Abstract
This work evaluated the efficiency and workability manteinance of a polyfunctional (lignosulfonate-based) and a superplasticizer (polycarboxylate-based) admixture in high-performance pastes (water/cement ratios of 0.30, 0.33 and 0.36) produced with two types of Portland cement (one pozzolanic and one high early strength). The workability of the pastes was evaluated by the mini slump test immediately after mixing, and after 20 and 40 minutes. Furthermore, the mechanical strength of pastes with fixed w/c ratio (0.36) and mini slump (130 mm), produced with the different combinations of cements and admixtures, was evaluated. The results showed that the CP IV+lignosulfonate and CP V-ARI+polycarboxylate mixes presented the best workability maintenance over time. The pastes produced with CP V-ARI presented 28-day strengths about 22% higher than those produced with CP IV, for both admixtures. In turn, the lignosulfonate-containing pastes showed strengths about 10% higher than those produced with the polycarboxylate-based admixture. Overall, it was possible to obtain the same workability for high-performance Portland cement pastes produced with both admixtures and cements; however, the workability mantainance and mechanical strength highly depends on the combination of cement and admixture used.
Highlights
Modern concrete contains components with very specific characteristics that give particular properties to the mix, as well as chemical admixtures that have even more specific effects [1]
Two types of cement were used: a Portland–fly ash cement (CP IV) and high early strength Portland cement (CP V-ARI), both standardized by NBR 16697 [14]
It is worth noting that it was necessary to use a lower admixture content (1.00%) for mixtures with w/c of 0.36. This is due to the fact that adsorption of the admixture dispersing molecules occurs more sharply in mixtures with higher w/c ratios, since these mixes have greater amounts of water available [17]
Summary
Modern concrete contains components with very specific characteristics that give particular properties to the mix, as well as chemical admixtures that have even more specific effects [1]. Water-reducing admixtures are known as plasticizers and superplasticizers, and act as cement particle dispersants. These products are used in concrete to increase its flowability without changing the mix composition, or by reducing the water consumption while maintaining the workability, reducing the water/cement (w/c) ratio and improving the mechanical strength and durability. The hydrating cement particles tend to flocculate due to the polarity of the water molecules and the Van der Waals attracting forces between the particles, which are electrostatic forces between opposite surface charge regions of cement. The main function of water-reducing admixtures is to disperse the cement particles, releasing the entrapped water between the grains, which increases the flowability of the mix [5]
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