Review of Chemical Admixtures for Improvement of Concrete and Mortar Applications

Review of Chemical Admixtures for Improvement of Concrete and Mortar Applications

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Introduction

Professionals in the field of Concrete Construction today are aware of chemical admixtures in concrete. With the emergence of tall and complicated structures in construction today, RCC is the material of choice. With a view to improving the properties of concrete in terms of workability, durability and mechanical properties, the use of admixtures [superplasticizers, specifically] becomes mandatory. The construction chemical industry found its roots in admixtures. Over the years, the situation has developed to the point where construction chemicals have become synonymous or recognizable with admixtures.

As an industry, we have much more to offer. Concrete today is no longer limited to using only superplasticizers for property improvement. Construction demands special requirements in terms of durability, workability and replacement of cement and sand. Here is where, special purpose additives come into the picture. With this in mind it is imperative to take a look at the materials available to improve mechanical, impermeability, durability and application aspects of not only concrete,but also mortars. Some of the materials available are:

– High Performance Integral Waterproofing Additives
– Additives for Improvement of wall plasters
– Additives for Concrete Products like pavers, pipes, manhole, etc. (Pressure cast dry-concrete or wet cast concrete)
– Additives to modify mortars for use in tile or block bonding
– Additives for placing screeds

We shall cover the working mechanisms and advantages of these additives in the sections below.

 

 

High Performance Integral Waterproofing Additives

Concrete has an affinity to water and therefore is a wettable material. Water enters the concrete through continuous capillaries either under hydrostatic pressure or by capillary action. The capillary suction forces in dry or partially saturated concrete can be equivalent to hydrostatic head of several metres. The water cement ratio has a direct bearing on capillary discontinuity. The porosity of concrete is one of the major factors contributing to ingress of water into the concrete. As waterproofing cannot be better than the base concrete, it is imperative that the concrete should be produced with a low water cement ratio and minimum permeability. That is where usage of Integral Waterproofing / damp proofing additives comes into the picture.

The ingress of water in concrete depends upon the degree of saturation of concrete and mechanisms of entry. The rate of water entry depends upon the microstructure of the concrete paste. If unsaturated concrete is exposed to water, it will be absorbed into concrete even in absence of pressure due to capillary suction. Permeation properties of near surface concretes are very important while determining the entry of water into the concrete. Liquid, Gas or Ions can migrate in concrete by diffusivity because of a concentration gradient. The cover should be highly impermeable as well as with a very low rate of water absorption. In most of the circumstances capillary action rather than permeability controls the passage of water and this should be a major consideration in design of waterproofing systems.

Functioning of Integral Waterproofing Additives

The aim of integral waterproofing is to densify the concrete to prevent water ingress and / or convert wettable capillaries to non-wettable types that would eventually lower the penetration of water into the system. Figure 1 shows the concept of hydrophillic and hydrophobic capillary action. The theory of capillary and the concept of capillary rise and capillary depression provide a basis for working of hydrophobic waterproofing materials. Wettable surfaces have low contact angles and difficult to wet surfaces have higher contact angles. The high contact angles have a two-fold effect on concrete. Firstly the pressure required to enter the concrete is positive where by capillary action is nil and a high water pressure (approximately 14 m head of water) would be required to penetrate the concrete surface. Hydrophobic surfaces exhibit high contact angles of water as shown in Figure 2. Integral Waterproofing Additives use this theory to affect permeability reduction in concrete / mortars.

 

 

In Short, waterproofing admixtures belong to a class of admixtures that improve concrete durability by controlling water and moisture movement and by reducing permeability and hence chloride ingress. This class of admixtures is referred to in IS: 2645. EN-934 refers them as water-resisting admixture and ACI 212.3R refers to the same as permeability reducing admixture. These admixtures are generally sub-divided into two categories (ACI Comm.212), viz., permeability reducing admixtures for concretes exposed to non-hydrostatic conditions (PRAN) and concrete exposed to hydrostatic conditions (PRAH).

Types / Bases Of Integral Waterproofing Additives

These admixtures can be based of different chemistries depending on the application of the base concrete, mortar or plaster. Damp-proofing admixtures or PRAN are water-repellents / hydrophobic. These are more suited for use in situations where the base concrete is not subjected to high hydrostatic heads. These for example can be used in external above ground walls in buildings, plasters, mortars, architectural concrete, concrete blocks, etc. Integral-waterproofing admixtures or PRAH may be hydrophobic (not necessarily). These are more suited for use in concrete / mortars subject to hydrostatic pressure heads. These for example can be used in water storage tanks, wet areas, basements, STPs, Tunnelling Concrete, Underground structures and other similar structures. Classification and materials used in Integral Waterproofing / Damp-proofing Admixtures is given in Table 1 below.

