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In October, 1994, CANMET in association with the American Concrete Institute sponsored a fourth conference on the superplasticizers and chemical admixtures in Montreal. The objective of this conference was to bring to the attention of the concrete community the new developments in chemical admixtures since the last conference in 1989. A total of 25 papers were accepted for publication in this special proceedings from the conference. If you are involved with superplasticizers and chemical admixtures, this special publication is a must. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP148

High-performance concretes (HPC) typically have low w/c to achieve the desired levels of strength and durability. As a result, HPC have a tendency to be stiff and lose their workability rather quickly. Often, high-range water-reducing admixtures (HRWRA) are used to improve the workability of HPC. Care must be exercised when using any admixture, or combination of admixtures, to insure that there are no detrimental side effects that might shorten the life of the concrete. Research has shown that, although retempering concrete with an HRWRA will generally improve workability and maintain the strength of low-w/c concretes, it may also reduce freeze-thaw resistance. Therefore, an experimental study was

A variety of new literature and data on the properties of cement pastes and concentrated slurries of various types of mineral particles are examined to elucidate the origin of the fluidification of cement pastes by superplasticizers. The influence of sodium poly-¯-naphthalene sulfonate superplasticizers (NaPNS) of different molecular weights on the rheological properties of pastes and on the early heat of hydration of cement, together with results from other physicochemical measurements (adsorption, zeta potential), suggests that the unique fluidification effect of these admixtures depends on at least four distinct phenomena. With reference to fluidification of slurries of "inert" minerals, the superplasticizer effect in concrete can be understood in terms of nonspecific (physical) and specific (chemical) effects. The "physical" effects comprise: adsorption of the superplasticizer molecules by van der Waals and electrostatic forces (direct or assisted by cations); surface charging that induces long-range interparticle repulsive forces; steric hindrance between adsorbed polymer molecules on neighboring particles, leading to added short-range repulsive forces. The "chemical" effect involves a reaction of the PNS superplasticizer molecules with the most reactive sites of cement particles (particularly C 3A), substantially reducing the initial surface hydration rate. This description is largely based on data relevant to PNS-type superplasticizers, but with proper allowance for specific chemical effects, it should also be valid for other types of superplasticizer.

Influence of an acrylic polymer on the rheology of mortars was investigated using a mixer in which the torque on the impeller shaft was continuously measured. The polymer was added to specific mortars either alone or in combination with aqueous solutions of sulfonated naphthalene formaldehyde condensate or sulfonated melamine formaldehyde condensate. Two cements were ground from two different clinkers to specific surfaces of 270 and 400 m 2/kg, respectively. The flow properties of these fresh mortars closely approximate the Bingham model, whatever the time after initial mixing may be. When used alone, the polymer decreases the plastic viscosity of the mortar. When used in combination with sulfonated melamine or naphthalene formaldehyde condensates, it decreases the yield value.

The rheological properties of fresh mortar and concrete were studied with five types of polysaccharide syrup added to either naphthalene or melamine sulfonate-based superplasticizer. The effects of the binary admixture system (superplasticizer and syrup) on the physicomechanical properties of the cementitious system, based on two types of cement (portland and pozzolan), were studied. The properties, studied at 25 and 40 C, were slump, loss of workability, setting time, and compressive and flexural strength. For one type of syrup added to the superplasticizer, synergistic effects were observed. In particular, it was demonstrated that, at 40 C, it is possible to increase the slump and reduce loss of workability using an admixture based on naphthalene sulfonate, plus Syrup G62 and melamine sulfonate, plus Syrup G62, compared to normal superplasticizer-retarding admixtures.