International Concrete Abstracts Portal

Three superplasticizers (SP) have been studied in this research: the first is based on modified polycarboxylic ether and is used to improve the workability of concrete and to obtain high mechanical characteristics at early age; the second, which contains naphthalene sulphonate, is used to reduce drastically mixing water in concrete and improve mechanical strength at early age; the third SP investigated is melamine-based and is used to improve the workability of concrete creating electrostatic repulsion between cement grains. The intention of the present investigation was to provide more information about the role of these SP in concrete at early age. The apparent activation, initial and final set times by Vicat needle, chemical and autogenous shrinkage were measured for cement pastes having a water/cement ratio of 0.25. The apparent activation energy has been determined by the "setting times method" at different temperatures: 10, 20, 30 and 40 °C. The volumetric autogenous shrinkage was measured at the same temperatures immediately after setting. The experimental results show that the apparent energy activation is slightly modified by the presence of SP. Also, the evolution of chemical shrinkage shows clearly that the SP acts on the hydration kinetic of cement. The effect of a particular SP on autogenous shrinkage at different temperatures can be correctly predicted by means of the maturity concept.

The old concept for optimum sulfate content in portland cement developed by Lerch in 1946 was further applied to study and explain cement admixture interactions related to the soluble sulfate content in systems with portland cement, mineral and chemical admixtures. It was shown that Lerch's criteria for optimum sulfate in portland cement based on isothermal calorimetry is applicable also for studies of modem systems with multiple binders and admixtures. Indirect evidence on the cause of some cement-admixture interactions was provided by means of testing for the effect of additional soluble sulfate added to the cement prior to testing.

The ability of lithium-based compounds to suppress deleterious expansion due to alkali-silica reaction (ASR) in mortar and concrete was first demonstrated over 50 years ago. Lithium nitrate solution is now marketed in North America as a chemical admixture for inhibiting ASR; the product currently available is a 30% solution of LiNO3. The purpose of the study reported here was to determine to what extent a chemical admixture based on lithium nitrate influences the properties of fresh and hardened concrete when used at the dosage levels required to suppress expansion due to ASR. This was done by comparing the fresh and hardened concrete properties of a number of mixtures with and without the admixture. The constituent materials and concrete mixture proportions used in the study were based on those currently approved for the Class AA Structural Concrete for a major highway project in Albuquerque, New Mexico. These materials included a Type II portland cement, Class F fly ash, a reactive aggregate, and water-reducing and air-entraining admixtures. The water-to-cementitious-material ratio of the concrete was in the region of W/CM = 0.35. All of the concretes tested had either no air entrainment, or air contents in the range of 5% to 7%, slump values between 100 mm to 125 mm and strengths in the region of 32 MPa to 36 MPa. The use of lithium nitrate solution, at the levels of addition necessary to effectively control expansion due to ASR, appeared to have no adverse effect on the properties of fresh and hardened concrete, even at dosages in excess of 10 litres per cubic metre. Changes in the measured slump, air content and setting time of plastic concrete were generally insignificant. Tests on sawn concrete samples confirmed that the use of lithium had no impact on the hardened air-void parameters. Physical testing of hardened concrete showed that the use of lithium had little significant or consistent effect on the concrete strength or its durability as defined by the resistance of specimens to the penetration of chlorides or cyclic freezing and thawing.

Model hydraulically inactive powders have been used to investigate the effect of polycarboxylate and lignosulfonate superplasticizers. A new rotational rheometer, avoiding the risks of sample separation, slippage and uncertain shear history, was used. Magnesium oxide can be used to model the rheology of cement pastes but an appropriate solution electrolyte composition must be used. Ground granulated blastfurnace slag can be used but results are sensitive to glass content and particle fineness. The effects of admixture chemistry can be studied using model paste systems.

This paper describes a part of a current European research programme (started March 2001) addressing superplasticizers of improved properties. The number of different cement types and blended cements is increasing. Incompatibilities of the cementitious compounds with admixtures can therefore not be excluded, especially at low water/cement ratios. Polycarboxylate superplasticizers with variations of side chain length, content of anionic groups and degree of polymerisation of the backbone were synthesised. The interaction of these polymers with different cements and cement blends was investigated by measuring adsorption, flow of cement pastes and mortar tests. Interesting performance differences can be seen between the different superplasticizer-cement combinations, especially at low water to cement ratios. The results of the paste flow can be explained with the polymer structure and the cement chemistry and can be used to predict polymer performance in mortar. The low alkali cement shows lower sensitivity to differences in structure of polycarboxylate superplasticizers than cements with higher alkali content. The adsorption of polycarboxylate polymers on cement is driven by the structural features of the polymer and can be directly related with the flow of the cement paste.