Mode of Action

Integral Waterproofing Additives generally function on the basis of one or a combination of the three mechanisms listed below.

1. Reduction of Capillarity by w/c ratio reduction
2. Hydrophobising Capillaries
3. Physical or Chemical Pore Blocking
1. Reduction of Capillarity: This is achieved using Conventional water-reducers or Air Entraining Agents or by using reactive pozzolans and silicates. These additives may improve workability (increases slump) at the same w/c ratio or provide a reduction in w/c at the same workability levels. This property helps concrete achieve excellent compaction and denser hydration (C-S-H Gel Formation) thereby reducing capillary pores. Less capillary pores means less transport of water through the cement matrix.
2. Hydrophobising Capillaries: This is achieved using hydrophobic additives such as Soaps, Bitumen, Veg. Oils & Fats, Mineral Oils (Old Technology) OR Long Chain Fatty Acids, Fine Wax Emulsions, Silicones, Silanes-Siloxanes, etc. (New Technology). These materials react with the cement constituents and form insoluble hydrophobic by-products that line the pores. This lining converts the water-absorbing capillary forces into water repellant capillary forces due to the hydrophobic effect of the constituent. Thus water is pushed out of the capillary pores and the concrete is kept dry. Fox e.g.: the stearates react with the calcium hydroxide in concrete to form insoluble calcium stearates that forms a hydrophobic layer on the walls of the pores in concrete. These admixtures are effective at reducing the capillary absorption under non-hydrostatic conditions only.
3. Pore Blocking (Physically or Chemically): Physical Pore Blocking can be achieved using inert powder fillers such as talc, bentonite, etc. or by using finely divided waxes, bitumens, or acrylic / SBR emulsions. When Hydrostatic Pressure is applied, these finely divided materials or emulsion globules are pushed into the capillaries, until they jam together, forming a physical plug, preventing further penetration of water.

Chemical Pore Blocking is achieved using advanced micro-fine latent hydraulic materials or pozzolanic Nanoparticles or reactive silicates to effect a structure of densest packing in the cement matrix on a nano-scale. The strength and permeability of a cement-based material is based on hydration of the clinker components, to form C-S-H gel and free lime (Ca(OH)2). The materials above use this Ca(OH)2, in presence of water, to recrystallize and form new more stable, foil-like C-S-H and C-A-S-H phases. These are also referred to as crystalline

For e.g. In case of use of specific high reactivity pozzolans, the cement matrix would be densified by a complex process called Dynamic SynCrystallization (DySC). Therefore the most porous part of the cementitious matrix, (Ca(OH)2), is converted to very stable, impermeable C-S-H and C-A-S-H type crystals. This crystalline mineralization process continues over time in presence of water, and the structure of the matrix becomes more refined, overall porosity decreases, micro-cracks are sealed and pore volume is minimized. This DySC process thus makes concrete watertight. Figure 3: shows the Working Mechanisms of IWP Admixtures

 

 

Effects On Properties Of Concrete

On Plastic Concrete

Integral waterproofing admixtures are formulated to affect the properties of the hardened concrete, and not those of concrete in its plastic state. Water repelling admixtures, conventional admixtures and air entrainers, may increase the workability of the plastic mixes slightly. On the other hand, inert pore fillers or reactive pore fillers, due to their finely divided particle sizes may decrease workability slightly. These materials mostly do not affect setting time or stability of the mixes. However, in all cases, it is advisable to test the material for required properties prior to use.

On Hardened Concrete

Integral waterproofing admixtures are designed not to have any significant impact on the mechanical properties of concrete. With most materials there is no significant changes in compressive / flexural strengths or on elastic modulus of the mixes. The only properties they impact are related to permeability of the concrete. This can be measured by means of initial surface absorption (BS: 1881 or Equivalent) or water penetration under pressure (DIN 1048 or equivalent). A comparison of properties for a control sample and a mix incorporating an integral waterproofing additive is given in Table 2.

 

 

Advantages

– Provides an efficient and durable barrier against rainwater, moisture and ground water and makes concrete waterproof
– No external application, simply mix it into your existing concrete
– Save costs on material and application costs for external membranes
– Speed up your construction, no waiting for external membrane application
– Permanent protection from water penetration and absorption
– Makes the mix more workable (improves the slump) at lower w/c
– Does not change setting time nor adversely effect the reinforcement.
– Disperses rapidly and makes a homogenous mix.
– Free from chlorides
– Certified for use in contact with water intended for Human Consumption
– The use of prevention as a means to waterproof structures can be a cost effective measure, especially for structural elements as it prevents future deterioration and repair cycles.

