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COmputationally Driven design of Innovative CEment-based materials

Final Report Summary - CODICE (Computationally driven design of innovative cement-based materials)

CODICE was an ambitious research project that brought producers and suppliers of cement-based materials together with universities and research institutes for the development of new and radically improved computational toolkits that monitor the formation and properties of cementitious materials by starting from macroscopic processing variables (size of the cement grains, water-cement ratio, temperature, cement grain composition).

The computational toolkits that have been developed in the CODICE project will radically boost the competitiveness of the European cement and construction sectors, since they will provide valuable and cheap tools to:
- other processing of cement (special attention is paid to the computational design of cements which entail lower carbon dioxide emissions and lower energy consumptions);
- decrease significantly the time and the costs of the quality control assessments;
- optimise the design of the cement-based materials (either in terms of their mechanical performance or their life-cycle analysis).

Apart from the knowledge of Tecnalia in the nano and sub-micro modelling of cementitious materials, the project has also incorporated the long-standing expertise of the TU Delft in the micro-modelling of cement-based materials and the superb know-how of University of Bonn in scientific computing. Besides, the project has run in parallel an experimental research programme devoted to validate the computational predictions. To this end the project also included the contribution of prestigious experimentalists from the University of the West of Scotland and the Instituto Eduardo Torroja. Finally, the partnership was completed by the solid commitment of CTG Italcementi, BASF SA and BIKAIN; three industrial companies which (all together) target the whole construction sector business and guarantee that their industrial feeling is properly addressed.

Cement-based materials, such as mortars, fibre-reinforced cementitious composites, and concrete are the most widely used construction materials in the world. Buildings, bridges, highways, airport runways, tunnels, quay walls, foundations, hazardous waste repositories, water retaining structures, and nuclear facilities are but a few examples of their wide range of applications. Cement-based materials, which are present in vast quantities in almost all built structures, are an essential part of our modern civilisation and, at present, they cannot be replaced by any other kind of material.

The omnipresence of cement-based materials belies their complexity. They are indeed complex composite materials with truly multi-scale internal structures that evolve over centuries: just by pouring cement powder to water, a myriad of chemical reactions take place to form a rigid, complex and porous structure that is called cement paste. This cement paste is a multiphase material which evolves over time. Basically it can be viewed as a composite in which calcium hydroxide crystals (portlandite), (sulpho)aluminate hydrates, and non-reacted cement powders are embedded into an amorphous nanostructured hydration product, the so-called calcium-silicate-hydrate (C-S-H) gel. This gel is the most important hydration product of cement based materials. It constitutes about 60-70 % of the cement paste, and is responsible for most of the properties of cement-based materials.

Albeit C-S-H gel constitutes the main ingredient of cementitious skeletons and the life-service of cementitious structures depends crucially on it, the possibility of tuning the intrinsic nature and properties of C-S-H gel has been obliterated from a technological point of view. The formation and properties of C-S-H gel rest on nanoscale processes and phenomena that until recently were out of reach. Fortunately, this long-standing impediment can be currently overcome by the complementary action of new experimental capacities which can resolve and operate at the nanoscale together with the development of stronger multi-scale simulations schemes which explicitly pay attention to the nanoscale.

Thanks to recent advanced nanoindentation experiments, there is now evidence that in terms of its mechanical properties the C-S-H gel can present itself either in a low stiffness and Low density (LD) C-S-H gel variety or in a variety with a high stiffness and High density (HD). This dissimilar bearing capacity is indeed much more pronounced in their resistance to osteoporosis-like degradation processes. Along their life-service, cementitious materials, akin to human bones, may lose calcium (the so called calcium leaching). This is a slow and long-term process, but it leads to a significant loss of strength / stiffness and impermeability, reduces the concrete's capacity to passivate the steel and ultimately causes the material to become significantly stress sensitive, which may eventually impair the structural integrity.

Thus, it is clear that there is plenty of room at the bottom of cementitious matrices. In fact the question which arises is straightforward: Could the formation of the stronger and more durable HD C-S-H varieties be promoted over the LD ones? If this is feasible both the mechanical performance and the resistance to the degradation of the cementitious skeletons would improve dramatically. Needless to say that if achieved, the whole construction sector would benefit tremendously from it. For instance, the cement industry could redefine its current market scenario, by including in their product range eco-cements or environmentally appealing cements (such as Belite Cements, which require lower temperature and lower carbon dioxide emissions to be produced) whose Achilles heel was their poor initial strengths. On the other hand, the envisioned more durable cementitious structures could make construction companies save important costs in terms of maintenance and repair.

The CODICE project aimed to give an answer to this challenging endeavour by taking advantage of the ever-growing simulation capacities. To this end, state-of-the-art simulation techniques were merged and smooth handshakes between the scales established.

The coarser description of the cementitious skeletons will be provided by the HYMOSTRUC three-dimensional model (micro-level simulations) developed by the Delft University of Technology (TUD). This well-tested code allows to trace the hydration of cement grains, the formation of interparticle contacts and to reproduce the micropore network. This coarse description of the hydration products will later be refined by both atomistic simulations (nano-level simulations) and Monte-Carlo simulations (sub-micro-scale simulations), from which the basic (nano)-building blocks of the hydration products will be identified and their colloidal aggregation reproduced. Furthermore, the mechanical properties of the cementitious skeletons will be predicted by Finite element model (FEM) simulations, in which the required information at the lowest level will be accurately fed by nano and sub-micro simulations.

