In early senile cortical cataract, the clear wedge-like spaces are clefts and vacuoles caused by hydration of the lens cortex. Water separates the lens fibers but, at first, the fluid itself is clear, so the spaces can remain transparent.

They become opaque because continued fluid and electrolyte imbalance damages the orderly lens-fiber structure:

So, the fluid-filled clefts do not simply “turn white” by themselves. Progressive hydration leads to fiber degeneration plus protein aggregation, producing the typical white, wedge-shaped or spoke-like cortical opacity.

Kanski describes cortical cataract as starting with clefts/vacuoles from cortical hydration, followed by cuneiform or radial spoke-like opacification. Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., cortical cataract section.

Correct. The fluid itself is initially clear, so it does not chemically “turn opaque.”

With ongoing cortical hydration, the surrounding lens fibers swell, separate, degenerate, and lose their precise orderly arrangement. Their altered refractive index, together with the fluid-fiber interfaces, scatters light. Later, crystallin proteins in damaged fibers can aggregate, increasing the scattering.

So the opacity is mainly due to damaged/disorganized lens fibers and protein changes around the clefts, not clear fluid becoming white. The fluid clefts are the early sign of the process. Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., cortical cataract section.