Limitations

– Not a substitute to bad mix design and manufacture, placing and curing practices.
– These may not be capable of preventing water ingress through deep cracks or structural cracks in the concrete.
– Requires high degree of quality control during concrete manufacture, placing and curing.
– In case of integral waterproofing or any other waterproofing treatment for underground structures, joints become the weakest link in entry of water and should be adequately treated.
– Application of Integral Waterproofing Compounds should be considered after characterizing its performance and requirements as per site conditions that the concrete would be subject to.

 

 

Areas of Application

– Basement slabs & Walls – Cast structural slabs
– Elevator pits – Concrete foundations – Underground pipes
– Tunnels – Deep pile foundations – Manholes – Dams
– Water-retaining structures – Swimming pools – Cooling towers
– Podium decks – Outdoor balconies – Potable water tanks
– Wet Areas

Water-resisting admixtures have a positive effect on durability of concrete by mitigating water ingress and hence chlorides, etc. Surface absorption is also reduced and when concrete is exposed to harsh environs this admixture has a beneficial effect on increasing the life span of a concrete structure. However, it is to be remembered proper mixing is essential for the performance of the waterproofing admixture. Excess dosage of the admixture has to be avoided and when used with other admixtures compatibility check has to be done.

Improvement of Construction Mortars

Construction Mortars play an important role in the completion of building projects, as much as a good concrete structure. Some of the mortars used in finishing works today include:

– Wall Plasters
– Tile and Block Bonding Additives
– Floor Screeds
– Mortars for Casting Concrete Goods

These mortars were traditionally made using simple polymers, plasticizers or air entraining agents. But today, more is demanded from these materials in terms of higher performance, extreme durability, easier application [sprayability] and better aesthetics. Some of the materials to meet these requirements are detailed in sections below. In the interest of readers, only the latest developments are being focussed on and traditional polymers and additives are not covered.

 

 

Additives for Plaster Improvement

Even with the advent of Aluminium Slip Formworks for Casting Building Walls [seen in major cities], building construction generally relies on wall plasters to level, beautify and protect the outer wall envelope of the building. Dampness entering the living space (especially through walls) is an indication that the structure has failed in durability and that structural elements have already begun deteriorating. With high demands on durability, we should look at preventing water ingress through walls, by providing the correct plaster as an envelope to the building. This ensures that the structure is protected against water and failure through its design life.

A high quality plaster for walls incorporates various properties such as Stability, Water Retention, Non Sagging, and Bonding to the substrate. Most factory made, ready-to-use plasters will incorporate these properties. However, these dry-mix, ready-to-use plasters, even though of high quality, do not make the cut due to initial cost of materials. To bridge the gap between ready-to-use plasters and site batched materials, special additives have been developed, which impart properties like stability, sprayability, water-retention, non-sag, excellent bond and resistant to water ingress to plaster, when added as a single additive.

These solutions can be customized for a variety of plaster options. The mix is to be mixed in a high mixing efficiency dry-mix production mixer for dispersing all the chemical ingredients suitably. Dosage is economical at 0.1 to 0.2 % by weight of the dry-mix or 0.5 to 1.0% by weight of cement. External building plasters are the first line of defence in providing the external envelope to the building walls. Plastering the external walls in the building, today involves multiple problems on its own. These Include:

1. Good Quality Cement
2. Availability of good quality Natural Sand
3. Wastage on account of Plastering Practices
4. Skilled Labour for Application

Today’s plaster additives combines many benefits into one well designed judicious product, which improves the following properties in any wall / ceiling plaster:

– Water Reduction
– Waterproofing
– Excellent Bonding, even to concrete blocks
– Thixotropy
– Smooth Finish and reduced shrinkage cracking
– Minimal to Zero Wastage
– Lesser Rebound
– Improved Sprayability
– Minimal labour requirement for Finishing
– Ability to use thin plaster layers for finishing

This material can be used universally for:

1. Site applications (site-made plaster) or
2. By dry-mix mortar manufacturers or
3. By masonry cement manufacturers to enhance the properties of plaster or similar mortars.

Theseadditives have the robustness to convert any cement-sand mix, to a sprayable, easily finishable, durable wall plaster. The robustness of this formulation, allows it to be used with multiple raw material sources including crushed sand, manufactured sand, GGBSF, Flyash, Natural sand, PPC and other base raw materials for plaster.

Additives for Precast Concrete Products like Pavers, Pipes, Etc.

Concrete Goods / Products are defined as “small or large goods made of Plain Cement Concrete (PCC) or Reinforced Cement Concrete (RCC) and manufactured on a large scale”. These concrete goods include products such as paver blocks, concrete tiles, extruded pipes, manhole covers, and hollowcore elements. These goods are characterized by their production methods, which includes casting dry / semi-dry concretes into moulds and forming them by application of pressure. This method is designed to provide early setting (due to the semi-dry concrete) and demoulding, and the pressure compaction / extrusion is designed to provide proper form and sharp edges to the goods.

