Controlled superplasticizer addition: effects on rheology, adsorption behavior and compressive strength

Rheological properties of mortar and concrete can be controlled by the mixing regime, the environmental conditions as well as the adjustment of cement and superplasticizer and the dosage time of the agent. The present work presents the influence of a delayed addition of different superplasticizers (2 PCEs, PNS, PMS) to cement pastes. The focus of the investigations was on the rheological parameters, on the adsorption behavior of the superplasticizers and on the compressive strength development of the pastes.
With delayed addition of all investigated superplasticizers, an expected decrease in yield stress could be observed. Furthermore, a time dependent effect on yield stress development as well as on slump loss could be monitored which is important especially for the investigated PCEs. The study in TOC content in the pore solution showed a high and time independent adsorption ratio of PNS and PMS. In contrast, the adsorption behavior of the investigated PCEs was more influenced by the addition time of the agent to cement paste.
The results of the compressive strength tests showed an increase with delayed superplasticizer addition. This development is due to the better dispersing effect with delayed addition and the finer microstructure of the hardened paste.
Altogether, the delayed addition of superplasticizers could be a fine possibility to reduce the active agent content or to improve the dispersing effect with the same superplasticizers ratio in mixture. The results of the present study will help to find the best addition time and dosage of the different superplasticizers to investigate mechanisms for controlled release afterwards.

Author: H. von Daake, D. Stephan

Section: Science and production

Keywords: superplasticizer, delayed addition, rheology, pore solution, superplasticizer adsorption, strength development