Superplasticizers (SPs), also known as high range water reducers, are additives used for making high-strength concrete or to place self-compacting concrete. Plasticizers are chemical compounds enabling the production of concrete with approximately 15% less water content. Superplasticizers allow reduction in water content by 30% or more. These additives are employed at the level of a few weight percent. Plasticizers and superplasticizers also retard the setting and hardening of concrete. According to their dispersing functionality and action mode, one distinguishes two classes of superplasticizers:

Ionic interactions (electrostatic repulsion): lignosulfonates (first generation of ancient water reducers), sulfonated synthetic polymers (naphthalene, or melamine, formaldehyde condensates) (second generation), and;

Steric effects: Polycarboxylates-ether (PCE) synthetic polymers bearing lateral chains (third generation).

Superplasticizers are used when well-dispersed cement particle

Modified poly(carboxylate ether)-based superplasticizer for enhanced flowability of calcined clay-limestone-gypsum blended Portland cement

Tunahan Aytas, Aurélie Rosine Favier, Amin Hodaei, Karen Scrivener, Buse Tatli

Supplementary materials that are utilized to replace ordinary Portland cement (OPC) decrease the workability of the cementitious mixtures and superplasticizers are usually added to cement to control their fluidity. In general, current superplasticizers are solely optimized for single component systems such as OPC. Here, we report the performance of a series of a modified poly(carboxylate ether)-based superplasticizers (PCEs) in a ternary OPC-calcined clay-limestone blend. We have utilized: i) a co-monomer with high ionic character, 2-acrylamido-2-methylpropane sulfonic acid (AMPS) to be incorporated into the acrylic acid backbone of PCEs and ii) low density of polyethylene glycol (PEG) grafted chains < 0.01 mol/mol. The optimized polymer does not intercalate into the layered structure of calcined clay and preserves its steric size in the presence of high concentration of sulfate ions.

Elsevier2017

Adsorption de polycarboxylates et de lignosulfonates sur poudre modèle et ciments

François Perche

Placing concrete requires much more water than the cement needs for its hydration. This results in an important porosity in the hardened concrete, which accentuates the degradation of this material. By adding small amounts of polymeric admixtures, called superplasticizers, to the fresh concrete one can significantly reduce the amount of water required to obtain the suitable workability. The plasticizing effect of superplasticizers has been studied for many years, and remains today, an ongoing field of research. This research led to a better understanding of the forces that act during the deflocculation and dispersion of cement grains induced by superplasticizers. This research work was performed within the framework of the "Superplast" European project. Its main objective has been to determine which parameters (molecular structure, induced charge, adsorption mode, molar mass) influence the plasticizing effects of superplasticizers. The results of this study will allow synthesis new polymeric admixtures with better performances than those currently available. For this study, two types of superplasticizers were selected: lignosulfonates and polycarboxylates. For each one, different samples were synthesised and characterised. For each sample, adsorption, rheological and interaction forces measurements were performed by our partners while we have performed adsorption and electroacoustic measurements on model powder suspensions. Different cement model powders were investigated and a magnesium oxide was selected. The particle size distribution and reactivity were carefully characterized. The inert model system (MgO) allowed us to study adsorption mechanisms without the complexity linked to cement hydration reactions that modify surface and solution of the suspension. Particle surface charge and pH are the two main parameters that influence the polymer adsorption and the polymer conformation. Magnesium oxide which has a high isoelectric point (around at pH 12.4) allows a surface charge similar to cement suspensions at high pH. First, we have measured the adsorption isotherms of all superplasticizers on model suspensions in NaOH (0.01M). This study allowed us to evaluate the affinity of each polymer for MgO and its adsorption plateau. These measurements showed that the lignosulfonates have a higher affinity than the polycarboxylates. They also showed that lignosulfonate adsorption is mainly influenced by their molar mass and their carboxylic group content (for similar sulfonate group content) while polycarboxylate adsorption is mainly driven by the backbone length, the side chain length and the carboxylic group content. Adsorption plateaux allowed us to calculate the surface coverage ratio and to estimate the superplasticizer conformation on the surface. Adsorption isotherms were finally measured on a Portland cement. The polymer adsorption is influenced by the same parameters as on MgO. The model system MgO is representative for the polymer adsorption on cement. In a second step, different electrolytes were added to model suspensions. This practice allowed us to study separately the effect of the main ions present in cement suspensions (Na+, Ca2+, SO42-, OH-) and to mimic the ionic composition of the aqueous phase of the cement suspension. Lignosulfonate adsorption is neither influenced by the studied ions or the pH. Only an increase of ionic strength increases the adsorbed polymer mass. Polycarboxylate adsorption is influenced by calcium and sulfate ions and by the particle surface charge. Finally, superplasticizer adsorption was studied on different cements by our partners and on a fly ash and a silica fume by ourselves. Lignosulfonate adsorption isotherms on cements showed that affinity and adsorbed polymer mass increases with the C3A content and decreases with the alkalis content. The isotherms measured on industrial by-products showed that superplasticizers adsorb on fly ash, but not on silica fume.

EPFL2004

Concrete Performance of Limestone Calcined Clay Cement (LC3) Compared with Conventional Cements

François Henri Avet, Karen Scrivener, Lionel Sofia

This study aims to demonstrate the robustness of concrete made of limestone calcined clay cement (LC3). Two campaigns were carried out using 375 kg.m(-3) and 500 kg.m(-3) of binder and targeting plastic and self-placing workability, respectively. The concrete performance of LC3 was compared with plain portland cement (PC), slag cement, and a blend containing silica fume. The density of concrete was found very similar between the different types of binders. The amount of superplasticizer required to reach the workability criteria was higher for the blend with silica fume compared with LC3. Moreover, the increase of workability required the same increment of superplasticizer, independent on the type of binder. In terms of strength, 57.2 MPa and 72.3 MPa were reached at 28 days for LC3 for the first and for the second campaign. Similar strength to PC is obtained for LC3 for all ages for the first campaign, outperforming slag concrete. For the second campaign, similar strength to PC is reached at 7 days. Very low absorption and resistance to freeze-thaw with deicing salt were measured. The air permeability was found lower for LC3 than PC. CO2 emissions per unit strength are significantly reduced for LC3 compared with PC, and with other blends for the first campaign.

AMER SOC TESTING MATERIALS2019

Adsorption de polycarboxylates et de lignosulfonates sur poudre modèle et ciments

François Perche

EPFL2004