The workability of concrete strongly depends on the rheological properties of cement pastes, which in turn are controlled by the complex system of reactions taking place during hydration process. Addition of organic additives used as superplasticizers modifies the kinetics of hydration reactions thus influencing chemical and physical properties of cement pastes. Many investigations have been carried out to disclose the effect of additives on rheological properties in cement and concrete [1-2]. Recently, detailed in-situ x-ray diffraction studies on the organic additives influence during hydration of Portland cement have been reported [3]. The present work extends the investigation to the behaviour of novel generation polymeric additives (poly-carboxylic-acid esthers based superplasticizer).
Cement pastes were prepared mixing ordinary Portland cement (CEM I 52,5) with water or solutions containing additives. To monitor the formation of gel-like (amorphous) phases during hydration process, a known amount of non-reacting internal standard (rutile) was added to the cement mixtures. Several glass capillaries were filled with the cement pastes and kept in a thermostatic bath at 40°C.  The hydration process was followed for the first 8 hours, taking snapshots at different times for each sample. Time resolved x-ray diffraction measurements were performed at the Swiss Light Source Material Science beamline (X04SA) equipped with a Mythen 1-D microstrip detector.
Phase quantification by Rietveld analysis indicates different rates of ettringite formation and C3S consumption between the cement pastes with and without additives (Figure 1). The refined values of the internal standard phase fractions record a slight increase with the hydration proceeding. This reveals the presence of an evolving amorphous component in the system.

Figure 1. Phase fractions time evolution of hydrating cement pastes with and without superplasticizer additive.

References

[1] S. Hanehara, K. Yamada, Interaction between cement and chemical admixture from the point of cement hydration, absorption behaviour of admixture, and paste rheology, Cem. Concr. Res. 29 (8) (1999) 1159–1165.
[2] C. Jolicoeur, M.-A. Simard, Chemical admixture–cement interactions: phenomenology
and physico-chemical concepts, Cem. Concr. Compos 20 (2–3) (1998) 87–101.
[3] M. Merlini, G. Artioli, C. Meneghini, T. Cerulli, A. Bravo and F. Cella, The early hydration and set of Portland ements: In situ X-ray powder diffraction studies, Powder Diffraction, 22, 201-208 (2007).

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