A. Acrylic cement modifiers
B. Polyvinyl acetate modifiers
C. Styrene butadiene modifiers
Other modifiers such as viscosity modifiers, flow modifiers, set-time modifiers, etc., will not be discussed here.
A. Acrylic modifiers: The acrylic resins and emulsions used in Portland cement mortar modification are from a chemical family called methyl esters of methacrylic acid. This family of acrylics is used to produce products from plexiglass to automotive finishes to concrete sealers to paint to dental repair materials to bone implant adhesion to mortar modification. The class we use for mortar modification is methyl methacrylate. You can tell by the smell. Acrylic emulsions smell like ammonia because ammonia is used to maintain the pH so the acrylic doesn't precipitate out like cottage cheese. If in doubt smell it.
The cool words don't matter but the chemistry does. In mortar modification it is a big deal that the acrylic version of the modifier (methyl methacrylate) in common use has both double carbon bonds and hydrogen to carbon bonds and a surplus of free hydrogen bondability. The carbon double bonds are incredibly strong. Think diamond. The carbon to hydrogen bonds are very short. Think about arm wrestling a guy with very short forearms. The excess or free hydrogen bonds will readily bond to anything else with an available electron particle. Think iron, aluminum and calcium, the metals that comprise the bulk of cement paste.
When water is added to Portland cement powder, complex reactions begin to change the blend of metallic oxides into a hard substance called a gel through a series of four main groups or phases of reactions. Each reaction produces byproducts that initiate the next reaction when the necessary energy threshold is reached. That is why you have initial set, a finisher set and 3, 7 and 28 day hardness. Tobermorite gel is the official name of the hard product that binds the aggregates to one another.
MAJOR COMPONENTS AND REACTIONS – WHY and HOW PORTLAND CEMENT WORKS
RAW MATERIALS, MINED, ASSAYED and ADJUSTED, FED INTO KILN AND BURNED TO FORM CLINKER to be GROUND
LIMESTONE = CaO + CaCO3 + DeltaE CO2
LIME Calcium Carbonate Heat Carbon Dioxide
CLAY = SiO2 + Al2O3 + Fe2O3 + Delta E + H2O
Silica Alumina Ferric Oxide Heat
CEMENT POWDER MIX WATER CONCRETE PRODUCTS and
BYPRODUCTS OF HYDRATION
2( 3CaO SiO2) + H2O = 3CaO SiO2 3H O + 3 Ca(OH)2
Tricalcium Silicate Water Tobermorite Gel Calcium hydroxide
2(2 CaO SiO2) + H2O = 3CaO SiO2 3H2 O + Ca(OH)2
Dicalcium Silicate Water Tobermorite Gel Calcium hydroxide
3 CaO Al2O3 + H2O + Ca(OH)2 = 3CaO Al2O3 Ca(OH)2 12H2O
Tricalcium Aluminate Water Calcium Hydroxide Tetracalcium Aluminate Hydrate
4CaO Al2O3 Fe2O3+H2O+2Ca(OH)2 = 6CaO Al2O3 Fe2O3 12H2O
Tetracalcium Aluminoferrite Water Calcium Hydroxide Calcium Aluminoferrite Hydrate
3CaO Al2O3 + CaSO4 2H2O + H2O = 3CaO Al2O3 CaSO4 12H2O
Tricalcium Aluminate Gypsum Water Calcium Monosulfoaluminate Hydrate
Acrylics for mortar modification were discovered in Germany in 1933 and have been in specialty use since the 1950's. German engineers developed special bridge spans that could deflect dramatically without failure in long spans. I have made 9 inch by 1 x 1 inch bars that I could deflect a full inch over time without failure. The primary contributions from acrylic additives are increased flexural strength which reduces cracking due to stress and improved bond strength. These benefits come about indirectly by holding onto the mix water so it is available for forming hydrogen bonds (hydration) instead of evaporating. In theory once this job is done the acrylic simply occupies space like an aggregate and is inert. In actual practice excess acrylic is used so there are additional considerations in the presence of solvents from sealers and cleaners which will soften the plastic film of excess acrylic. The bond strength is again an indirect result from the hydrogen bonds being able to form with the aggregates or other substrate prior to the water evaporating and not being available for forming bonds.
The technical writings you can find in the modified concrete literature usually say that acrylic modification does not increase compressive strengths. My tests show increased compressive strengths that, while not dramatic, are probably related to the principle of using low water concrete and a fog cure. See the test data I have included with this article. Much of the testing was done by Rohm and Haas, the dominant supplier until they were purchased by Dow Chemical who is now the dominant supplier.
The other benefit that makes acrylics superior as modifiers is that inert character. Once cured, the acrylic will not re-emulsify. In the presence of water and other chemicals common to concrete, acrylics will not "blush". That is a very good thing. I once maintained a 36 inch column of water on an acrylic modified mortar base 1/8 inch thick for three years. When the column was removed and the surface dried it was again completely serviceable, no effect on bond.
A note about powdered resins vs. aqueous or resin emulsions. It is, of course, more convenient to have everything available in a bag and just add water; however, you cannot get the same product by adding water. Here's why. In order to use a dry resin it must go into solution in a reasonable amount of time, say 5-15 minutes. A hard resin like the typical emulsions on the market will not do that. Additionally, the system has to be modified with other chemicals to get the acrylic to dissolve and the whole mix becomes too chemically sensitive. If you cannot get into solution timely you have production problems that are unacceptable. Acrylic resins once dry do not go back into solution easily. To cope with this reality a dry acrylic resin is a blend of at least two resins, one soft that will go into solution reasonably fast giving trowel or placement properties, and a hard or harder resin that will provide the end structural properties desired.
The theory is that the soft resin will go into solution quickly and give the modified properties desired for placement and finishing and the hard resin will go into solution before the product is fully set so the structural properties are achieved. Here's the problem: Solution time varies directly with temperature. A dry resin may perform well at 80F but not at all at 50F. Additionally, you have no dependable method to measure these factors. I used to get job failure calls on several manufacturers products every year when the weather started to change in Canada, spring and fall, but never a call from Southern California. I wonder why. These poor guys had called the manufacturers and were told they didn't follow instructions. I think the manufacturers probably didn't know their polymer would not perform at low temperatures.
B. Poly vinyl acetates are easily recognized by the smell, in emulsions at least. If you are working with a dry system you can sometimes smell it when wet but not always. If it smells like Elmer's Glue instead of ammonia it is poly vinyl acetate. Do not confuse acetates with polyvinyl alcohol. Polyvinyl alcohol is often a component of polyvinyl acetate to give it elongation and solubility features but is not used as a mortar modifier.
Poly vinyl acetates have a couple of advantages over acrylics but that doesn't mean you will want to use them.
1. They cost less. Acrylics achieve optimum performance at a dose of about 10% of the cement weight on a solids to solids basis. Often a 5% dose is plenty and you would rarely for any reason exceed 15% of the cement weight on a solids basis. That can be expensive. A poly vinyl acetate can probably give the same appearance of modification at less than one-third, maybe one-fifth those levels on a solids basis. For a lot of mortar that is a lot of money.
2. Poly vinyl acetate modified mortars have better workability and trowel properties. In technical terms that is things like fatness, flow and hang.
3. Initial structural properties are good to excellent. Cement hydration is well protected for the same reasons it is when using acrylics. Poly vinyl acetates mortars do not develop as good flexural properties as acrylics by a long shot but they yield excellent compressive strengths, they get very hard. Bond strengths are good to excellent but typically not as good as using acrylics. I think this is more of an index of hardness. Things that are brittle are more vulnerable to breaking bond, not because the bond is not good but because it is vulnerable to shock from the brittleness.
Poly vinyl acetate modified mortars have one glaring weakness. That is blush. When exposed to water or even high humidity they will soften and fail under stress. Even being sealed will not stop this problem if there is a high humidity environment for a prolonged period. Sealers will transmit water vapor at a rate of 4-5 pounds of water per 100 square feet. That is plenty of water to cause a problem if it goes on very long. In the Southwest US you will usually get away with it unless something goes wrong and as the plumber says in the movie Moonstruck, something always goes wrong. In the Northwest you will never get away with it. I was involved in the ICBO hearings for cementitious EIFS plaster substitutes and the stucco manufacturers got permission (along with other well financed concessions) to abandon acrylics in the mix design and not install vapor barrier behind the EPS board because, after all, it was Phoenix. That fall it rained 30 inches in Phoenix.
I am involved consulting on a job failure with a complicated terrazzo overlay labyrinth design. The zinc strips may have set up a cathodic (electrical) flow with the rebar in the slab causing the oxide of zinc to fracture the bond since zinc oxide takes up many times the volume of zinc. The overlay material is from a well known provider and trainer and their polymer is poly vinyl acetate and once the fracture allowed moisture to penetrate the overlay appears to have blushed, the bond failed, delamination began and is not repairable. A dollar catastrophe for sure but worse, a heartache, since it is a memorial to her dead son.
C. Styrene butadiene mortar modifiers work almost identically to acrylic mortar modifiers. They cost about the same, the dosage is about the same, the trowel properties and structural properties are about the same. The smell identification is much like acrylic with a “gas leak” odor. So you ask, “Why does it matter which I use.”
It matters a lot. Styrene butadienes will deteriorate in the presence of ultraviolet light. Some styrene butadienes are sold as styrene acrylics. That just means they will fail in the presence of UV light a little slower, but they will still fail. So where does styrene butadiene have a use. Underlayment. If you want to top it with an acrylic modified mortar it will perform equivalent to the acrylic. My opinion? Since there is little if anything to be gained over an acrylic modified mortar it only results in double inventory and risk of misuse. Stay with acrylics.
Conclusions? Unless you have a specific reason to use other modifiers stay with acrylics. They won’t let you down.
Good luck and good concreting.
The Doctor
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