I wish Todd were here so I could argue with him

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Fred, if you are still with us, the only thing that a latex additive in your grout will do that is NOT good is make clean up more difficult. Other than that , the use of the proper latex additive will INCREASE all the properties of the cement grout, including water resistance, compressive strength, stain resistance, flexibility, color retention, moisture retention (enhances the cure but does not accelerate it), and reduces or eliminates efflorescence.