However, this method of production has its limitations. Concrete used to make concrete products has different properties. Semi-dry concrete with lower water content is by nature difficult to compact. In addition products must be free of efflorescence. A particular challenge in the concrete products industry is the achievement of sufficient early strength to enable the rapid removal from forms. Only a perfectly matched combination of admixtures and casting techniques is able to produce the desired results. Basic requirements of concrete goods are:

– High green strength (instant formwork removal)
– Structural stability, no swelling or shrinkage
– Sharp edge formation, smooth side surface
– Compact / Dense Structure
– No bonding on tools, moulds or machinery in green state
– Free of Cracks and Efflorescence
– Resistance to Water / frost

To countermand these limitations and meet requirements, a series of additives were developed, which on one hand modify the rheology of the semi-dry mix by lubricating it and densifies the mix on the other hand to provide optimum compaction and hydrophobicity.

The general trend of adding water to a semi-dry mix for concrete goods is shown in Figure 4. The figure shows the effect of rising water content on the stability of the mix when compacted under pressure. As can be seen, the mix is very sensitive to the amount of water added. Add Less water (dry branch) and the concrete would not compact properly and add more water (wet branch) and the mix would collapse after demoulding.

Using a compaction aid / densifying additivereduces the amount of water that can be added, while improving compactability of the concrete by lubricating the mix as well as interface between the mix and the mould. This additive also makes the mix more robust by reducing sensitivity of the mix to added water, which allows the manufacturer to produce concrete goods with much more efficiency. In short adding a compacting aid expands the limit of water addition in the mix from a narrow point to a range of values. Lesser water means better green strengths of the concrete, better stability, better compaction and sharp edges in the concrete goods.

Similar solution is also available for wet-cast concrete, which combines a variety of materials to provide water reduction, acceleration in hardening and good surface Finish.

Advantages of Using Compaction Aids:

– Improvement of Compactness and Density of Concrete in Produced Goods
– Complete Hydration of Cement Grains
– Increased Compressive Strength at Demoulding and 28 Days
– Increased Stability of Produced Goods
– Enhanced Surface Finish and Edges
– Decreasing adhesion of concrete to Moulds and Pressure Head
– No changes in water demand of the mix
– Decreased Efflorescence and Water Permeability
– Minimal Dosage at 0.2 to 1.0% by weight of cement
– Reduced Rejection Rate
– Better Quality – therefore higher returns on product
– Improving production efficiency, due to quicker demoulding

Areas of Application:

– Paver Blocks – Manhole Covers – Extruded Concrete Pipes
– Hollowcore Slabs and Walls – Concrete Tiles and Roof Tiles
– Other Miscellaneous Concrete Goods

Additives to Modify Mortars for use in Screeds or Tile & Block Bonding

Normally SBR or Styrene Acrylic based polymer additives were used for screeds and tile or block bonding mortars. Improvement in polymer and additive technologies has helped improve these polymers to provide a slew of improvement to properties of bonding mortars. These new generation additives are designed using the latest technologies like bonding, wetting, water retention, self-levelling, etc. to provide floor screeds or tile and block bonding mortars with a variety of properties such as:

– High bonding and adhesion strength
– Excellent Cohesion to prevent bond loss
– Provides good adjustment / workability time
– Improved Water Retention
– No shrinkage or cracking in screed / bonding bed
– Improved water resistance to avoid leaks through screeds / masonry joints / tile bed
– Reducing saponification and subsequent weakening of bonding mortar in water
– No pre-wetting of tiles / blocks becomes necessary
– Economical and labour saving
– Thin bed mortars, improves work rhythm
– Material Saving, so little to no wastage
– No batting for tile / block is required, thereby minimizing breakage waste

Areas of Application

– Laying Floor Screeds [self levelling or high-flow]
– Fixing of ceramic and other tiles in bathrooms, kitchens and facades
– Fixing tiles on floors, walls etc.
– Laying of mosaic tiles, natural stones, marble tiles, boards etc.
– Fixing of Concrete Blocks

Conclusion

This article explored the working mechanisms and applications of the latest construction chemical technologies in improvement of concrete and mortars. Of course, improved mix design techniques and placing techniques too play a very important role in the production of high quality, durable, concrete or mortars. Construction chemicals significantly broaden the range of possibilities of construction to achieve different aims such as acid resistance (for pipes), improved durability, surface finish, faster cycle times, faster work rhythms in production and adding robustness to the construction. Correct usage of construction chemicals would be vital in all areas of construction to enhance benefits. Therefore, it is imperative that Construction Chemicals be a vital component of improving concretes and mortars to promote sustainable construction.

 

Sunny Surlaker
Head Admixtures Division
MC-Bauchemie (India) Pvt. Ltd.

 

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