How soon can we grind/polish new concrete?

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We've been asked to bid polishing the floors of a new pump station. There is only about 5,000 sq.ft. but as soon as the floors are poured, all the trades would like to get on with putting up block walls and bringing in all massive pump and pipework.

How soon could we start grinding? The floor will be 4,000psi or better mix. I would like to at least be able to hit the entire floor with the metals before everything gets turned into a massive hand-polishing job. I think we could talk them into staying off the slab for seven days. For some of the most congested areas we may even be able to get a bit more time.

if your going to wet cure it

if your going to wet cure it you will be able to get on the floor in 7 days

We would be grinding with wet

We would be grinding with wet metals.
I know concrete cure time varies based on conditions. Assuming proper curing, would we be able to polish to a final 800 finish?

Opps. Silly me. You said wet

Opps. Silly me. You said wet cure, not wet grind. Yes I understand what you are saying, there is a difference. Thank you.

yeah if you wet cure a slab

yeah if you wet cure a slab you should have no prob getting on it in 5 to 7 days

Alaska's picture
I like to have a test

I like to have a test cylinder broken to show compressive strength at or above 4500 psi before beginning to polish. With the right mix design and wet curing we've been able to achieve this at day three. I wait until after day seven to densify. No topping "guard" coats until the slab moisture is below 75% Rh.

Alaska, I'm curious as to why

Alaska, I'm curious as to why to wait with the densifier? Also wondering what the reason is for waiting with guard?

Sorry to ask so many questions but I'm getting to be an old guy and I don't know if I will live long enough to learn all I need to learn, the hard way. ;)

Alaska's picture
uncleshred, Don't worry about

uncleshred, Don't worry about the age issue. I'm old enough to be the G.Grandfather to the younger kids on the job site and I still don't know it all either! However, degrees in chemistry and physics help me understand some of the process variables.

1. Wait for the densifier so there is no reactive competition. I want as much natural reactant formation within the mix using only water as is possible. The densifier reaction should take place later. I wait as long as is consistent with the construction schedule.

2. If the slab Rh is too high the guard may fail due to moisture gradient migration upward. While the guard should be vapor permeable, provided it is a good product and not applied too heavily it still retards surface evaporation, thus allowing an increase in moisture at the surface. I have not noticed any of the guard manufacturers providing restrictive guidance about slab moisture so I use 75% Rh as a max. I have seen guard failure with extremely high moisture content, e.g. greater than 85% Rh and vapor transmission rate above eight pounds/1000.

Cori's picture
So it sounds like the waiting

So it sounds like the waiting is pretty much just for the chemicals... both densifier & guard. Are there really any other reasons for a wait beyond 7 days? I’ve mostly seen specs calling for a 28 day wet cure. Anyone know if this is just an overkill spec or sort of a dummyproof way to ensure the floor is ready for the chemicals?

Alaska's picture
'tis overkill in my opinion.

'tis overkill in my opinion.

28 days is an old "magic" number for fully cured concrete. We know now it is simply an arbitrary number with no significance other than being of historic interest.

However, Cori ... to clarify. Waiting before applying the guard is for the moisture content (Rh is relative humidity) to be low enough to avoid trouble.

We currently are grinding new

We currently are grinding new pours on a 34,000 sq. ft. project. At 5 days it was to soft, wet grind with 40g metals large agg to easy and was hard to control quality. At 7 days it worked well- med agg pretty easy and we could control the quality.

Densifier reacts with the calcium hydroxide of concrete to create CSH Gel. The theory is it takes about 28 days for the right amount of calcium hydroxide to be present for the densifier to react with.

28 days is the most widely accepted idea for proper hydration to occur.

Jim Cuviello
28 days if you want to cover

28 days if you want to cover all your bases. We have ground at 5, 7, 15, 30 wet curing, curing compound and most other combinations. Only at 28 days or older have we pretty much never run into any problems. The issue we run into is the little cement fines and very small sand aggregate releasing from the surface causing micro pitting.

As far as densifiers go it is my understanding that you want the majority of the chemical reaction that takes place when concrete cures to be finished before applying them. So they they react with the left over CSH sitting there unreacted. Although, I would think you could apply them immediately providing the additional chemical to react with the CSH. But then again you really want the reaction to take place in the open capillaries of the mostly cured concrete to help fill them. Not sure if this would happen if applied immediately after the pour since you will still have vapor coming through the slab since it is still it is still curing.

Densifiers seem to be vodo magic and all the manufacturers say something a little different other than they all react with the CSH.

This reminds me of something.

This reminds me of something.
The 28 day (magic number) in application of the "polishing chemical".

