Understanding the Mechanism of Drying Shrinkage in Alkali-Activated Binders

Abstract

Cement production for concrete is responsible for 5-8% of the global anthropogenic emissions, making it essential to search for alternative binders. This study focuses on alkali-activated materials (AAMs) as a potential replacement for Portland Cement (PC) to reduce carbon emissions. The practical implementation of AAMs in the construction industry faces challenges, particularly related to volume instability caused by drying shrinkage. Drying shrinkage is linked to both water loss through evaporation and the hydration process. The underlying mechanism of drying shrinkage in AAMs is not well understood yet, hindering their widespread application.

The aim of this study is to advance the understanding of the drying shrinkage mechanism in AAMs, by considering the contribution of the pore size distribution and gel characteristics of AAMs. A detailed analysis is performed to identify the governing parameter related to the drying shrinkage mechanism. This is done by controlling the pore size distribution and total porosity at moment of exposure, while differences in the gel composition were obtained. Moreover, the impact of different mix design parameters of blended AAMs on drying shrinkage behaviour, weight loss, microstructural development, flexural and compressive strength are investigated. The selected mix design parameters in this study include slag-to-fly-ash ratio (1, 0.7 and 0.5), curing time (3, 7, 14 and 28 days) and Na₂O wt.% content (4 and 5 wt.%).

Results from the study indicate that drying shrinkage in AAMs is influenced by factors beyond water loss, diverging from the observed correlation in PC. The study highlights important findings related to mix design parameters. In terms of the slag-to-fly-ash ratio, an increase in ground granulated blast furnace slag (GGBFS) content correlates with reduced drying shrinkage, weight loss, refinement of the pore structure and total porosity. Regarding curing time, prolonged curing durations lead to decreased drying shrinkage and weight loss, coupled with improved flexural and compressive strength. As for activator content, an increased amount of activator refines the pore structure, resulting in reduced total porosity, weight loss, and increased compressive strength. However, drying shrinkage remains relatively constant over the 56 days exposure.

Based on the starting point that the pore size distribution of selected samples at the moment of exposure to drying was controlled, it is suggested that gel characteristics, i.e. reaction products, rather than pore size distribution, govern the drying shrinkage phenomenon in AAMs. Comparative analyses underscore the influence of homogeneous reaction products, a higher atomic Ca/Si ratio and the availability of sodium and silicate from the activator. At the moment of exposure, the type of gel is more crucial than the quantity, as demonstrated by the sample with more reaction products exhibiting greater drying shrinkage in the analysis.

The drying shrinkage mechanism in AAMs is strongly correlated with microstructure and the nature of reaction products. The comprehensive results of this study suggest that the gel characteristics have an crucial role as a driving force in the mechanism of drying shrinkage. The study underscores the substantial influence of mix design parameters on drying shrinkage, offering valuable insights for the practical implementation of AAMs in construction. This research marks a significant step forward in enhancing our understanding of this complex phenomenon for AAMs.

The study provides several recommendations for future research, including extending the range of mix design parameters and curing times to evaluate the findings of this study, assessing the rate of reaction of mixtures and considering the impact of drying on the exposed pore structure. Furthermore, the application of N₂ adsorption to detect the smaller range of pores in AAMs, the determination of the influence of internal relative humidity on drying shrinkage, and the investigation of cracking potential of AAMs related to drying shrinkage are also suggested.