binding capacity.There are several types of energetically modified cements, depending on the raw materials used; all contain some proportion of conventional Portland cement, which may itself undergo EMC Activation, and many contain supplementary cementitious materials. EMCs can be produced with less energy and carbon dioxide production than traditional cements.
Each type of energetically modified cement has its own performance characteristics, including mechanical load. The most frequently used EMCs are made from fly ash and natural pozzolans; these are relatively abundant materials, and the performance characteristics are relatively close to those of Portland cement while providing energy and carbon dioxide savings. EMC products have been extensively tested by independent labs.
History
The term "energetically modified cement" was first used in Sweden, where the EMC Activation process was developed in 1992 by Vladimir Ronin at Luleå University of Technology. The term was introduced in a paper by Ronin et al.. The process was refined by Ronin and others, including Lennart Elfgren. Continuing academic work and research "self-healing" properties of energetically modified cements is ongoing at LTU.At the 45th World Exhibition of Invention, Research and Innovation, held in 1996 in Brussels, Belgium, EMC Activation was awarded a Gold Medal with mention by EUREKA, the European inter-governmental organisation.
The research work connected with EMCs has received awards from the Elsa ō Sven Thysells stiftelse för konstruktionsteknisk forskning of Sweden.
Chemistry
Through the use of pozzolans in concrete, porous Portlandite can be transformed into hard and impermeable 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.EMC Activation is a process which 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 highly reactive pozzolans can yield further stabilisation benefits upon the pozzolanic reaction-pathways.
In concrete, 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 and calcium hydroxide. This reaction can be noted in three ways, as follows:
The underlying hydration reaction forms two products:
- Calcium silicate hydrate, 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, which in concrete chemistry is known also as Portlandite. In comparison to calcium silicate hydrate, Portlandite is relatively porous, permeable and soft. 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.
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. 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 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 that may be represented as follows:
Further, many pozzolans contain aluminate that will react with Portlandite and water to form:
Pozzolanic 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:
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 may of itself yield stable calcium aluminium garnet. This can be represented per the following equation:
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. However, adding high-reactivity pozzolans into the concrete mix prevents such a conversion reaction. In sum, whereas pozzolans provide a number of chemical pathways to form hardened materials, "high-reactivity" pozzolans such as blast furnace slag 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" according to U.S. standard ASTM C989.
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.
Self-healing properties
Natural pozzolanic reactions can cause mortars and concretes containing these materials to "self-heal". The EMC Activation process can at times increase the likelihood of the occurrence of these pozzolanic reactions. The same tendency been noted and studied in the various supporting structures of Hagia Sophia built for the Byzantine emperor Justinian. 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.Range of characteristics
Concretes made from energetically modified cements can be designed to exhibit superior strength and durability or to exhibit rapid and ultra-rapid hardening. This depends upon the "pozzolanic" characteristics of the raw material that is employed to make it. For example, fly ash in its natural state is typically, but not always, more "pozzolanic" than volcanic ash.An important consideration in choosing a concrete material is its strength-development within a specified time period, which must match or exceed a project's specifications. According to Ronin, the current upper-limit for using EMC made from natural pozzolans is 60% for practical large-scale usage.
Less noxious emissions and low leachability of EMCs
The EMC activation of fly ash is a mechanical process, and does not involve heating or burning. Leachability tests were performed by LTU in 2001 in Sweden on behalf of a Swedish power production company. These tests confirmed that EMC made from fly ash "showed a low surface specific leachability" with respect to "all environmentally relevant metals."EMCs using California volcanic ash
Energetically modified cements have been used in large infrastructure projects in the United States.> EMCs made by replacing at least 50% of the Portland cement with have yielded consistent field results in high-volume applications. This is also the case for EMC made from natural pozzolans.Volcanic ash deposits from Southern California were independently tested; at 50% Portland cement replacement, the resulting concretes exceeded requirements. At 28 days, the compressive strength was 4,180 psi / 28.8 MPa. The 56-day strength exceeded the requirements for 4,500 psi concrete, even taking into account the safety margin as recommended by the American Concrete Institute. 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. The surface smoothness of pozzolans in the concrete was also increased.
Durability of concretes produced and high-performance concretes (HPCs)
Treating Portland cement with EMC activation will yield high-performance concretes. These HPCs will be high strength, highly durable, and exhibiting greater strength-development in contrast to HPCs made from untreated Portland cement. Concrete made from ordinary Portland cement without additives has a relatively impaired resistance to salt waters. 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.Energetically modified cements also exhibit high resistances to chloride and sulphate ion attack, together with low alkali-silica reactivities. Like all concretes comprising pozzolans, they are more durable than concretes made from Portland cement.
Projects using EMC
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.In the United States, energetically modified cements have been approved for usage by a number of state transportation agencies, including PennDOT, TxDOT and CalTrans.
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. These projects include sections of Interstate 10. In these projects, EMC replaced at least 50% of the Portland cement in the concrete poured. This is about 2.5 times more than the typical amount of fly ash in projects where energetic modification is not used. Independent text data showed acceptable 28-day strength requirements in all projects.
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 was a factor.