SERVAAS LE ROUX – PRODUCT MANAGER FOR ADMIXTURES AT MAPEI SOUTH AFRICA – DISCUSSES THE APPLICATION OF BESPOKE ADMIXTURE SOLUTIONS FOR UNIQUE APPLICATIONS
In layman’s terms it could be said that concrete consists of fine and course aggregates mixed with water and cement and that the strength of the concrete depends on the ratio of water to cement (W:C ratio), the compaction applied after placing and the curing applied after compaction. Whilst this description is true in the broader sense it falls short of describing the many chemical and mechanical issues affecting the fit for purpose of a specific concrete mix. These factors affecting the suitability of a concrete are often taken for granted because most concrete users think only of eventual strength when defining the nature of their concrete mixture even though the workability of concrete and the initial strength are often stronger determining factors.
To further complicate matters the workability retention and initial strength of concrete oppose one another when workability retention is achieved in the conventional manner which is to add water or retarding admixtures to the concrete. It is because of this tension between workability and strength that concrete admixtures have become a pivotal point when optimising the various issues around concrete manufacturing.
Workability Retention in a Nutshell
It is helpful to first consider the mechanisms allowing initial workability to also understand the ways in which this workability is lost later in a concrete mixture’s life. This also highlights the necessity of concrete admixtures to optimise workability retention.
In this regard “workability” is achieved by allowing for enough paste (binder and water) between aggregate particles to prevent mechanical interlock whilst also allowing enough space between binder particles in the paste to prevent the hydration process from occurring too rapidly.
Hydration commences as soon as the cement particles are brought into contact with the water. Very close to the onset of the hydration process the gypsum that is present in OPC (Ordinary Portland Cement) creates a substance called ettringite, which covers the cement particles to stunt the growth of the hydration crystals. The ettringite layers are broken down with time as hydration crystals starts breaking through, this sets in motion a process where the concrete mixture becomes increasingly alkaline.
At first, the hydration crystals do not touch, but eventually they intertwine to form a dense matrix. It is in between these two states that workability is lost.
Workability is lost in a number of ways – the most common being:
- Evaporation of the water in between cement particles due to excessive ambient heat. This brings the cement particles closer together which allows hydration crystals to touch much sooner.
- Absorption of water and plasticizing agents into aggregates. This has the same effect as evaporation.
- Hydration of cement particles which eventually develops into a dense crystal matrix.
- Breakdown of plasticizing agents due to alkalinity – this is discussed in more detail below.
Plasticizers – The Underpinning of Concrete Admixtures
Plasticizers are chemical substances that attach to the ettringite layers covering the cement particles which then polarises these particles. The effect is that the cement particles are distributed more evenly in the paste and so the state in which hydration crystals do not yet touch is prolonged.
Most plasticising agents tend to break down when the concrete mixture eventually becomes highly alkaline. This has a rapid slump loss effect. There are, however, plasticizing agents on the market that remain dormant until the concrete mixture becomes alkaline. It therefore reacts in a relay fashion with the normal plasticizers.
Figure 1 below depicts typical slump loss relative to time when various plasticizing products are applied in the same concrete mixture. The terms “Normal Plasticizer” and “Super Plasticizer” are both references to families of products as the distinction is based on the product’s ability to reduce water.
Necessity of Unique Admixture Solutions to Concrete
Concrete admixtures do, however, extend beyond the scope of only “plasticizers”. The term refers to a collection of chemical substances used to modify the characteristics of concrete in order to meet the demands of a specific project. Whilst the “demand of a project” typically relates to the strength, workability and workability retention of the concrete the limiting factors are often set by the cost or character of one or more of the raw materials used in the project.
If, for instance, extenders are exceptionally costly at a specific project’s location it could force the project team to use pure cement which would imply much faster hydration and thus quicker slump loss. The project team could then be forced to specify workability extension admixtures (such as Mapei’s Dynamon EW) over and above the standard plasticizing agents.
If the aggregates available on site show a significant gap in the consolidated grading then the concrete would tend to show mechanical interlock at low paste contents whilst segregating at high paste contents. The solution to this problem would be to specify the use of a viscosity modifying agent (such as Mapei’s Viscostar 3K) to make the concrete mixture more cohesive at the point of segregation.
In some cases, the aggregates available on site show significant levels of absorption. A solution to this dilemma would be to utilise a “sacrificial plasticizer” (such as Mapei’s Dynamon MS100) that is formulated to be drawn into the pores of aggregates and in so doing stunt the absorption process to allow for an extension in workability retention.
Mapei’s concrete admixture product offering provides solutions for every possible problem set that could arise on site due to the limitations set by site conditions. We are confident that our expertise and service offering will match the quality of our products.
Please feel free to contact us for more information.