Why are my smooth hard trowel concrete floors always darker than my clients like?


Your question is often asked in our industry. The answers may be a combination of materials and events that lead to the darkened surface about which your client comment upon. The first thing to minimize or avoid is the use of calcium chloride. However, I suspect this is not the problem. Calcium chloride will cause more mottle or spotting than overall darkening.

Overall darkening has been traced to the following causes: The type of discoloration produced is influenced by both the alkali content of the cement and the calcium chloride content of the cement paste. Trowel burning causes extreme darkening discoloration. I suspect this is the coloration your clients are commenting about.

A blackening of the surface may result from attempts to hard trowel concrete after it has become much too stiff to trowel properly. Tool metal rubbing off onto the stiff concrete is a concern, and is used as a conventional explanation for blackened coloration. In truth, some of the coloration does come from the abraded metal particles. However, tests have shown a concrete's cement paste can be compacted and smoothed with a plate glass tool and result in similar blackening of the surface as with a metal tool.

The blackening is attributed to the over compaction of the plastic cement paste and the pressing out of the water in the plastic paste. The loss of the water causes the water to cement ratio to become low in water content and the cement paste begins to darken.

To minimize trowel burns, have an adequately staffed finishing crew. Reduce water evaporation losses with monomolecular films (L&M E-CON). Avoid the use of calcium chloride where possible. Lastly, do not "heel" the finishing blades out on to their edges.

Follow curing procedures to limit the effect of coloration on the concrete. Apply the specified curing membrane as soon as the conditions allow and repair any surface tears in the cure membrane surface.

Please contact us if the conditions you have are unique and the floor specifications are demanding, L&M wants to be a resource to our customers.

Thanks for Asking!