For many years the wear enhancing benefits of concrete surface treatments, such as sealers, shake-ons and embedded aggregate hardeners have been accepted and used without question. On the other hand, chemical hardeners and densifiers have been met with some misgivings. The question most often asked is: “How can an already hard concrete be hardened further after it has set up?” The answer I hope to point out in this paper is: “By reinforcing the cement paste’s internal structural weakness.” – L&M Construction Chemicals
Harder Concrete: The back-story
The methods by which concrete’s intrinsic properties can be enhanced are very well known. A good, long lasting floor begins with concrete produced from high quality and properly proportioned raw materials that are placed in a timely manner at a low water cement ratio. Then, at the proper time, the surface is hard troweled to a dense finish. Finally, proper curing and the timing of joint cutting are a final yet essential part of the process. (See related article in this issue of Concrete News)
By using proper design criteria for the concrete mix we will likely have a concrete that is the best that nature and science has to offer. However, all concretes have inherent weaknesses that must be overcome to maximize their performance. For example, it is a well-known fact that concrete is strong in compression yet weak in flexure. Therefore, it must be reinforced with steel to overcome this weakness. Equally true, but less known, is that concrete is not uniformly hard. While this does not present a problem for structural concrete designed to carry a load, it does present a problem for concrete designed to resist wear caused by the daily operations of a business.
Micro-pitting, small voids mean big problems.
The big question is where do you apply the fix? In the case of structural members the fix is applied at the point of maximum flexure. In the case of floors, it is at the point of maximum wear, or as we call it: the near-surface wear zone. This is the part of the floor that is just under foot, approximately the top 1/8 inch. The near-surface wear zone is made up of a high percentage of cement paste and very fine aggregate that have been tightly toweled into a hard, dense surface. This hard, very dense surface has a flaw, however, known to be its propensity to micro-pitting. Micro-pitting is the creation of microscopic voids in the near-surface wear zone, generally through some sort of use. Micro-pits at the surface of a concrete floor appear in much the same fashion as potholes in a highway—starting at the edge and growing in size until the total surface is worn away. Eventually their presence renders the working elevation troublesome and even hazardous to the normal operations of many businesses.
Calcium hydroxide: Free lime ain’t free. It costs.
One of the primary causes of micro-pitting is the presence of calcium hydroxide (free lime), which is a by-product of the hydration process by which cement paste hardens. Microscopic deposits of calcium hydroxide can be found throughout the cement paste, but they are particularly troublesome when they are present in the near-surface. These deposits are very soft and do not contribute to the strength of the cement paste or to the surface durability of the concrete.
Methods for converting soft calcium hydroxide into a very hard and strong material have been known to the concrete industry from the very beginning. One of the first admixtures was volcanic ash and was placed in ancient concrete mixes by the Romans. In addition to naturally occurring pozzolans like volcanic ash, other sources are products that are the result of man’s activities, such as fly ash produced in the process of burning coal, or silica fume, which is a by-product of the silicon metal industry. All pozzolans have one thing in common, they are very rich in silica and when mixed in concrete they chemically react with calcium hydroxide, producing a stronger and denser concrete.
How do concrete chemical hardeners work?
Most chemical hardeners are also rich in silica in the form of soluble silicates, siliconates and other proprietary, reactive ingredients which, when placed on hardened concrete, chemically react with weak calcium hydroxide and produce a stronger and denser concrete surface.
One could refer to these hardeners as post-hardening admixtures since they react in much the same way as pozzolans when added to a concrete mix as admixtures. Silica, in the form of soluble silicates in a water solution, penetrates into the surface of the concrete and, as long as moisture is present, remains chemically active. Once it dries completely it will harden into a dense, insoluble mineral.
L&M Construction Chemicals recommends a maintenance program that requires frequent floor washing with water during the first six to twelve months following treatment. We have found that by using this simple washing procedure the uptake of water by the concrete is sufficient to keep the silica chemically active, reacting with calcium hydroxide as it becomes available over time as the cement hydrates. The end result is a much harder, denser concrete floor surface.
Timing of placement:
Two good reasons to wait before you apply a chemical hardener.
Both the time and method of installation play key roles in determining the effectiveness of a chemical hardener. New concrete should be properly cured before installation of a chemical hardener. It is a mistake to fall prey to the lure of claims that chemical hardeners and densifiers can be used as effective curing compounds, or that their early application has no detrimental effect on the resultant concrete. Early application of any chemical hardener, while convenient, does not maximize the hardening potential of the treatment for two strong reasons.
