Cerdanyola del Vallès, 13th February 2019. Mortar and concrete preparation is a complex process, affected by different chemical reactions difficult to replicate. The properties of concretes improve when the amount of water is reduced. The cement paste with less water has higher density, and thus higher compressive and flexural strength, lower permeability and higher resistance to weathering; as well as it yields a reduction on the volume change from drying and wetting that minimizes shrinkage cracking tendencies. However, when the content of water is reduced the mortars and concretes become more viscous reducing its workability and increasing potential placement issues.
Polycarboxylate (PCE) superplasticizers make concrete more fluid with less water, resulting in a substantial enhancement in workability, allowing the reduction of the water content of the paste. However, polycarboxylate superplastizers are very sensitive to the clays that may contain the sand used for the production of mortars and concretes.
Therefore it is of prime importance to understand the interaction between polycarboxylate superplastizers and clays to improve the performance of this complex mixtures. The behavior of fresh paste has been investigated at beamline NCD-SWEET of ALBA Synchrotron by researchers from BASF Chemical Company, Universitat Politècnica de Catalunya (UPC) and ALBA Synchrotron. This research has served to develop an in-situ methodology that properly characterizes the interaction between Polycarboxylate and clays. Previous methodology used centrifugation and gentle heating of the pastes and this research has shown that it led to artifacts due to this sample preparation. Synchrotron radiation allows to measure the cement pastes without any sample preparation step.
"The presence of clays in the cement pastes inhibits the dispersing capability of polycarboxylate superplasticizers, thus diminishing its capability to make concrete and cements fluid. The methodologies previously used for the characterization of the interaction PCE-clay did not considered the alterations induced by the centrifugation and drying processes in the PCE admixture, thus the true interaction was not correctly measured. So it was necessary to measure the interaction in the fresh samples, avoiding the drying process", says Pere Borralleras from BASF, which is also exemplified in the figure below.
"Synchrotron light in this experiment was key as it allowed measuring the intercalation of superplasticizers in clays in the fresh paste. This way the study is not biased for the sample preparation step", says Ignacio Segura from the Department of Civil and Environmental Engineering from the UPC.
This new methodology will allow the development of better PCE superplasticizers (more resilient to ubiquitous clays) in the years to come. It should also help to sustainability as washing sand is a common practice that it consumes precious fresh water. "Sustainability is at the core of every possible research at ALBA" says Prof. Miguel A.G. Aranda, ALBA scientific director and co-author of this publication.
This research has not only provided novel results but is also an example of collaborative efforts between industry and academy and has originated a publication in the second-highest impact factor journal of the construction and building technology.
Figure: X-ray powder diffraction pattern for clay with increasing PCE content: a) fresh paste measured using synchrotron light, b) dried paste collected with Cu Kα diffractometer.
