Effect of chemical structure on the efficiency of shrinkage reducing admixtures in alkali activated systems

Alkali activated binders, especially those based on alkali activated blast furnace slag (AAS), have the potential to become an alternative construction material to ordinary Portland cement binders. Nevertheless, AAS has some disadvantages which prevent its broader practical applications. An extensive shrinkage is one of the main limiting factors. Therefore, the study of chemical admixtures mitigating especially the drying shrinkage is necessary to be performed. The efficiency of suitable shrinkage reducing admixtures depends on the chemical structure of used surfactants. The study is consequently focused on the molecular architecture of amino alcohol surfactants which are closely associated with their ability to effectively reduce shrinkage. The molecular structure of used chemical compounds is shown in Figure 1. The influence of different substituents bounded to the secondary amine group was studied in terms of their effect on alkali activation, mechanical properties, microstructure arrangement and in particular on the enhancement of drying shrinkage reduction. It was determined that the addition of any tested admixture delayed the CASH gel formation which negatively influenced the flexural as well as compressive strengths in the early stages of hydration process (1 – 7 days). However, only slight decrease in strengths compared to reference sample was measured after 28 days of curing. The deeper insight into the microstructure (Figure 2) confirms previous results. It is obvious that in the case of reference sample the consistent matrix of binding phase is created after 24 hours. On the other hand, only thin layer of hydration products is formed in samples containing the admixture, which increases the porosity of material and tends to the deterioration of mechanical properties. Finally, the study confirms that the reduction of surface tension in pore solution occurs primarily with admixtures containing branched substituents, which further decreases the capillary tension responsible for the shrinkage according to Young-Laplace equation. The presented study highlights the essential role of molecular structure of shrinkage reducing admixtures contributing to the development of a new range of additives designed especially for alkali activated materials. Please click Additional Files below to see the full abstract

Identification of Mechanical Fracture Parameters of Alkali-Activated Slag Based Composites During Specimens Ageing

The aim of the paper is to present the results of the experiment focused on the development of the mechanical fracture characteristics of alkali-activated slag (AAS) based composites within the time interval from 3 days to 2 years of ageing. Two AAS composites, which differed only in the presence of shrinkage reducing admixture (SRA), were prepared for the purpose of experiments. The composites were prepared using ground granulated blast furnace slag activated by water-glass with silicate modulus of 2.0, standardized quartzite sand with the particle grain size distribution of 0−2 mm, and water. Commercially produced SRA was added into the second mixture in an amount of 2 % by weight of slag. The test specimens were not protected from drying during the whole time interval and were stored in the laboratory at an ambient temperature of 21 ± 2 °C and relative humidity of 60 ± 10 %. The prism specimens made of the abovementioned composites with nominal dimensions of 40 × 40 × 160 mm with the initial central edge notch were subjected to the fracture tests in a three-point bending configuration. The load F and displacement d (deflection in the middle of the span length) were continuously recorded during the fracture tests. The obtained F−d diagrams and specimen dimensions were used as input data for identification of parameters via the inverse analysis based on the artificial neural network, which aim is to transfer the fracture test response data to the desired material parameters. In this paper, the modulus of elasticity, tensile strength, and fracture energy values were identified and subsequently compared with values obtained based on the fracture test evaluation using the effective crack model and work-of-fracture method