Long-Term Shrinkage and Mechanical Properties of Alkali-Activated Slag

Abstract

Alkali-activated slag (AAS) binders offer a sustainable alternative to Portland cement, yet their long-term mechanical properties remain uncertain. Despite reported declines in strength and stiffness over time, the mechanisms behind these reductions are not fully understood. Existing research largely focuses on short-term properties or isolated factors, and there is a lack of integrated studies spanning paste, mortar, and concrete. The role of dry shrinkage in longterm degradation and the development of effective mitigation strategies has also received limited attention.
This thesis addresses these gaps by investigating the causes of long-term reductions in the strength and elastic modulus of AAS mortars and concretes and evaluating potential solutions. An experimental program examined the influence of activator type (sodium silicate vs. sodium carbonate), curing regime (ambient, 7-day fog, and fog–sealed curing), and gypsum addition. Mortar and concrete specimens were monitored for their compressive strength, flexural strength, elastic modulus, and dry shrinkage over a six-month period. Results were also compared to traditional Portland cement systems (CEM I and CEM III) to benchmark performance. Moreover, correlations between mechanical properties and dry shrinkage were analysed, and the elastic modulus and shrinkage were compared with model predictions. Findings indicate that early fog curing substantially reduces shrinkage and increases mechanical properties by limiting moisture loss and microcrack formation. Sodium silicate activation produces denser microstructures, higher strengths, and stiffness than sodium carbonate systems. Incorporating 6% gypsum mitigates shrinkage by over 30% through ettringite formation and pore refinement, while slightly delaying early strength gain.
In AAS mortars and concrete, increasing dry shrinkage reduces both elastic modulus and flexural strength, while compressive strength may still rise under ambient curing, whereas Portland cement mortars (CEM I and CEM III) exhibit simultaneous gains in compressive strength, elastic modulus, and flexural strength due to ongoing hydration, and early-age fog curing in AAS mitigates shrinkage and microcracking, maintaining higher stiffness and flexural strength with long-term stabilisation as the microstructure densifies. Comparisons with Standard predictive models indicate that OPC-based codes overestimate the AAS elastic modulus, although long-term shrinkage trends are reasonably captured.
This study provides new insight into the mechanisms linking shrinkage to long-term degradation in AAS and demonstrates that combining sodium silicate activation, gypsum addition, and early fog curing offers a practical route to durable, shrinkage-resistant, and sustainable AAS concretes suitable for structural applications.