International Concrete Abstracts Portal

The diversity of concreting methods in recent years has led to a demand for long retardation of concrete setting. Conventional research has achieved retardation for up to about 3 days by the addition of a set retarder, but few studies have dealt with concrete that is placable after a much longer period. This paper reports the effects of the set-retarder dosage and temperature on the slump, period of set retardation, time limits for superplasticizing and compressive strength of concrete. The use of concrete with long-retarded setting is discussed as well.

It is broadly recognized that the adsorption of super-plasticizers on cement particles is a key factor in determining the rheology of concrete. In order to avoid the problems linked to the hydration of cement, the adsorption of super-plasticizers is often studied on unreactive model powders. However, in order for the model system to remain as close as possible to cement, the surface should have a similar charge and a similar chemical nature. Furthermore, the pH of the solution should be close to that of the hydrating cement (about 12.5). Under these conditions, cement has been shown to have a positively charged surface. The model powders used in this study were Mg(OH)2 and dead burnt MgO, which have nominal isoelectric points of 12.0 and 12.4 respectively, and which are chemically similar to Ca(OH)2 and CaO. The surface charge of such model suspensions was studied as a function of added superplasticizer. These were either commercially available or currently under development, ranging from strongly to very weakly ionic. Adsorption isotherms for two polymeric super-plasticizers, with similar structures but with different ionic group spacing, have been measured for both MgO and Mg(OH)2 at pH 12 and 11.3 respectively and between 10 and 40°C. Results showed a strong temperature dependence for the adsorption of the less ionic polymer on MgO.

Results describing investigations of the kinetics of hydration and struc-ture formation of cement systems (C3 S, C3 A, C3 A + gypsum, and portland ce-ment) in the presence of naphthaleneformaldehyde type superplasticizers (SNF ) of different compositions are discussed Using the individual components of SNF sodium polymethylenenaphthalenesulfonates ( PNS ) with polycondensation degrees ranging from 2 to 17, the basic dependences of the kinetics of the processes on the average molecular mass of the admixture ( Mn were shown In the case of C3 S the retarding effect of SNF on hydration and structure formation processes depends lineally on Mn , as a general rule, whilst for C3 A the retarding effect is more pronounced for mid-condensated components of superplasticizer. This difference is due to the different adsorption mechanism of the superplasticizer on the above-mentioned minerals. The unusual form and the analogy of ther-mokinetic curves for C3 A and C3 A - gypsum hydration in the presence of PNS are the first experimental proof that organic-mineral compounds are formed in these systems.

The importance of molecular weight as a parameter influencing the performance of water-reducing admixture in cement paste was investigated. The sulphomethylolated ALCELL@ lignin sample was divided into four fractions of different molecular weight. The fractionation was performed by membrane ultrafiltration on a small laboratory unit. Advanced hydrophilic membranes with a 1, 10 and 50 kd nominal MW cut-off and effective area of 63 cm2 were used. Molecular weight distribution, average molecular weights (Mn , M,, Mz and Mz+1 and polydispersity of the original sample and its fractions were determined by high-performance aqueous size-exclusion chromatography. The five samples, prepared as 20% aqueous solutions, were used as water-reducing admixtures. Their influence on fresh cement pastes was examined using the torque and mini-slump tests. The torque test was performed to investigate the fluidifying and retarding effects of the samples. Changes in torque resistance were monitored by a computerized system giving an indication of the degree of dispersion and set retardation. The mini-slump test was used to determine the workability and air-entrainment of the fresh cement paste. The dependence of the properties of the fresh cement pastes on the lignosulphonate molecular weight was studied. It was found that the lower the molecular weight of the lignosulphonate sample the higher the set retardation of the cement paste. The highest fluidification and lowest air-entrainment were observed when the intermediate molecular weight samples were used. The optimum molecular weight fraction in each particular experiment was always more effective than the original unfractionated sample. Fractions with extremely low efficiency were also identified. The data obtained can be utilized in optimization of lignosulphonate admixture performance.

A deflocculation test based on the method proposed by A. Verhasselt and J. Pairon has been applied to different mixes of cement-admixtures in order tc distinguish quickly between plasticizer and superplasticizer. The admixtures used were commercial products containing different proportions of lignosulphonates (L), sulphonated naphthalene formaldehydecondensate (SNF) and sulphonated melamine formaldehyde condensate (SMF). Three different cements were employed: a low tricalcium aluminate (C3 A = 2.5 %), a high alkali content (Na2 O equivalent = 0.95%) and a high fineness Portland cement (S.A. Blaine = 5002 cm2/gr). In addition, the minimum active dosage of superplasticizer was calculated. The influence of the type of cement in the minimum active dosage is clearly shown by the results obtained through this method.