Which ingredient can be added to shorten the working time of Coe-Pak?

The correct ingredient that can be added to shorten the working time of Coe-Pak is zinc oxide. Coe-Pak is a commonly used material in periodontal surgery for its properties as a periodontal dressing. The addition of zinc oxide works through its ability to influence the setting characteristics of the material, effectively accelerating the polymerization process. This results in a faster setting time, allowing clinicians to work more efficiently during surgical procedures.

In contrast, other substances do not have the same effect on the working time of Coe-Pak. Calcium carbonate is often used as a filler or buffering agent but does not significantly change the working time properties. Aluminum hydroxide may function as an abrasive or for other chemical properties, but it does not serve the purpose of reducing the time Coe-Pak remains workable. Silica gel is utilized typically to control moisture or as a desiccator, rather than actively shortening the setting time of dental materials. Thus, zinc oxide is specifically chosen for its ability to accelerate the setting process in this context, making it the correct answer.