Influence of Lignosulphonate Molecular Weight Fractions on the Properties of Fresh Cement

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, Mw, 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.