- 23 - 24 April 2018 Empa, Dübendorf, Switzerland
Projects 1, 2, 3, 4 and 5 are transversal and provide important input for many of the other projects. The first three of these are concerned with the use of supplementary cementing materials, SCMs, which are being increasingly used in combination with Portland cement clinker in the interests of sustainability. Examples are fly ash, a by-product of coal fired power stations, slag from iron blast furnaces, silica fume from ferrosilicon production, fine limestone and natural pozzolanic minerals.
These, by-products of other industrial processes or natural materials which are not thermally treated, reduce the amount of Portland cement clinker so reducing raw materials and energy consumption and CO2 emissions. The current level of clinker replacement by SCMs in Europe is around 20% . There are three main factors which currently limit this level of substitution:
Cements are not at equilibrium, either internally or with their service environment. Reactions are driven by thermodynamics but are mediated by kinetics, for example, by restraints on chemical fluxes through an inhomogeneous and porous matrix. The challenge is to relate equilibrium and kinetics to predict performance in an increasingly diverse range of cements with supplementary cementing materials.
Deterioration of cementitious materials by sulfate ions is a concern for concrete in contact with ground water in many parts of Europe, and is an important issue for underground construction – identified as a major area for progress in the construction industry. Sulfate resistance relates closely to testing and standardisation: a European standard is urgently needed. But existing national standards lack credibility. Most importantly the performance of cements containing SCMs is not agreed between different countries.
Over a decade of work under the auspices of standards committee TC51 has shown that none of the present methods are suitable as a performance test, due to poor reproducibility and important discrepancies between performance in the field and in the laboratory. The research theme to be studied by the RTN has been articulated through discussions with members of TC51, several of whom belong to the institutional and industrial partners of the Network (VDZ, Aalborg, BAM, IETCC, DTI, ZAG).
The programme (Training researchers to generate knowledge on deterioration of cement matrices) proposed by Nanocem was endorsed by a resolution of TC51 in October, although this committee has no funds to support research:
"Resolution 392 (Lisbon): 7/8 October 2004 -Subject: Research programme on sulfate resistance CEN/TC 51 supports the Nanocem Network sulfate resistance research programme as an essential pre-normative activity and as a precondition for a performance orientated standard on sulfate resisting cement. The decision was taken by unanimity"
The prime function of cementitious materials is structural. While materials can be formulated to achieve a specified set of long-term properties, evolution of properties at early ages cannot be well predicted. Concretes are usually specified by their strength, after 28 days wet curing, but are frequently put into service at much younger ages, thus the rate of strength development and particularly its predictability are very important to efficient and safe construction and to durability.
Models will be developed to relate microstructural and strength development; particularly to support the use of SCMs. A number of modelling approaches have been developed, but progress has been limited through failure to bring together the critical mass of research expertise and experimental data needed to develop robust predictive models, applicable across a wide range of cementitious materials.
For the first time the necessary expertise will be available within the Network to generate the underlying understanding of the material properties; together with the industry, who can provide materials to a high level of specification. The research teams combine experts in mechanics and modelling, with experimentalists and the characterisation methods needed to make progress.
Mechanical properties should be divided into elastic properties of the bulk material and the fracture mechanical properties. Together these two sets of properties determine damage development (micro-cracking) as ultimately strength.
Despite the traditional image of cementitious materials, there is considerable potential for further development of innovative, knowledge based materials. For example visible concrete may be structural but, structural or not, concrete still has an aesthetic function. This function often deteriorates with time; surfaces become dirty and drab and improvement in aesthetic durability would be an important advance.
This theme brings together several projects with explore the next generation of multifunctional cementitious materials. The aspects which will be examined are: