Influence of Aggregate Fineness Modulus on the Electrochemical Properties of Cement-Based Materials Containing Natural and Recycled Aggregates

The utilization of recycled materials has become a fundamental necessity across all industries, particularly in construction, as part of global efforts to preserve natural resources for future generations. This research was conducted to investigate the electrochemical properties of cement-based products containing natural and recycled aggregates with varying fineness moduli of 2.5, 3.0, and 3.5. For this purpose, six mortar mix designs were prepared with a constant water-to-cement ratio, incorporating both natural and recycled aggregates according to the specified fineness moduli. In addition to compressive strength testing, the specimens were subjected to half-cell potential measurements, linear polarization resistance, and potentiodynamic polarization tests to evaluate the corrosion behavior of embedded steel reinforcements. The experimental results revealed completely contrasting behavioral patterns between mortars containing natural aggregates and those incorporating recycled aggregates. In mortars containing recycled aggregates, a decrease in the fineness modulus (finer sand particles) led to a reduction in compressive strength. This phenomenon was attributed to the weakened bond between the recycled aggregate particles and the new cement paste matrix. The highest compressive strength values were recorded for specimens with a fineness modulus of 3.5, while the lowest values were associated with a fineness modulus of 2.5. Conversely, mortars containing natural aggregates exhibited a completely opposite trend. As the fineness modulus decreased, the compressive strength showed a marked increase due to the increased surface area available for bonding between aggregate particles and cement paste. In this category, specimens with a fineness modulus of 2.5 demonstrated the highest compressive strength values, while those with a fineness modulus of 3.5 recorded the lowest performance. This contrasting pattern was consistently observed throughout the electrochemical investigations. Half-cell potential measurements showed that in recycled aggregate mortars, decreasing the fineness modulus resulted in more negative potential values, indicating an increased probability of corrosion. In recycled aggregate mortars with a fineness modulus of 2.5, the embedded reinforcements exhibited a 90% probability of corrosion after the first exposure cycle. In contrast, natural aggregate mortars with the same fineness modulus only reached similar corrosion probability conditions after the third exposure cycle. Linear polarization resistance measurements further confirmed these patterns. In recycled aggregate mortars, decreasing the fineness modulus was associated with reduced corrosion resistance. Specimens with a fineness modulus of 3.5 showed the highest corrosion resistance, while those with a fineness modulus of 2.5 exhibited the lowest values. This trend was completely reversed in natural aggregate mortars, where decreasing the fineness modulus led to improved corrosion resistance. Potentiodynamic polarization tests provided additional confirmation through analysis of electrochemical parameters. The anodic Tafel slope decreased with decreasing fineness modulus in recycled aggregate mortars, while increasing in natural aggregate mortars. The cathodic Tafel slope showed increasing values with decreasing fineness modulus in recycled aggregate mortars, whereas natural aggregate mortars showed decreasing cathodic Tafel slopes. In conclusion, a consistent behavioral pattern was observed across all experimental investigations. In mortars containing recycled aggregates, decreasing the fineness modulus consistently led to deterioration in both mechanical properties and corrosion resistance. However, in mortars containing natural aggregates, the same modification resulted in improved performance across all evaluated parameters. This fundamental difference in behavior is attributed to the inherently porous nature of recycled aggregates and their weaker bonding characteristics with the cement paste matrix compared to natural aggregates. These findings have significant implications for the practical application of recycled aggregates in sustainable construction.