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Los nanotubos de carbono (NTC) son estructuras de carbono que se obtienen en forma cilindrica de escala nanométrica. Este artículo presenta una evaluación de la adición de NTC en hormigón de cemento Portland, centrándose en la importancia de llevar a cabo la dispersión de los NTC en el agua con el uso del ultrasonido antes de su incorporación en la masa de hormigón. Para ello, tres mezclas de hormigón se prepararon con un NTC libre (referencia) y dos NTC (con una serie previa de dispersión en agua y el aditivo con el uso de ondas ultrasónicas y otras sin dispersión). A continuación, se analizó las propiedades de fluidez, resistencia a la compresión, tracción y la absorción del agua. El NTC añadió contenido de 0,30% a respecto de la masa de cemento. En todos los casos, la dispersión previa de NTC usando ultrasonido potenció el efecto de la adición de nanotubos de carbono, siendo importante para la eficiencia de este material cuando se añade al cemento

Cement is considered the basic component with the highest environmental impact in construction, in terms of CO2 emissions. As for the aggregates, the process of comminution of rocks, in addition to artificial sand, generates stone powder that ends up being stored outdoors, generating environmental damages. Thus, the replacement of cement by stone powder appears as an attractive alternative towards the sustainable concretes. In this context, the objective of this paper is to determine the maximum packing density in Portland cement, silica fume and stone dust pastes, to determine the optimal cement substitution content for the stone powder. In addition, it is intended to verify the influence of excess water on the consistency of the mixtures produced. The substitution was done in contents equal to 0%, 7%, 14% and 21% by volume and, for each content, the packing density was determined analytically by CPM model and combinations were reproduced experimentally. Excess water was checked by the mini Kantro cone test. The results showed that the higher cement substitution content of the stone powder obtained the higher packing density, experimental and analytical, and the higher workability, allowing economic and environmental advantages. Analyzing each material, the stone powder resulted in the highest packing density and silica fume is the lowest one. Therefore, finer particles resulted in lower packaging densities, due to the greater specific surface area, which demands more water. The agglomeration resulted in more empty gaps between the grains. In addition, mixtures flowability increased with the increase of the stone powder content. As the excess water is responsible for mixture lubrication, a higher packing density for a given volume of water improves the flowability.

Silica fume (SF) is the most commonly mineral admixture used for the production of high-strength concrete (HSC) due to its chemical characteristics of pozzolanic reactivity and physical filling effect. The objective of the present work is to compare the SF content by the chemical analysis of the pozzolanic reaction and the physical behavior by particle packing techniques. The first step of the study was to analyze the SF content to consume the calcium hydroxide (CH) produced during the hydration of Portland cement (PC), based on stoichiometric calculations. Then, the SF content for maximum packing density was obtained using analytical particle packing models and experimental tests. The compressive strength of the pastes was also measured. The results showed that the theoretical SF content for consuming CH is 15.6%, replacing PC. According to the packing density analytical models, the ideal SF content is 15% of the total fine materials. However, the experimental results indicated the use of a smaller SF content (10%). This difference between theoretical and experimental results is probably due to the high specific surface of the SF, which results in important surface forces between the grains, particle agglomeration and difficulty in SF densification with water.