Construction chemistry

Our approach to construction chemistry is anchored in the belief that the environmental footprint of concrete can be substantially reduced through the combined optimization of both cement hydration and chemical admixtures. For a reference text on the subject, you may consult our book “external pageScience and Technology of Concrete Admixtures”.

Cement

Concrete is the most used material worldwide, after water. It’s binder, cement, represents only about 15% of the concrete mass, but its production accounts for 5-8% of manmade CO2. To reduce this represents a major objective for the construction sector.

Unfortunately, this is not trivial, in particular if one does not want to compromise on any properties of ordinary cement. In this respect, the most used approach consists in replacing part of the cement by Supplementary Cementitious Materials (SCM), which must have a lower environmental impact, be cheap and available in large amount, in addition of course to reacting as well with water as cement and being durable.

While a variety of promising solutions explored around the world, they most generally struggle in terms of early strength development and often rheology control. Our interest in cement chemistry lies in addressing these issues through the use of combinations of chemical admixtures, of which the working mechanisms must be carefully orchestrate to get the most out of the complex coupled chemical systems.

Hydration
Schematic representation of hydration of model cement as depicted in Marchon et al. J. Am. Ceram. Soc. 2017

Chemical admixtures

Chemical admixtures present a great opportunity to achieve big effects with small additions to concrete or other particulate systems. They can be described as the “spices” of concrete. As previously mentioned, they operate by modifying interfacial properties in the system. This is illustrated in Fig. 1, where the properties influenced by the different phases in the system are presented in a ternary diagram.

Triphase
Illustration of the role played by different interfaces in the cementious materials. S-S = Solid-Solid, L-L = Liquid-Liquid, V-V = vapour- vapour and S-L-V = Solid-Liquid-Vapour.

In regard to research in this area, it should be noted that the developments in nano-sciences have opened completely new possibilities. For example, it is thanks to detailed and systematic studies by atomic force microscopy that it has been able to understand the conformational behavior of polycarboxylate superplasticizers at solid-liquid interfaces [external pageFlatt el al. (2009)].

The interfaces of greatest interest are those between solids and liquids. They control for example the state of flocculation that in turn strongly impacts the rheology of particulate systems as concrete. They also determine the extent to which dispersants can adsorb and modify flocculation. While many basic issues in this field have been clarified over the past few years, several of unknowns subsist. Two examples concern either performance at low dosages (case mostly found in practice) or the behavior of admixture blends (case of most commercial products). These issues are very relevant to practice and their underlying scientific questions cover a variety of very interesting and basic issues as the competitive adsorption of organic admixtures in relation to their molecular structure].

One factor that perturbs the working mechanisms of dispersants is their incorporation into layered minerals either pre-existing in the mix (clays) [external pagePlank et al, (2006a,external pageb)] or formed by hydration (calcium aluminate layered double hydroxides) [external pageGiraudeau et al. (2009); external pagePlank et al, (2006a,external pageb)]. Such issues will become of growing importance due to the increased use of blended cements (higher aluminate contents) and the need to use local aggregates (clay minerals cannot be easily removed when present). Understanding how to prevent or engineer admixture sequestration by these layered minerals presents a challenging topic in which surface science has a major role to play.

Another area where solid-liquid interfaces have recently found to be more important than previously suspected is in (early) hydration kinetics [external pageJuilland et al., (2010)]. Building on this, it can be expected that the effect of many admixtures on hydration can be rather directly related to either dissolution kinetics or hydrate nucleation. We intend to push the degree of comprehension of these phenomena down to the molecular level and understand for example how chlorides enhance slag reactivity. Such work would provide new insight into alternative ways of how to enhance the reactivity of blended cements, which could involve molecular design of organic admixtures.

