No abstract

The effects of fine limestone powder on the early hydration of cementitious systems accelerated by means of alkali-free aluminum sulfate based products, commonly used for shotcrete applications, were investigated in the course of laboratory and real scale tests. In binary (CEM I + limestone) and ternary (CEM I + limestone + slag) systems the addition of fine limestone led to an enhancement of the hydration degree and strength development at early times (<24 h). The formation of ettringite, aluminate hydrates, and C–S–H is affected by the joint action of the setting accelerator and the fine limestone. Accelerator and limestone, in combination with the cement, can be optimized to enhance ettringite and silicate reaction, in some cases coupled with aluminate reaction inhibition, to produce mixes suitable for sprayed concrete applications. Such optimization can help to reduce the cement content in the mixes without compromising the early strength development of the shotcrete.

The reduction of clinker use is mandatory to lower the negative environmental impact of concrete. In shotcrete mixes, similarly to the case of conventional concrete, the use of supplementary cementitious materials (SCMs) and proper mix design allow for the substitution of clinker without compromising the mechanical properties. However, the impact of the substitution on the durability of shotcrete needs to be further assessed and understood. The results from the present study, obtained from real-scale sprayed concrete applications, show a reduction of the Ca2+ leaching and sintering potential of clinker-reduced shotcrete mixes due to the presence of SCMs. This positive effect, crucial for low maintenance costs of tunnels, is mainly related to a reduced portlandite content, which on the other hand negatively affects the carbonation resistance of shotcrete. Additionally, the hydration of SCMs positively influences the chloride penetration resistance presumably due to a combination of microstructural changes and changes in the chloride binding capacity. Differences found in the pore size distribution of the various mixes have low impact on the determined durability parameters, in particular compared to the effect of inhomogeneities produced during shotcrete application.

El hormigón proyectado es un tipo de hormigón especial utilizado como soporte de rocas durante la construcción de túneles y excavación de minas, entre otras aplicaciones. Las principales características de este tipo de hormigón son el fraguado rápido y el desarrollo de una muy alta resistencia mecánica a edades tempranas, lo cual permite que el hormigón se fije a la base sin necesidad de un soporte extra, endureciéndose en pocos minutos. Las reacciones de hidratación que ocurren en las primeras horas en el hormigón proyectado determinan el desarrollo de las propiedades mecánicas y su estudio es de crucial importancia para entender y optimizar el comportamiento de este tipo de hormigones. El estudio de estas reacciones, sin embargo, resulta relativamente complicado por varias razones: (i) el hormigón proyectado endurece muy rápido, lo cual dificulta su manejo durante los ensayos experimentales, y (ii) composiciones similares mezcladas en el laboratorio no se comportan de forma similar a las proyectadas. En este trabajo se presenta una metodología experimental desarrollada para el estudio de las reacciones de hidratación que ocurren en el hormigón proyectado durante las primeras horas y su correlación con la resistencia mecánica.DOI: http://dx.doi.org/10.4995/HAC2018.2018.6907