Title:
The Use of Phosphonates as Dispersants and Rheology Modifiers in Mortar and Concrete
Author(s):
M. Bellotto and F. Faini
Publication:
Symposium Paper
Volume:
262
Issue:
Appears on pages(s):
237-248
Keywords:
hydration; microstructure; rheological modeling.
DOI:
10.14359/51663235
Date:
10/1/2009
Abstract:
During the early stages of hydration, cement paste develops a structure which will ultimately lead to setting through a depercolation process. This structuring
process is reversible until setting, meaning that it can be destroyed by imposing a mechanical shear stress but it will rebuild with approximately the same kinetics. The driving force for this process lies in the attractive forces acting among the cement particles in the course of hydration.
A wealth of organic admixtures is used to modify the interparticle forces and
consequently altering the fresh state properties of cement paste, and we will
concentrate in the following on superplasticizers and retarders. Both these classes of admixtures modify the microstructure of cement paste and yield different mechanical properties and different kinetics of the structuring process. The evolution of mechanical properties is followed by rheological measurements in the oscillatory mode, which enables to determine the storage and loss modulus of cement paste. These properties can be interpreted along a modeling scheme referring to a heterogeneous composite material, similarly to what has been done with nanoindentation and ultrasonic measurements.
It is shown that the difference between superplasticizers and retarders lies only in a different grading of the same basic interaction with the hydrating surfaces, and in fact it is possible to devise molecular structures yielding a dispersing retardant or a slump-retaining superplasticizer, i.e., the intermediate member between the two families. In a similar way, it is possible to tailor the interaction of the organic molecules with the hydrating surfaces in order to develop more robust admixtures with regards to variations of the interstitial solution chemistry.