Thermal expansion mismatch occurs when different materials within a composite, such as concrete, expand and contract at different rates due to temperature changes. In concrete, this is typically seen between the cement paste and the aggregates. Since aggregates constitute a significant volume of concrete and generally have different thermal expansion coefficients compared to the cement matrix, temperature fluctuations can lead to internal stresses. This mismatch can cause microcracking and, over time, may lead to macroscopic cracks, significantly impairing the structural performance and durability of the concrete.