A promising solution for reducing the carbon footprint of concrete is the use of alkali-activated concretes (AAC). Before this material can be widely applied, its long-term behaviour needs to be understood, especially since some studies reported a decrease of mechanical properties over time. Similarly, Prinsse et al. reported decreasing mechanical properties, especially elastic modulus and flexural and splitting tensile strength for the studied slag-based AAC (S100) and the blended slag- and fly-ash-based AAC (S50) up to the tested age of 2 years. They hypothesized that these decreases could be only temporarily. To test that hypothesis, this study continued to monitor the mechanical properties of both AACs up to the age of 5 years. As a reference, two OPC-based concretes (OPCC), with different strength classes, are monitored up to the age of 3.5 years. In addition, the internal structures of the concretes are assessed for carbonation and internal micro cracking. S100 shows stabilization of the elastic modulus and the compressive strength, whereas the tensile splitting strength continued to decrease up to 5 years. This is attributed to a combination of carbonation and drying, since the microscopic analysis showed increased porosity around the ITZ and in the carbonated region. In addition, S50 shows an ongoing decrease of all tested mechanical properties, which is attributed to carbonation. No decreases in mechanical properties are found for OPCC.

Although alkali activated concretes (AACs) are promising for reducing the carbon emissions of concrete, in order to enable their wide application it is vital to understand their long-term behaviour. Herein, we report the development of mechanical properties of a ground granulated blast furnace slag (GGBFS)-based AAC and a binary fly ash (FA) /GGBFS-based AAC exposed to 55% relative humidity and 20 °C up to the age of 5 years. For comparison, two ordinary Portland cement (OPC) concretes were monitored for 3.5 years. For the GGBFS-based AAC, after an initial decrease within the first 6 months the elastic compressive modulus stabilized, while its tensile splitting strength continued to decrease for the tested period of 5 years. The binary AAC showed a continuous decrease in its tensile splitting strength for 5 years and a reduction in its compressive strength after 2 years. No decreases in mechanical properties were observed in OPC-based concretes. To reveal underlying mechanisms, additional analyses were performed. Permanent degradation was observed in both AACs; the binary AAC mainly suffered from carbonation, and the GGBFS-based AAC showed microcracking. These cracks were probably caused by drying shrinkage and drying-induced chemical changes. Based on the measured mechanical properties of AAC, crack widths and stiffness of reinforced AAC beams under bending were analytically evaluated and compared to experiments. Decreases in bending stiffness and increases in crack width were observed for reinforced AAC beams tested at later ages. A bimodular approach is proposed to predict the reduction of bending stiffness in the studied AACs over time. These findings are relevant to understand serviceability limit states of reinforced AACs.