Unfortunately, concrete can become discolored due to conditions present within the concrete itself or as a result of external influences. Below are general comments on the subject. More detailed information can be found in the publications listed at the end of this article.
Regarding calcium chloride: it accelerates the hydration process. It also retards the hydration of the ferrite compound in the Portland cement. This compound usually becomes lighter with hydration. But, in the presence of calcium chloride the retarded, unhydrated ferrite phase remains dark. In other words, and in my experience, increased contents of calcium chloride can darken the concrete. To make the explanation even more lengthy; in the absence of moist curing, the ratio of alkali content of the cement (all cement contains a percentage of alkali ) compared to the quantity of calcium chloride, dictates weather we see light sport on a dark background or dark spots on a light background.
Blackening of the surface is common when the concrete is “burned by late and vigorous troweling”. The more the troweling effort, the more compacted and dense the concrete surface becomes. The result is a reduction in the moisture content (water cement ratio) at the surface. Also, abrading of the metal from the trowel on the hardening concrete surface has a darkening effect.
Uneven curing of the surface will result in uneven color. When using plastic sheeting over concrete use care to either entirely “tent” the surface or entirely lay the plastic sheet directly and evenly over the surface. Both efforts are extremely difficult, time consuming, and costly. There must not be any occasional “tents, bubbles or ripples”. If this occurs, a “pronounced mottled” (color from light to dark, etc.) appearance will result. What happens is that on warm days, each ripple becomes a miniature greenhouse where the evaporation-condensation cycle is repeated several times. The buildup of condensate water around the ripple and in low spots causes the variation in curing and therefore variations in color. This is especially true in concrete containing calcium chloride.
Air cured concrete (which is to say… lack of curing) exhibits the greatest amount of discoloration. Thoroughly moist-cured concrete exhibits the least. Uneven curing will result in uneven coloring. In my opinion, the final color of the surface of the concrete slab is greatly dependent on 1) the moisture content (water-cement ratio) at the surface during the time of the final finishing effort, 2) the intensity of that effort, and 3) the curing conditions.
Lastly, avoid changes in raw materials in the concrete, such as cementitious material or sand sources. These individual materials may differ in color. Stay with the same concrete supplier and the same mix design. Avoid heavy or varying dosages of calcium chloride and varying dosage rates or the intermittent use of high range water reducers (superplasticizers) that alter the water-cement ratio. Minimize inconsistencies.
FAQs
Dark spots can be removed with a single washdown of water if concrete does not contain calcium chloride. In concrete containing calcium chloride, it is often necessary to wash the area several times. It appears that best results are obtained when such discolored concrete is washed soon after the spots appear.
Can you fix discolored concrete? ›
Cleansing — For mild discoloration, repeatedly flushing the hardened concrete surface with hot water, then scrubbing with a stiff brush, can reduce or remove discolored blotches. Acid treatment — If the discoloration is more severe, a mild acid solution of 1-3% muriatic acid to water can help remove the discoloration.
Is concrete discoloration normal? ›
Answer: You should expect some discoloration with typical variations in trowel effort, ambient conditions, and concrete setting. You should not expect concrete slabs to be perfectly uniform in color, as they do not have manufacturing controls like floor covering materials.
Why did my concrete turn brown? ›
Inconsistent concrete mixtures and improper water content during pouring are two of the most common causes. Variations in ingredients or water ratios lead to poor cement hydration and strength gain, affecting curing and color.
Will dark spots on concrete go away? ›
This discoloration is usually removed by high-pressure washing with high-pressure water and, if necessary, chemical cleaning agents. The use of calcium chloride on concrete can discolor the surface.
How do I get my concrete back to its original color? ›
WAYS TO MAKE STAMPED CONCRETE LOOK NEW AGAIN
- Add highlights with an antiquing stain.
- Apply a tinted sealer.
- Remove old sealer, restain and reseal.
- Apply a product specifically designed for restoration, like Brickform Refresh or Increte Renovate.
Apply vinegar or bleach to the discolored areas. If you use vinegar, do not dilute it. Pour it onto the concrete and scrub it into the surface with a scrub brush. If the discoloration starts to fade, repeat until it disappears completely.
How do you fix concrete ghosting? ›
Antiquing Concrete Stain is a great option to blend the colors on more porous floors. Concrete dye can also be used to cover imperfections and accent the floor as needed. Topical stains can disguise the tile ghosting still visible after grinding.
Why does my new concrete look dirty? ›
Factors found to influence discoloration are: the use of calcium chloride, variation in cement alkali content, delayed hydration of the cement paste, admixtures, hard-troweled surfaces, inadequate or inappropriate curing, changes in the concrete mix, and concreting practices / finishing procedures that cause surface .. ...
What makes concrete look new again? ›
Seal the Surface
Allow the sealer to dry and cure according to the recommended time before using the surface. For the Do-It-Yourself-ers, restoration, overlay, staining, and painting are all realistic ways to make your old, worn-out concrete look like new.
Concrete may appear discolored in certain areas as it takes up to 60 days for the concrete to be fully cured. This may cause the concrete to look spotty in areas because there are different amounts of moisture evaporating from each section of your concrete.
What is concrete mottling? ›
Mottled concrete is irregular shapes of different shades of white- gray to gray-black. These blotches are restricted to the surface, normally only 1/32" to 1/8" deep.
How do you remove oxidation from concrete? ›
Look for a rust remover that contains oxalic acid, as that's the ingredient that will really get the job done. Other acids that are effective include trisodium phosphate, which you'll mix with water, or hydrochloric acid.
What is spalling concrete? ›
Spalling is a term used to describe areas of concrete which have cracked and delaminated from the substrate. There are a number of reasons why spalling occurs including freeze thaw cycling, the expansive effects of Alkali Silica Reaction or exposure to fire.
Can stained concrete color be changed? ›
Yes, the color of stained concrete floors can be changed. There are several methods available to alter the color of stained concrete, including acid staining, water-based staining, and concrete dyeing. These methods allow for the application of new colors to the existing stained concrete surface.
Can dark stained concrete be lightened? ›
If the desire is to lighten the color, the best results I have seen come from pulling some of the initial stain out of the concrete. By using the carrier on which the stain is based (water, acidic water, acetone, alcohol, etc.), you can usually pull some or all of the color back out.
Can concrete color be changed? ›
Since the color comes from a chemical reaction, there is no easy way to change the color of acid stained concrete; you can add another layer of concrete on top and stain this a completely new color, or you can add to the existing stain to deepen it and possibly change the hue slightly.
Will colored concrete lighten as it cures? ›
Be certain to use curing, sealing, and cure-and-seal products approved for use with colored concrete and follow manufacturer's recommendations for application. Concrete will continue to lighten until it is fully cured. Allow the concrete to cure at least 30 days before inspecting it for color match or appearance.