BUT- you can apply ashford or sealhard or any other hardener to concrete that is a day old to 10000 days old.

Why- exactly the same chemistry, actually less % of solids.

I call BS.

Alaska's picture
CSX, As you observe the


As you observe the silicates you mentioned are sanctioned by the mfg for application to new concrete for the purpose of hardening. And Jim C. provides the valid caution that if you want to cover all your bases you wait out the 28 days, if it calls for that in the mfg's tech data sheet.

As I mentioned in my 3/13 response to your inquiry, wait as long as possible within the constraints of the construction schedule for the natural water reactions to occur. Cement does this:

Tricalcium silicate crystals form most rapidly, therefore they are responsible for most of the concrete strength in the first week. Dicalcium silicate, which is formed in a slower crystalization reaction, contributes continuously to strength development as it forms, generally becoming the most dominant strength component in mature concrete. Concrete is arbitrarily defined as mature at 28 days, although so long as it is moist it will continue its hydration reaction forming more bonding crystals and gaining strength nearly indefinitely. Somebody, sometime had to pick a number so there could be an agreed “STOP” the concrete is “DONE”, an 28 days was born.

The equation for the hydration of tricalcium silicate is
2 Ca3SiO5 + 7 H2O ---> 3 CaO + 2 SiO2 + 4 H2O + 3 Ca(OH)2 + 173.6 kJ

The equation for the hydration of dicalcium silicate is
2 Ca2SiO4 + 5 H2O---> 3 CaO + 2 SiO2 + 4 H2O + Ca(OH)2 + 58.6 kJ

Notice that the hydration of both dicalcium silicate and tricalcium silicate have the same reaction products. That is both calcium silicate hydrate crystals (CaO–SiO2–H2O) and Calcium Hydroxide (Ca(OH)2-Ca(OH)2) crystals.

If the cement were perfectly balanced chemically, there would be no opportunity for a densification reaction. However, like most of the real world in chemistry it is imperfect and generally there is an imbalance leaving some unreacted calcium hydroxide available to react in the future with the densifier you apply. Here you can pick your favorite way to write the compound as these are exactly the same: Calcium Hydroxide = Slaked Lime = Hydrated Lime = Ca(OH)2 or CaH2O2

So what happens if you introduce a silicate compound early in the reaction? This additional reaction competes actively with the innate hydration reaction and in my opinion results in a surface condition that is less than ideal by biasing the innate reaction toward the formation of CaSO4 , if there happens to be any reactively available sulfur (S) available in the mix water or as impurity in the cement itself.

What is the densifier hardener reaction itself you may ask? The densifier’s or hardeners react with the unreacted calcium hydroxide to form additional calcium silicates. The exact silicate formed depends upon the manufacturer’s formulation, which is usually proprietary and thus “…. your mileage may vary.…” with respect to actual effectiveness in both densification, hardness and abrasion resistance. Not all are created equal! Also, note that the densification reaction also takes time to form the calcium silicates. IF the densification reaction time (that is the time the floor is actually wet with the densifier) is short then less reaction product can be formed even if there is an excess of reactants available. So, while some silicate compounds tend to react a little bit faster (lithium silicate is generally faster) than others, IF the reaction time is short then they will actually produce less total calcium silicate and your floor will not be fully densified, hence have a weak surface, poor wear characteristics, etc.

Therefore …. I suggest you wait as long as possible prior to densification and keep the floor wet with densifier as long as you can regardless of the silicate formulation you choose. All within reason of course. The fruits of this approach won’t look any different a few days out, but will look much different when compared to your competitors years later.

James Vermillion, ACI, ASTM, ICRI, ASCC
Master Artisan
NOT recognized by CPAA as a Master Craftsman
……. but who’s work photos as of February 10, 2011 compromised nearly 10% of the CPAA’s examples of excellent polishing!

Alaska, Very informative

Very informative post on the chemical reactions. You mention that the lithium silicates take less time to react than sodium silicate products. Why is that? Is the lithium silicate chemical structure less stable and therefore more willing to react with the calcium hydroxide? Just curious. Along those same lines, the lithium manufacturers typically suggest a coverage rate of 500 s.f./gal whereas the sodiums recommend a lower coverage rate of 200 s.f./gal. My concern with this has always been that the sodiums typically have a higher solids content. For instance, Retroplate is close to twice that of Consolideck. Do the lithiums react more? By that I mean can you only expect say 20% of the reactive solids in a sodium silicate densifier to react whereas a lithium reacts at 80% of it's reactive solids, or is this just a way for lithium manufacturers to make their product costs seem lower on a per square foot basis than sodiums. I typically seem to get better results with retroplate when I apply it. And if a slab is being difficult I will usually apply the retroplate to help keep things together as it performs better than the lithium in these instances. With that being said, I've never flood coated a lithium on a difficult piece of concrete so I can't say that the sodium necessarily does a better job, just that the lower coverage rates of the sodiums seem to provide better densification. Following this same logic however, my concern is that years down the road I may find the slabs I've used the sodium silicates on will superior to those that I've densified using the lithiums. Any input you have on this would be appreciated.

