What is the role of silica sol in phosphate-bonded investments?

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It is coated on the investment walls to cause hygroscopic expansionIt enhances both the strength and setting expansion of phosphate-bonded investmentsIt functions as a setting retarder in silica-bonded investmentsIt serves as a refractory material in silica-bonded investments

Explanation

Silica sol is used to improve the mechanical strength and increase the setting expansion specifically in phosphate-bonded investments. It is not applied to produce hygroscopic expansion, nor does it act as a retarder or refractory in silica-bonded investments.