International Concrete Abstracts Portal

This paper reports on part of a substantial research programme on properties of condensed silica fume (CSF) concretes cured in temperate and climates, carried out in the Department of Civil Engineering at Loughborough. The hot The research approach was to investigate mixtures proportioned to have equal workability and 28 day strength (when water cured at 20°C). This paper examines the effect of superplastizer, curing method (water and polythene) and curing environment (temperate and hot) on the compressive strength, permeability and pore structure of 40 MPa concretes. More specifically, the paper contrasts the performance of two 15% CSF mixtures (replacement by weight of cement) where workabilities were controlled by the addition of extra water or superplasticizer. The development of the concretes’ strength and subsurface permeability index (air and water) with age (from 7 to 180 days) is described, together with the intrinsic permeability (air and water) and pore structure of their equivalent mortar fraction. The use of superplasticizer to control workability increased the compressive strength of CSF concrete mixtures by around 18% and 10% in the temperate and hot environments respectively. The super-plasticized concrete had lower air and water permeabilities which is attributed to an improved pore structure as confirmed by mercury intrusion porosimetry date. The improvements were more marked in the CSF concretes cured in a hot environment.

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.

The influence of a phosphonate-based admixture called aminotri (methylene phosphonic acid) (ATMP) on the hydration of C 3A, C 3A and 25 percent gypsum, and C 3S and Type I cements (normal, high-alkali, and low-alkali) was evaluated. Isothermal conduction calorimetric investigations of various mixes containing 0.03 to 0.05 percent of phosphonic acid, at water-cement ratios of 0.35 and 1.0, were carried out up to 72 hr. The ATMP acted as a super-retarder for all mixes studied. At a dosage of only 0.05, the exothermal effect for one of the high-alkali Type I cements was delayed by about 16 hr. The hydration of C 3A and C 3S was also retarded by about 40 to 45 min and more than 50 hr, respectively, at a dosage of 0.05 percent. In the C 3A and gypsum system, the third peak corresponding to the reaction of C 3A with ettringite was extended by about 8 to 9 hr.

The corporate use of mineral admixtures, such as slag, silica fume, fly ash, and superplasticizer, in concrete is steadily rising for reasons of economy, enhanced strength, low heat generation, increased durability, and better rheological control. This study reports results of the influence of a cross-linked and NSF type of superplasticizer on the flow properties of blended cements. The cross-linked superplasticizer was comprised of polycarboxylic ether and cross-linked polymer, whereas the NSF type was a modified lignin, alkysulfonate, and polymer. In view of the difference in their molecular structures, their effect was studied on two types of cement: a normal portland cement and a moderate heat portland cement (belite-rich, low in C 3A), to which different proportions of slag, silica fume, and Class C and F fly ash were added to simulate binary and ternary blended cement compositions. Following a detailed chemical and mineralogical characterization of these blending components, the slump flow of 25 mortar blends were tested at a sand-binder ratio of 1.5, with the superplasticizer dosage varying from 2.5 to 3 percent by weight of cement. The water-binder ratio of these mixes ranged from 0.31 to 0.35. Marked differences in flow characteristics (determined by different methods) were recorded as a function of the cement type, blending component, and superplasticizer composition. Viscometric measurements made on the corresponding cement paste mixtures using a rheometer also exhibit pronounced differences in terms of their apparent viscosity. The possible superplasticizer interactions that occur in these blended cementitious systems are discussed. This study reiterates the cement-superplasticizer compatibility factor currently under intense discussion among researchers.