We have studied the rheological behavior of concentrated cement suspensions in the absence and presence of comb polymers comprised of a polyacrylic acid (PAA) backbone and charge-neutral, poly(ethylene oxide) (PEO) teeth. These species possessed a uniform backbone molecular weight and graft density, with varying teeth molecular weight. Both PAA, a linear polyelectrolyte, and PAA/PEO comb polymers imparted initial stability to concentrated cement suspensions above a critical weight fraction, w* of 4 mg/(g of cement). Cement-PAA suspensions, however, set prematurely. Their rapid, irreversible stiffening stemmed from deleterious interactions between PAA and multivalent counterions in solution. Interestingly, the presence of PEO teeth comprised of only a few monomer units in length mitigated such interactions. The rheological property evolution of concentrated cement-PAA/ PEO suspensions exhibited complex behavior ranging from the reversible gel-like response observed at short teeth lengths to a remarkable gel-to-fluid transition observed during the deceleratory period for systems comprised of longer PEO teeth. At longer hydration times, all cement-PAA/PEO suspensions exhibited initial elastic modulus values, G i ϳ exp(t/ c ) before the onset of the acceleratory period, followed by initial set. Their characteristic hydration time, c , and set time depended strongly on the concentration of "free" carboxylic acid groups [COO ؊ ] arising from non-adsorbed polyelectrolyte species in solution.

We have studied the effects of poly(acrylic acid)-poly(ethylene oxide) (PAA-PEO) comb polymers on the stability of aqueous BaTiO 3 nanoparticle suspensions over a wide pH range in the presence and absence of mono-and divalent salt species. The comb polymer architecture consists of charge-neutral PEO teeth attached at random intervals along an ionizable PAA backbone. Potentiometric titrations, light scattering, and turbidity measurements were conducted on pure PAA and PAA-PEO solutions to assess their degree of ionization, radius of hydration, and stability. Adsorption isotherm and rheological measurements were conducted on BaTiO 3 nanoparticle suspensions to determine the effectiveness of both PAA and PAA-PEO dispersants. Our observations indicate that the presence of PEO teeth effectively shield the underlying PAA backbone from ion interactions, e.g., counterion-screening or ion-bridging effects, thereby allowing PAA-PEO dispersants to impart stability to BaTiO 3 nanoparticle suspensions over a wide range of pH, ionic strength, and ion valency conditions where pure PAA fails.

Polyelectrolyte species, known as superplasticizers, dramatically affect the rheological properties of dense cement suspensions. We have studied the influence of sulfonated naphthalene formaldehyde condensate (SNF) and carboxylated acrylic ester (CAE) grafted copolymers of varying molecular architecture on the surface (e.g., adsorption behavior and zeta potential) and rheological properties of concentrated cement suspensions of white portland cement and two model compounds, ␤-Ca 2 SiO 4 and ␥-Ca 2 SiO 4 . The adsorption of SNF species was strongly dependent on cement chemistry, whereas CAE species exhibited little sensitivity. The respective critical concentrations (⌽*) in suspension required to promote the transition from strongly shear thinning to Newtonian flow (flocculated 3 stable) behavior were determined from stress viscometry and yield stress measurements. Theoretical analysis of interparticle interactions suggested that only colloidal particles in the size range of <1 m are fully stabilized by adsorbed polyelectrolyte species. Our observations provide guidelines for tailoring the molecular architecture and functionality of superplasticizers for optimal performance.

No abstract