Cement stabilization is a useful and widely adopted method to improve the engineering properties of soils. However, characterization of the unconfined compressive strength, a simple and useful design property, is not straightforward due to complex interactions of various influence factors. This study investigated the effects of water content on the unconfined compressive strength of cement-stabilized clayey sands. The results show that the strength of the cemented binary mixtures increases with water content and water-to-cement ratio until a threshold value is reached and then decreases with further increase in water content and water-to-cement ratio. The unconfined compressive strength is correlated with ultrasonic wave velocity and shear wave velocity, respectively, showing two nearly unique correlations. Microscale analysis based on the coated sphere model revealed that the strength of the sample is affected by the bonding area and the strength of the binder material (cement–clay mixture). An empirical equation is also proposed based on the microscale analysis so as to capture the effects of water content on the strength.