Alaska's picture
Clarity, Generally speaking


Generally speaking the further upward one moves on the periodic table, the greater the potential reactivity. Our common densifiers are in the first group of the periodic table, alkaline metals. In general sodium densifiers react a little faster than potassium and a little slower than lithium. However, there is a very small difference in time and of little if any practical sequella.

Solids content we assume to be the stuff we want to have in our products. However, unfortunately there is no explicit guarantee that the solids hyped by a manufacturere are the available reactive compounds we want and need. We have no way of knowing exactly what we are getting as the manufacturers are generally guarded about the actual content of their products. However, you are correct that we wish to have more reactive components in the bucket. Unfortunately we don’t know from the literature exactly how much more or less of this we have from one brand to another nor from one type to another. It would be nice if the manufacturers were more transparent and forthcoming with this information, but I don’t expect that to happen within my lifetime. So, our options are to apply as much as we can discern from the labels, pay for laboratory analysis ourselves, reverse engineer the products and learn from experience.

I can’t support the statement that one type of densifier has more reactive component than another. I believe the reactive content varies from brand to brand and varies between products within a brand. For example, Prosoco is open enough to disclose that their LS product has 16% total solids while their LS/CS product has only 6% total solids. The Prosoco specification sheet indicates that the LS/CS product has an active content of 6% which is the same as the total solids content of LS/CS. However, the specification sheet for the LS product indicates an active content of only 14.5% for the LS product, which is not the same as the total solids content of 16%. See the difficulty? Not enough information is easily available to make a comparison even within one product line, let alone between manufacturers.

As to how much of the active compound is actually used in a particular slab, that is a whole ‘nuther can of worms. Chemical reactions are influenced by a couple things which we can control, e.g. temperature and the amount of time we allow the reactants to remain in contact. Densification reactions are not fast, they take time sort of like baking bread. You can’t slap the densifier on the floor, clean it off and say the job is done. The reaction only occurs while the densifier is on a warm floor and still wet. The longer the warm floor is wet with densifier, the more opportunity there is for reaction products to form. The warmer the floor is while it is wet with densifier the more reaction product you will probably get. This is simple chemistry, this is how it works. Once the floor is dry, the reaction stops. It will start again when the floor is re-wetted if there are soluble reactants still available within the voids of the slab.

While I can’t make any definitive statement about which is absolutely better in the long term, my personal anecdotal experience has been that I have had better apparent long term results using older sodium based densifiers and their longer application protocols. I too, have not used any of the lithium based products for flood coating due to their higher cost.

I share your concern for the longevity of slabs which have received short duration applications of any densifer. I doubt they will endure as well as an equivalent slab given a longer application of any type densifier. Again however, wild cards …. how much reactive compound is available in the densifer used and how much reactive compound is available in the slab? My best counsel on this is use a quality densifer and keep it wet on the floor as long as you can afford to do so. I’ve personally had the best success using L&M’s SealHard for flood coating and FGS Hardener Plus for final densification. With proper maintenance I have floors in heavy use that look great all the way back to eight years ago.

I don’t work for L&M, nor do I sell densifier.

Mark - Nufinish
I would be careful which

I would be careful which advice you chose to follow when it comes to forums, always air on the side of caution.
There's a reason why a 28 day rule exists, I would recommend following it. If you are going to break that rule I would not go below 21 days because of a high risk of pop out and it's not just aggregate but sand and you could have a low quality floor/polish as a result.
Keep in mind when you grind the floor at say 7 days as some are recommending you are allowing the concrete to lose moisture more rapidly because you have opened it up before it has properly cured. This results, from my experience, in a lower quality concrete slab and sometimes a very poor quality floor.


Mark Rossi
Nufinish Corporation

Alaska's picture
28 days covers the legal

28 days covers the legal bases, as was correctly pointed out by Jim C. As Mark R. points out, starting too early will not produce a good result.

Compression strength testing covers the real world of aggregate bonding. If the aggregates are not sufficiently bonded, as shown by low compressive strength, then you must wait before polishing or take steps to improve the bonding. This is true regardless of the age of the concrete.

Testing is always a better starting point than rules of thumb. Make your decisions based on testing plus experience.