Reologia de pastas cimentícias com cinza de casca de arroz

Abstract

With urban expansion and population growth, currently the consumption of Portland cement has increased, as well as environmental concerns and the search for solutions to mitigate its use. Rice husk ash is an agro-industrial residue, which is often discarded inappropriately in the environment, and presents itself as an alternative material for use in cementitious matrices. In order to reduce this environmental liability and bearing in mind that the production of Portland cement is highly polluting, the use of rice husk ash (RHA) in partial replacement for cement was analyzed in this research. This study dealt with the rheological analysis of thirteen cement pastes, maintaining the residue substitution content equal to 10% and a constant water-to-cement ratio w/c = 0.45 with different contents of two chemical additives. A Silicon® NS viscosity modifier type admixture was used - an aqueous solution of nanosilica, in the range of 1.2% to 2.8% w/w (on the weight of the cement), and a type III superplasticizer, MasterGlenium® ACE 409 composed of modified polycarboxylate ethers, in the range of 0.2 to 0.6% w/w in cement pastes. These different formulations were evaluated by response surface methodology in order to find a feasible composition, which presents good workability, low plastic viscosity, with reasonable yield stress, for future applications in self-compacting mortars and concretes. For that, both the oscillatory and rotational rheometry tests were performed, as well as the results of the mini-slump test, the flow time in the Marsh Funnel and the compressive strength of the thirteen formulations. A feasible composition was obtained with 0.497% w/w of superplasticizer (additive 1) and 2.444% w/w of the viscosity modifying admixture. This formulation was prepared and tested, and elastic modulus values = 1630 Pa, yield stress = 16 Pa, plastic viscosity = 0.19 Pa.s, thixotropic recovery after 1 minute = 65%, spreading diameter = 126.5 mm and flow time through the Marsh Funnel = 8.06 s. It was found that the chemical admixtures used did not affect the compressive strength, while replacing the cement with 10% CCA increased the compressive strength by ≈ 29.4% at 07 days of curing and by ≈ 16.06% at 28 days of curing in relation to the control group.