Adsorption of plasticizers – Influence of plasticizer and cement type

Adsorption of the plasticizing agents sulfonated naphthalene formaldehyde condensate, lignosulfonate and polyacrylate was measured by UV spectroscopy and Total Organic Carbon (TOC) on five different cement slurries. The rheological properties of the slurries were measured by a parallel plate rheometer as a function of plasticizer and cement type, time and plasticizer dosage. INTRODUCTION The performance of superplasticizers in cementitious systems has been found to depend on cement fineness, cement composition, mode of introduction into the mixture, as well as the type and chemical composition of the superplasticizer itself . This paper investigates how the adsorption behaviour of the three superplasticizers sulfonated naphthalene formaldehyde condensate (SNF), lignosulfonate (LS) and polyacrylate (PA) depend upon various cement characteristics and how it affects rheological properties of cementitious pastes. The experimental work was done partly at EMPA in Switzerland and partly at SINTEF in Norway. EXPERIMENTAL The ultraviolet absorption spectrum of dilute aqueous solutions provides a method for the detection of aromatic and other UV-active (conjugated bonds) groups in superplasticizers. The UV spectra of different superplasticizers (SNF, sulfonated melamine formaldehyde (SMF) and lignosulfonate) are quite different and can be used for chemical identification purposes. Polyacrylate-type products generally do not contain aromatic groups and do not show UV-adsorption in a readily accessible spectral region, i.e.>200 nm. The cementitious pastes where prepared from the cements 1-5 which are described in Table 1. Mixtures with SNF and LS were prepared with a w/c ratio of 0.40. The blending of the pastes was performed in a high shear mixer from Braun (MR5550CA) by adding solids to the water and mix for 1⁄2 minute, resting for 5 minutes and blending again for 1 minute. The superplasticizers were either added with the water first (immediate addition) or 5 minutes after water addition (delayed addition). Rheological parameters were recorded 20 minutes after water addition by a parallel plate (1 mm gap, upper plate serrated to 150 μm depth) rheometer MCR 300 produced by Physica. The measurements were done isothermally at 20C. A shear stress shear rate curve was measured with linear sweep of shear rate from 200 down to 2 s in 30 points lasting 6 s each. Hysteresis, gel strength and oscillation sequences were included in the sequence, but will not be discussed further. The full measurement sequence lasted 25 minutes and was repeated 4 times to monitor time dependencies: