HOW CAN ADMIXTURES INHIBIT THE FORMATION OF EFFLORESCENCE ON/IN CONCRETE

Introduction

 Here is another compilation over the years of my self-study and research that become my reference to my product developments. This compilation will discuss efflorescence formation on or in cementitious substrates and how to prevent its formation using the right type of concrete admixtures.

 Concrete or any cementitious mineral substrates are the most versatile building materials used in civil infrastructure. Concrete consists mainly of cement binder, sand aggregate, and additives.  Water is also required – for hardening the cement and attaining the desired processing consistency. Concrete is a porous material with a wide range of pore sizes. Concrete bulk properties and quality are dependent on concrete mix design.  Concrete should be workable, finishable, strong, durable, watertight, and wear-resistant. These qualities can often be obtained easily and economically by the selection of suitable materials rather than by resorting to admixtures (except air-entraining admixtures when needed). One way of determining concrete quality is by the water/cement value, the ratio of mixing water to cement. Excess water results in an increased number of capillary pores in the cement and thus a loss in rigidity.

 What are Concrete Admixtures?

 Admixtures are those ingredients in concrete other than Portland cement, water, and aggregates, which are added to the mixture immediately before or during mixing to produce some desired modification to the properties of fresh and hardened concrete.

 The major reasons for using admixtures are:

1. To reduce the cost of concrete construction

2. To achieve certain properties in concrete more effectively than by other means

3. To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions

4. To overcome certain emergencies during concreting operations

 Admixtures can be classified by function as follows:

1. Air-entraining admixtures - are used to purposely introduce and stabilize microscopic air bubbles in concrete. Air-entrainment will dramatically improve the durability of concrete exposed to cycles of freezing and thawing.

2. Water-reducing admixtures - are used to reduce the quantity of mixing water required to produce concrete of a certain slump, reduce the water-cement ratio, reduce cement content, or increase slump. Typical water reducers reduce the water content by approximately 5% to 10%. Adding a water-reducing admixture to concrete without reducing the water content can produce a mixture with a higher slump. An increase in strength is generally obtained with water-reducing admixtures as the water-cement ratio is reduced.

3. Plasticizers - often called superplasticizers, are essentially high-range water reducers meeting ASTM C 1017; these admixtures are added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete (Fig. 6-9). Flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction while still remaining essentially free of excessive bleeding or segregation.

4. Accelerating admixtures - used to accelerate the rate of hydration (setting) and strength development of concrete at an early age.  Accelerators are designated as Type C admixtures under ASTM C 494 (AASHTO M 194).

5. Retarding admixtures - are used to delay the setting rate of concrete. Retarders are sometimes used to: (1) offset the accelerating effect of hot weather on the setting of concrete; (2) delay the initial set of concrete or grout when difficult or unusual conditions of placement occur, such as placing concrete in large piers and foundations, cementing oil wells, or pumping grout or concrete over considerable distances; or (3) delay the set for special finishing techniques, such as an exposed aggregate surface.

6. Hydration-control admixtures - They consist of a two-part chemical system: (1) a stabilizer or retarder that essentially stops the hydration of cementing materials, and (2) an activator that re-establishes normal hydration and setting when added to the stabilized concrete. The stabilizer can suspend hydration for 72 hours and the activator is added to the mixture just before the concrete is used. These admixtures make it possible to reuse concrete returned in a ready-mix truck by suspending the setting overnight. The admixture is also useful in maintaining concrete in a stabilized non-hardened state during long hauls. The concrete is reactivated when it arrives at the project.

7. Corrosion inhibitors - are used in concrete for parking structures, marine structures, and bridges where chloride salts are present. Corrosion-inhibiting admixtures chemically arrest the corrosion reaction.

8. Shrinkage reducers - have potential uses in bridge decks, critical floor slabs, and buildings where cracks and curling must be minimized for durability or aesthetic reasons. These admixtures have negligible effects on slump and air loss, but can delay setting. They are generally compatible with other admixtures

9. Alkali-silica reactivity inhibitors - Chemical admixtures to control alkali-silica reactivity (alkali-aggregate expansion) were introduced in the 1990s (Fig. 6-18). Lithium nitrate, lithium carbonate, lithium hydroxide, lithium aluminum silicate (decrepitated spodumene), and barium salts have shown reductions of alkali-silica reaction (ASR) in laboratory tests

10. Coloring admixtures - Natural and synthetic materials are used to colour concrete for aesthetic and safety reasons. Generally, the amount of pigments used in concrete should not exceed 10% by weight of the cement. Pigments used in amounts less than 6% generally do not affect concrete properties.

11. Miscellaneous admixtures such as: Workability, bonding, dampproofing, permeability reducing, grouting, gas-forming, antiwashout, foaming, and pumping admixtures.

                a. AIR DETRAINERS - Air-detraining admixtures reduce the air content in concrete. They are used when the air content cannot be reduced by adjusting the mix proportions or by changing the dosage of the air-entraining agent and other admixtures.

               b. GAS-FORMING ADMIXTURES - Aluminum powder and other gas-forming materials are sometimes added to concrete and grout in very small quantities to cause a slight expansion of the mixture prior to hardening. This may be of benefit where the complete grouting of a confined space is essential, such as under machine bases or in post-tensioning ducts of prestressed concrete. These materials are also used in larger quantities to produce autoclaved cellular concretes. The amount of expansion that occurs is dependent upon the amount of gas-forming material used, the temperature of the fresh mixture, the alkali content of the cement, and other variables.

               c. ANTIWASHOUT ADMIXTURES- Antiwashout admixtures increase the cohesiveness of concrete to a level that allows limited exposure to water with little loss of cement. This allows placement of concrete in water and underwater without the use of tremies. The admixtures increase the viscosity of water in the mixture resulting in a mix with increased thixotropy and resistance to segregation. They usually consist of water-soluble cellulose ether or acrylic polymers.

               d. GROUTING ADMIXTURES - Portland cement grouts are used for a variety of purposes: to stabilize foundations, set machine bases, fill cracks and joints in concrete work, cement oil wells, fill cores of masonry walls, grout prestressing tendons and anchor bolts, and fill the voids in preplaced aggregate concrete. To alter the properties of grout for specific applications, various air-entraining admixtures, accelerators, retarders, and nonshrink admixtures are often used.

               e. FUNGICIDAL, GERMICIDAL, AND INSECTICIDAL ADMIXTURES – Bacterial and fungal growth on or in hardened concrete may be partially controlled with fungicidal, germicidal, and insecticidal admixtures. The most effective materials are polyhalogenated phenols, dieldrin emulsions, and copper compounds. The effectiveness of these materials is generally temporary, and in high dosages, they may reduce the compressive strength of concrete.

               f. BONDING ADMIXTURES AND BONDING AGENTS - Bonding admixtures are usually water emulsions of organic materials including rubber, polyvinyl chloride, polyvinyl acetate, acrylics, styrene-butadiene copolymers, and other polymers. They are added to Portland cement mixtures to increase the bond strength between old and new concrete. Flexural strength and resistance to chloride-ion ingress are also improved. They are added in proportions equivalent to 5% to 20% by mass of the cementing materials; the actual quantity depending on job conditions and type of admixture used. Some bonding admixtures may increase the air content of mixtures. Non-re-emulsifiable types are resistant to water, better suited to exterior application, and used in places where moisture is present.

               g. PUMPING AIDS - Pumping aids are added to concrete mixtures to improve pumpability. Pumping aids cannot cure all unpumpable concrete problems; they are best used to make marginally pumpable concrete more pumpable. These admixtures increase viscosity or cohesion in concrete to reduce dewatering of the paste while under pressure from the pump. Some pumping aids may increase water demand, reduce compressive strength, cause air entrainment, or retard setting time. These side effects can be corrected by adjusting the mix proportions or adding another admixture to offset the side effect.

               h. PERMEABILITY-REDUCING ADMIXTURES - Permeability-reducing admixtures reduce the rate at which water under pressure is transmitted through concrete. Some supplementary cementing materials, especially silica fume, reduce permeability through the hydration and pozzolanic-reaction process. Other admixtures that act to block the capillaries in concrete have been shown to be effective in reducing concrete corrosion in chemically aggressive environments. Such admixtures, designed for use in high-cement content/low-water-cement ratio concretes, contain aliphatic fatty acid and an aqueous emulsion of polymeric and aromatic globules Table 1 provides a much more extensive classification of admixtures.

               i. DAMPPROOFING ADMIXTURES - Admixtures known as dampproofing agents include certain soaps, stearates, and petroleum products. They may, but generally do not, reduce the permeability of concretes that have low cement contents, high water cement ratios, or a deficiency of fines in the aggregate. Their use in well-proportioned mixes may increase the mixing water required and actually result in increased rather than reduced permeability. Dampproofing admixtures are sometimes used to reduce the transmission of moisture through concrete that is in contact with water or damp earth. Many so-called dampproofers are not effective, especially when used in concretes that are in contact with water under pressure.

*Superplasticizers are also referred to as high-range water reducers or plasticizers. These admixtures often meet both ASTM C 494 (AASHTO M 194) and ASTM C 1017 specifications.

 What is efflorescence and its Causes?

 Efflorescence of concrete masonry is another problem associated with water penetration that is particularly important for decorative concrete masonry. Efflorescence is caused by moisture moving through the capillaries and carrying the calcium hydroxide produced from cement hydration to the surface. The carbonation of the calcium hydroxide on the surface results in insoluble white calcium salt efflorescence. Efflorescence may also be caused by the transport of soluble salts, due to the permeability of concrete masonry, either from the sub-soil or from ground water and re- crystallisation at the surface after water evaporation.

 Efflorescence does cause discoloration of the affected surface.it should be determined whether it is a primary efflorescence (whitish bloom or colour fade), which develops during setting and curing of the any cement and mineral based substrates, or a secondary efflorescence (uniform discoloration or localized encrustations), which develops later where moisture or water exits the concrete or any cement/mineral based substrates. Alternatively, it may be a cryptoflorescence, which is a salt crystallized within the pore structures of the concrete. It forms below the surface and is not visible until the crystal growth develops enough to cause surface scaling and spalling. If the sources of efflorescence--soluble salts, water/moisture in contact with the salts forming salt solutions, and a passage allowing salt solution to migrate to the surface--are not minimized or at least one is eliminated, efflorescence may possibly reoccur.

 Admixtures containing sodium and potassium may increase the risk of soluble efflorescence. Admixtures affect the pore structure of the hardened cement paste, and can make it more or less resistant to the movement of water depending on the concrete mix design.

Permeability of concrete is influenced by its porosity and interconnectivity of pores in the cement paste, and microcracks especially at the interface between paste and aggregate. Porosity and interconnectivity are mainly controlled by the water/cement ratio (w/c), degree of hydration, and the degree of compaction. The permeability and hydrophilic nature of concrete products leads to easy water penetration. Water penetration is a well-known factor affecting the performance and the durability of concrete masonry.

 How To Prevent Efflorescence Occurrence?

 When concrete and other mineral building materials come into contact with water, they absorb an amount, which depends on their porosity. This contributes to the Efflorescence and salt damage by hydration and crystallization.

 Efflorescence of concrete substrates is one of the problems associated with water penetration that is particularly important for decorative concrete. Efflorescence is caused by moisture moving through the capillaries and carrying the calcium hydroxide produced from cement hydration to the surface. The carbonation of the calcium hydroxide on the surface results in efflorescence of an insoluble white calcium salt.

 One of the best methods of decreasing permeability in concrete is to increase the moist-curing period and reduce the water-cement ratio to less than 0.5. Most admixtures that reduce water-cement ratio consequently reduce permeability.

Some supplementary cementing materials, especially silica fume, reduce permeability through the hydration and pozzolanic-reaction process. Other admixtures that act to block the capillaries in concrete have been shown to be effective in reducing concrete corrosion in chemically aggressive environments. Such Admixtures may influence efflorescence depending on their composition, how they are used and, of course, the dominant water transport mechanism.

 Water resisting (Water repellent) admixtures can minimise water movement within the concrete and hence reduce problems associated with water absorption and efflorescence of the concrete.

Efflorescence control admixtures in particular are designed to fight efflorescence before it forms Admixtures may be used to improve concrete compaction, for example by reducing its porosity (capillary pores) and water permeability in general, but also allow the optimisation of the water/cement ratio while maintaining the required processability of the concrete. Water reducers and other admixtures that are able to retain or to release water can act in this way, but should not contain large amounts of highly soluble ions such as sodium, potassium, chloride or sulphate.

 Admixtures such as integral curing agents or waterproofing agents may be effective against efflorescence thanks to the hydrophobisation of all surfaces (inside and outside) or capillary stabilisation; provided that their soluble salt content is low and they do not entrain excess air when the concrete is mixed. By avoiding the evaporation of the mixing water and stopping ingress of external water, they strongly affect the relevant material transport mechanisms of concrete.

 Conventional oil-based water repellent admixtures such as stearates or oleates have been used in pressed concrete for many years .The performance of these admixtures is not satisfactory because they are not permanently bonded to the substrates. The water repellency is due to the hydrophobic deposit of calcium stearate or oleate within the concrete. These admixtures are subject to breakdown due to weathering and biological deterioration over a period of time.

 The most advanced admixture technology has developed specific polymers suitable to chemically stabilise the pore network, for example by ion sequestering. Optimising polymer parameters like trunk chain length, side chain length, side chain and packing density and molecular charging provide new possibilities to actively influence the pore surface of concrete on a nanoscale level.

 How Effective is Silicone Water Repellent (Water Resisting) Admixture?

 A silicone water repellent admixture usually contains reactive silane/siloxane. The advantage of this admixture is that the silane and siloxane reacts with concrete ingredients via a strong siloxane bond and it also crosslinks to form polysiloxane that lines the capillary wall of the concrete matrix. Due to the chemical bonding, the polysiloxane lining becomes part of the concrete substrate resulting in long-term durability compared to that of a traditional oil-based admixture that relies on a hydrophobic calcium stearate or oleate salt deposited within the capillaries. Whereas the silane reaction forms polysiloxane within a masonry capillary wall. The alkylalkoxysilane or siloxane first hydrolyses in the presence of water to form a silanol that then reacts with the masonry substrate forming crosslinking nano-polysiloxane structures via siloxane linkages within the concrete matrix resulting in a permanent hydrophobic attachment to the treated concrete substrate.

 Because the silicone water repellent admixture effectively forms a nanomolecular polysiloxane lining within the capillaries rather than blocking the capillaries, small amounts of silicone are required to achieve molecular lining compared to that of capillary blocking. However, such molecular lining by small amounts of silicone can effectively reduce the surface tension of the substrate and therefore convert the hydrophilic nature of masonry capillaries to a hydrophobic state.

 Addition of the silicone water repellent admixture into concrete imparts significant water repellency. Efflorescence of the concrete was effectively controlled. Due to a permanent siloxane linkage and polysiloxane nanomolecular lining within the concrete, the addition of the silicone admixture significantly improves the durability of the concrete.

 The application of silicone water repellent admixture is simple and cost effective. The admixture is simply mixed into the concrete pre-mix at the final stage of mixing before being laid or pumped  into the mould or framework.  No further procedure or extra posttreatment is required for the substrate. However, due to the immediate surface water repellency, further spaying of water onto the ready-formed substrate in order to help cement curing becomes difficult. Therefore, it is important to retain the moisture level within the concrete substrate during the curing period in order for the concrete to cure completely. Retaining the moisture level for complete cement hydration is achieved by spraying curing compound membrane to the exposed surface of the freshly laid concrete.

 Silicone water repellents may appear to control efflorescence by stopping water intrusion, however, they can cause greater damage if they prevent the escape of crystallized efflorescing salts, thus forcing the building material to absorb the expansive pressure of internal crystallization and lead to spalling of surface material.

 Some Silane Emulsion is used as a Concrete Admixture (Water Resisting Admixture). The recommended admixture range of a 1: 3 dilution of Silane Emulsion is 1.0 % to 5.0 % of the cement content. A significant reduction in water uptake can already be achieved at a concentration of 1.0 % of the cement. Silane Emulsion is added either simultaneously with or immediately after the mixing water – it should never be added along with other additives. To keep a constant w/c value the total mixing water is reduced by amount required earlier for dilution. A longer mixing time will thoroughly distribute the product within the overall system, which in turn will make it highly effective.

 Choosing the right grade of water resisting admixture is dependent on the functionality of the concrete structures or substrates. Some water repellents affect the adhesion of subsequent coatings. If the concrete substrates need to be coated over with coatings or membranes, a water- resisting admixture with a primer functionality must be used. Wacker Chemicals has a range of  silicone/Polysisoloxane emulsion that is recommended to be used as Concrete Admixtures that does not affect the adhesion of the subsequent coatings.

 How to test the Admixture Efficacy to Efflorescence Resistance?

 Salt deposits may accumulate on the surfaces of masonry or concrete substrate. This deposition can affect the integrity of the substrates and the subsequent coatings. Encrustation between coating and substrate greatly increases the cost of repair since it is necessary to remove the coating (this generally looks like blister), efflorescence and get to a sound substrate.

 Salt formation typically occurs during the initial cure of cementitious substrate (e.g. Render and screeds) as water moves through and is driven out as a result of the rate of hydration. The water carries out salts that are not bound with in the cement structure, once the water evaporates; the salt is left behind and forms a white deposit. Further efflorescence can occur if water penetrates the cementitious substrate and carries free salt out, and the formation of efflorescence will continue to occur with water travelling through the coating system as the free salts being carried out. The following  test methods  are accelerated laboratory  efflorescence testing  that are designed for coatings  but   are also  useful in evaluating  the effectiveness of the efflorescence control admixture:

 1. Efflorescence Resistance using 10% Sodium sulphate solution - The test is conducted by laying the test substrates into a tray containing 10% sodium sulphate solution at a depth of 10 mm (less than 30% of the substrate total height) for 7 days. The top surface of the substrate above the solution is visually examined for evidence of efflorescence. The efflorescence of the sample containing the silicone admixture is compared to that of the control substrate. This test is also useful to determine the silicone admixture resistance to rising damp. The rising damp in a masonry building is generally caused by moisture rising through the building footings and is a difficult problem for remedial treatment in buildings.

 2. Efflorescence Test (BASF Method) –  It is also a salt bath method  which uses a   20% of Sodium Hydroxide (NaOH) solution in water and with addition of  5  Grams of Sodium Sulfate (Na2SO4) into 1-liter NaOH solution. A sponge is wetted with NaOH/ Na2SO4 salt solution ensuring the sponge is filled up with the salt solution and the soaked sponged is then placed the soaked sponge into the plastic tray. The tray is filled up with the salt solution as higher as the sponge.

 The test substrates are then placed on top of sponge in salt bath.  The test substrates periodically evaluated according to requested test periods. Any discoloration, whitening, salt formation/blooming on the surface or any surface changing is reported.

 3. TOMBSTONE TEST (Rohm & Haas Method) – Using a 2% aqueous sodium Sulfate solution, the shallow plastic tray is filled with about 1 gallon for above tray. The test substrates are placed upright in the test rack and the test rack with test substrates is place on the tray ensuring all the  test substrates has reach the bottom of the tray. Test substrates are then removed from the test racks depending on the requested test periods (usually 24hr to 48 hrs) and any salt formation is rated accordingly.

 All The test methods are semi-quantitative. Results will vary from operator to operator based on differing judgements of severity of efflorescence.

 Conclusion

Concrete   is the most versatile building material in civil engineering. However, it is vulnerable to harmful substances that penetrate into the concrete by means of moisture.  Only optimized concrete mix design to suit its purpose and effective preventive measures like using a suitable efflorescence control admixture can provide reliable protection for any concrete structure.  Admixtures affect the pore structure of the hardened cement paste, and can make it more or less resistant to the movement of water depending on the concrete mix design. Integral water repellents such as stearates reduce capillary suction but have little effect on water permeability, so they may reduce primary efflorescence caused by premature drying, but will not reduce efflorescence caused by the transfer of water under gravity or pressure. On the other hand, water-reducing admixtures and integral waterproofers (based on cement and other particulates) used to create a finer pore structure may reduce water permeability, but not capillary suction.

Once the concrete is optimized for its purpose from a mix design perspective, protecting the product from external influences after concreting is still possible. There are generally two methods available: hydrophobic Impregnation and film-forming coatings. In both cases, protection against moisture is central since water plays a key role by transporting dissolved salt substances.  Silanes, siloxanes, and silicones are suitable water repellents for surfaces exposed to moisture. Siloxanes and silicones are bigger molecules and are more suited for porous materials such as pavers and architectural blocks, especially as they have lower evaporation pressure and are able to penetrate the surface easily and sufficiently to provide long-lasting protection. Film-forming treatments must be impermeable to carbon dioxide. Acrylic emulsions and solvent-based solutions are suitable, too.

The most efficient way of protecting concrete is to prevent water uptake. The past decades have shown that silanes with long alkyl chains (e.g. iso-octyl) are the ideal product class for this. Their current dominance in masonry protection stems from their outstanding water-repellency and durability. Silanes outperform rival product classes in their resistance to physical, chemical, and microbiological attack. If the right product is chosen, treatment with silane will preserve a structure for a long time.

REFERENCES

1. Dreyer, Uwe, "How to reduce efflorescence in concrete", Brockhues AG, Walluf Winter Seminar 1995.

2. Russell, Peter, "Concrete Admixtures", A Viewpoint Publication, 1983, p. 14.

3. Design and Control of Concrete Mixtures EB001, Chapter 6 Concrete Admixtures

4. CONCRETE PROTECTION FOR CONCRETE ADVANTAGES by Wacker

5. Patrick Loughran, AIA, PE, LEED AP,” Efflorescence in Masonry: Understanding the Problems and Solutions”

6. Efflorescence by BASF

7. ASTM C979 / C979M - Standard Specification for Pigments for Integrally Colored Concrete

8. ASTM C260 / C260M - 10a(2016) - Standard Specification for Air-Entraining Admixtures for Concrete

9. ASTM C494 / C494M - Standard Specification for Chemical Admixtures for Concrete

10. AASHTO M 154 - Standard Specification for Air-Entraining Admixtures for Concrete

11. AASHTO M 194M/M 194 - Standard Specification for Chemical Admixtures for Concrete

12. ASTM C1017 / C1017M - Standard Specification for Chemical Admixtures for Use in Producing Flowing Concrete

Compiled by:

Engr. Zenith F. Czora, BSChE, ATSC, is a licensed chemical engineer with technical expertise in researching, developing, and formulating coatings products, coating specification, SDS and GHS labeling, production scale-up, manufacturing process, quality control, good laboratory practice, and products accreditation and certification. She has 28 years of experience in the coatings industry and specializes in the product development and formulation of architectural coatings; texture coatings; floor/sports surface coatings; membrane/elastomeric coatings; roof/waterproofing coatings; wood coatings; and automotive, industrial, and marine coatings and adhesives and grouts.