We are studying such phenomena down to the molecular level and have for example made substantial progress in relating quantitatively the retardation of cement hydration to the molecular architecture of PCE superplasticizers [external pageMarchon et al. (2017)]. Such work, along with other studies on the impact of chemical admixtures on hydration kinetics, is providing new insights into alternative ways to enhance the reactivity of blended cements.

jaceImage
Schematic representation of the retarding effect of comb- copolymers on cement hydration kinetics from Marchon et al. J. Am. Ceram. Soc. 2017

The liquid-vapor interfaces have also been used for decades to entrain air in an effort to enhance freeze-thaw resistance. More recently, modifications of the same interface have been exploited to reduce drying shrinkage and the cracking that can result. While taken individually, the use of such technologies is relatively well understood, a real challenge appears when attempting to combine them. Problems are also reported to arise in some blended cement systems. Here one can envision that to understand and control at a molecular level the interactions between these competing admixtures.

Finally, it should also be noted that solid-air interfaces should not be neglected since they impact agglomeration in the dry state. This is of great technological importance during the grinding of mineral powders or simply their handling. For example, the cement industry is has been using additives defined as grinding aids for several decades. On this topic, we have conducted a collaborative multiscale investigation of the mechanisms at stake [external pageMishra et al. (2015)], with from our group a particular focus on molecular modelling [external pageMishra et al. (2017)]. Moreover, in this sector there is a growing interest to move from grinding aids towards cement improvers. In addition to improving the grinding, these admixtures are also intended to positively affect later stage properties such as rheology or strength. This is of particular interest for improving the performance of cements with a low carbon footprint and a natural connection to our more general interest on manipulating cement hydration through tailored admixtures.

Reference:

Giraudeau C., d'Espinose de Lacaillerie J.-B., Souguir Z., Nonat A., Flatt R.J. “Surface and intercalation chemistry in cementitious materials and its implications in rheology”, J. Am. Ceram Soc. 92 [11] (2009) 2471-2488.

Jimenez-Gonzalez I. Efecto de los ciclos de humedad-sequedad en el deterioro de rocas ornamentales que contienen minerales de la arcilla (Effect of wetting and drying cycles on the deterioration of clay-bearing stones), PhD University of Granada, 2008.

Juilland P., Gallucci E., Flatt R.J. and Scrivener K.S., “Dissolution theory applied to the induction period in alite hydration”, Cem. Concr. Res. 40 (2010) 831-844.

Plank J., Dai Z., and Andres P. R., ‘‘Preparation and Characterization of New Ca–Al–Polycarboxylate Layered Double Hydroxides,’’ Mater. Lett., 60 [29–30] (2006) 3614–7.

Plank J., Keller H., Andres P. R., and Dai Z. M., ‘‘Novel Organo-Mineral Phases Obtained by Intercalation of Maleic Anhydride–Allyl Ether Copolymers Into Layered Calcium Aluminum Hydrates,’’ Inorg. Chim. Acta, 359 [15] (2006) 4901–8.

Scherer G.W., Flatt R.J. and Wheeler G.W. “Materials science research for the conservation of sculpture and monuments” Mat. Res. Bull., 26 [1] (2001) 44-50.

Marchon, D.; Juilland, P.; Gallucci, E.; Frunz, L.; Flatt, R. J. Molecular and Submolecular Scale Effects of Comb-Copolymers on Tri-Calcium Silicate Reactivity: Toward Molecular Design. J. Am. Ceram. Soc. 2017, 100 (3), 817–841.

Flatt, R. J.; Schober, I.; Raphael, E.; Plassard, C.; Lesniewska, E. Conformation of Adsorbed Comb Copolymer Dispersants. Langmuir 2009, 25 (2), 845–855.

R. K. Mishra, D. Geissbuhler, H. A. Carmona, F. K. Wittel, M. L. Sawley, M. Weibel, E. Gallucci, H. J. Herrmann, H. Heinz & R. J. Flatt (2015) En route to multi-model scheme for clinker comminution with chemical grinding aids, Advances in Applied Ceramics, 114:7, 393-401

Mishra, R. K.; Weibel, M.; Müller, T.; Heinz, H.; Flatt, R. J. Energy-Effective Grinding of Inorganic Solids Using Organic Additives. Chimia (Aarau). 2017, 71 (7–8), 451–460

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