Hence, the main objective of the CODICE project was to develop serial parameter-passing multi-scale modelling toolkits to:
1. monitor and predict the nano / microstructural formation and evolution of cementitious matrices as a function of macroscopic processing variables, such as the size of the cement grains, water / cement ratio, cement grain composition;
2. monitor and predict the degradation (ageing) of cementitious structures subjected to osteoporosis-like chemical degradation processes, as a function of macroscopic processing variables;
3. predict the mechanical performance of cementitious skeletons given their nano / microstructure (accurate structure-performance linkage).

Success in achieving CODICE's objectives is expected to:
1. Guide the design of new, stronger and much more durable cementitious materials by promoting the HD-C-S-H nanostructures over the LD- C-S-H ones. In particular, we envisage improvements on the mechanical properties of about 50 % and 600 % for non-degraded and degraded cementitious scaffolds respectively, when compared to conventional designs.
2. Boost the competitiveness of the European construction sector, since it will provide valuable and cheap tools to:
- optimise the processing of cement;
- decrease significantly the time and the costs of the quality control assessments;
- optimise the design of the cementitious materials (either in terms of their mechanical performance or their life-cycle analysis).

1 - CODICE breakthrough discoveries

Along the CODICE project an intensive experimental and computational research programme has been carried out. Several outcomes of our research are certainly noteworthy. The most outstanding ones briefly are:
A) quantitative study of hydration of C3S and C2S by thermal analysis;
B) synergy of T1-C3S and b-C2S hydration reactions;
C) deciphering the smallest units OF C-S-H gel;
D) the textural and mechanical characterisation of C-S-H gels from the hydration of synthetic pure T1-C3S, b-C2S and their blends;
E) hydration of C3S, C2S and their blends, micro and nanoscale characterisation;
F) mapping of mechanical properties of cement paste microstructures;
G) a model for the C-A-S-H gel formed in alkali-activated slag cements.

2 - CODICE toolkits
All the knowledge gained from our intensive computational and experimental activities have enabled the development of three validated computational toolkits. It should be noted that the main objective of CODICE was in fact the development of three serial parameter-passing multi-scale modelling toolkits to monitor and predict:
- the nano / microstructural formation and evolution of cementitious matrices as a function of macroscopic processing variables, such as the size of the cement grains, water / cement ratio, cement grain composition (CODICE_HYD toolkit);
- the degradation (ageing) of cementitious structures subjected to osteoporosis-like chemical degradation processes, as a function of macroscopic processing variables (CODICE_DEG toolkit);
- the mechanical performance of cementitious skeletons given their nano / microstructure (accurate structure-performance linkage) (CODICE_HYD toolkit).

CODICE socio-economic impact
CODICE strengthens the competitiveness of the European industry! The construction sector is of the utmost importance for the European economy. It represents 850 billion turnover, 10 % of Gross domestic product (GDP), and 12 million jobs, as well as a key sector in job generation (one additional job in construction is considered to generate two more in other areas). Unfortunately, the construction sector is mainly based on resource-intensive production schemes.

The CODICE project significantly contributes to transform this sector from a resource-intensive scheme into a knowledge-intensive one noticeably reducing its efforts (in time and money) to produce new high added value products. Moreover, the project results provide a cement processing optimisation leading to an essential reduction of carbon dioxide (CO2) emissions and energy consumption. The green paper on energy efficiency points out that the European Union (EU) could save at least 20 % of its energy consumption in a cost-effective way contributing to: security of supply, supporting the Lisbon agenda and the environmental protection and the EU's Kyoto obligations. The 'Energy efficiency' action plan (COM-2006-545) defines important energy saving potential that the results of the CODICE project will help to reach.

The impacts of CODICE on the whole construction sector are the following:
- The cement industry can benefit at least in a threefold way:
1. First, it can optimise its production to adjust the composition and the grains size distributions so as to fit better to the needs of its clients: flexible products for stringent conditions and clients!

2. Second, the developed modelling / simulation-based toolkits provide a more economical and efficient method (i.e. virtual laboratory) to test the feasibility and development of new cementitious possibilities. The production of cement grains entails high electric power consumptions and huge CO2 emissions. Unfortunately, the rule of thumb is that the higher the temperature for the production of the cement grains, the better the mechanical performance they offer. The simulation schemes developed in CODICE offer an unbeatable solution to the cement sector to assess and improve the efficiency of cheaper cement formulations (i.e. those requiring lower energy consumption and lower CO2 emissions for their production).

3. Lastly, the cement industry can save considerable time and costs in terms of testing and quality controls. Prior to be sold, cements must pass stringent quality tests. Current testing on cementitious material takes long time and is expensive.
- Suppliers of admixtures, formulations and cement-based materials, together with recast and building companies can similarly benefit from the advent of the herein designed multi-scale simulation toolkits. For example various production / processing parameters, chemical / mineral admixtures can be easily studied to find the best combinations for a specific requirement (e.g. early strength, durability). Improved designs for specific applications!

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