The proper mixture of all the physical ingredients in grout is vital to the overall strength of the grout once it has cured. Too much water results in the elements separating, while too little water can make grouting a near impossibility due to the thickness. If you are mixing grout and it is a little too runny there, are only a couple of acceptable ways to thicken it up and still retain its structural integrity.
The normal grout mixture includes sand, cement, pigments for color and various additives that are part of the dry mix. The entire dry mix is thoroughly combined before it is packaged for sale and mixed together with water to form the grout that is used for various projects. The amount of water to dry mix is vital to the overall strength of the grout, and manufacturer's directions should always be followed to the letter in terms of quantities.
Importance of Mixture
During the mixing process you are blending all of the various dry elements together and letting them chemically react to each other as well as the water you have introduced into the mix. Slaking is the period after you first mix the grout when you let it sit for 10 or 15 minutes to soak up the water and then remix the grout before applying it to your installation area.
Latex Additive Versus Water
One of the ways you can try and thicken up your grout, if you have mixed just a little too much water and it's not necessarily runny but just on the thin side of creamy, is to add some latex additive into the mix. Latex is sticky and will add a certain element of creaminess to the grout, which can help so long as the grout hasn't been completely watered down into a soup-like mixture.
More Dry Mix
The only acceptable way to conform to the manufacturer's method of thickening runny grout is to add more dry mixture to the batch and mix it until a proper consistency is achieved. So long as you are using measuring cups, you will never use too much water, but on the off-chance you somehow spill a little too much into the mix, it's an easy fix, as you just need to include a little extra dry.