Lignosulfonatos de sódio e magnésio: uma avaliação comparativa entre matérias-primas fundamentais para aditivos plastificantes

Abstract

Motivated by low cost, lignosulfonate based plasticizers are the most widely used type of admixtures in the concrete industry worldwide. Due to the plurality of types of lignosulphonates and consequent variations in their chemical, structural and compositional characteristics, this study evaluated the influence of basic admixtures consisting of sodium lignosulphonates (LS-Na) and magnesium (LS-Mg) combined in different dosages, (0.40% and 0.80%) to two different types of cement: CP II F 40 and CP III 40 RS. The following tests were carried out: determination of adsorption curves, isothermal calorimetry in cement pastes, hydration kinetics through microstructural tests (TG/DTG and XRD) and, finally, concrete applications - comparison of properties obtained in the fresh state (reduction of water, loss of slump, entrained air content) and hardened (compressive strength and tensile strength by diametral compression, modulus of elasticity). The results clearly demonstrated that there was difference in the performance of each lignosulfonate, influenced especially by intrinsic characteristics such as sugar content, molar mass and free sulfate content. In the presence of lignosulphonates, different rates of hydration of both cements were observed, impacting on delayed dissolution of silicate phases (C3S and C2S) and consequent portlandite contents. At 90 minutes of hydration, for both cements the amount of water combined chemically and accumulated heat were increased with the increase in lignosulfonate content, being globally higher for LSMg at the dose of 0.80%, indicating the accelerated development of the ettringite. No adsorption curve was fitted into the Langmuir model, with linear behavior, being higher for LSMg (lower sulphate and molar mass) and the most pronounced difference when combined with CP III 40 RS. In the concrete application, the LSMg obtained higher compressive strength, tensile strength and modulus of elasticity, although the LSNa obtained better performance in the fresh state - greater reduction of water, less loss of slump, but greater development of entrained air. It is evident to the chain of the concrete admixtures industry the importance of optimization through the appropriate and specific choice of types of lignosulfonates as raw material.