First of all, during the first days after placement the capillary pores of the concrete are saturated with mix water that has not been used to hydrate the cement. This free water is affecting the concrete in two ways. First, free water aids the on-going hydration of cement and secondly, it causes the pore structure to become swollen, and therefore causes a restriction in the size of the capillary pore. The fact that the capillary pores are filled with water and swollen greatly reduces the ability of any applied liquid (chemical hardener) to penetrate into the concrete near-surface. By the 6th or 7th day after placement, however, the free water in the capillary pores is greatly reduced, thus allowing the pore structure to be more open. At this time an applied liquid experiences greater penetration into the concrete.
Secondly, it is better to have more calcium hydroxide available at the time the chemical hardener is applied to the surface of the concrete in order to make more complete the long term chemical reaction. Under normal conditions most concrete will achieve generally about 60-65% percent of its hydration within 7 days and in the process will produce a proportional amount of the calcium hydroxide. L&M has found that waiting 7 days or longer before application of chemical hardeners will greatly increase the long term hardening results of a chemical hardener, because by waiting there will be a significant increase in the total number of potentially reactive sites available where chemical hardening and densification can take place immediately.
New chemical hardener/densifers require new installation methods.
Regarding application methods, many chemical hardeners are applied simply by spraying. No effort is called for and none given to work them into the surface of the concrete. While spraying a chemical hardener on the surface and allowing it to dry may be easy, it does not do an adequate job. The proper application of the new generation of chemical hardeners and densifiers, saturating the surface and using automatic or riding floor scrubbers, not only forces the reactive ingredients into the pores of the cement paste, it also purges the near-surface wear zone of structurally weak impurities. Removing these impurities allows for more contact between the chemical hardener and the calcium hydroxide compounds, thereby increasing the effectiveness of the chemical hardener. With the removal of these weak impurities from the wearing surface, the surface is now filled with a structurally sound material; thereby further densifing the concrete and producing a surface that resists micro-pitting.
The proof for concrete hardeners is in the back-up testing.
Owners have known the benefits of a chemically hardened floor for many years. Until recently the affirmation of these benefits has been solely from claims and myths. That has all changed. L&M Construction Chemicals completed tests performed by Construction Technology Laboratories, Inc. of Skokie Illinois. Data from these series of tests show that L&M’s SEAL HARD, when properly applied to the surface of concrete, can increase the wear resistance of that surface approximately 2-1/2 times. These same tests also show that SEAL HARD is the clear industry leader when compared to other well-known chemical hardeners.
Myths and misconceptions, #1:
The role of Tobermorite Gel
There have been many myths and misconceptions that have surrounded these products. One myth has it that chemical hardeners harden the tobermorite crystalline structure of the hydrated cement paste. This is not true. Tobermorite is the major crystalline structure produced during hydration. It is this crystalline structure that gives concrete its strength. Chemical hardeners chemically react and harden calcium hydroxide compounds that are not part of the tobermorite crystalline structure but are found in the cement paste.
Depth of penetration, protection where it counts
Another major misconception concerning chemical hardeners deals with the depth of penetration. There are those that claim a remarkable depth of penetration of several inches. This is just sales puffery and has no benefit in the real world. As in designing reinforced concrete beams the placement of the reinforcement is the key to success. The proper use of chemical hardeners should be to primarily reinforce (harden and densify) the near-surface wear zone.
It is this surface that must resist the wear imposed on it by traffic. After this surface is worn away the concrete floor becomes a candidate for replacement and the depth of penetration past this zone is of little benefit to the owner.
The use of chemical hardeners along with shake on aggregate hardeners: More than belt and suspenders.
Another long held misconception is that when a shake on hardener is used a chemical hardener is not needed. This belief springs from a lack of understanding of the internal workings of a concrete wearing surface. The wearing surface is made up of two elements, the cement paste and aggregate. Simply put, the cement paste is there to hold the aggregate in place.
The hardness and amount of aggregate is increased in the near-surface wear zone, the wear resistance of the concrete surface is increased. In order for the aggregate to resist wear it must remain in the near-surface wear zone. Aggregate that is dislodged or pulled out cannot increase wear resistance and leaves the remaining exposed hardened cement paste unprotected. It should be noted that the cement paste bonding the small, hard aggregate of the shake on hardener has the same shortcomings as the cement paste holding the aggregate in the concrete. When the cement paste is hardened using a chemical hardener, the shake on hardener benefits from an increased resistance to micro-pitting, similar to the benefits to concrete as explained earlier. The longer the cement paste survives in the near-surface wear zone, the longer the aggregate in the shake on hardener will remain in place and the more wear resistant the surface of the floor will remain.
Chemical hardeners and densifiers, a thing of the future.
It was not long ago that chemical admixtures for concrete were labeled a witches’ brew. Today they are regarded as essential to the industry. Chemical hardeners have to make the same trip as concrete admixtures, but this trip is being made in record time. With the ever-mounting evidence of good performance and supporting technical data, new generation chemical hardener/densifers (the post-hardening admixture) are now being thought of in the area of wear resistance in much the same way as reinforcing steel is in the area of structural concrete.