cement-properties
Reading Time 5 minutes
Share:

Cement Properties: What is Permeability? How to Prevent it?

Permeability is an important parameter that has a direct effect on the useful life of cemented systems. Permeability, which is related to the structure and amount of the voids inside the concrete, decreases durability of the structure against chemical attacks such as sulphate and chlorine. In order to provide durability in concrete, it is quite important to apply cemented systems that conform to the usage area and to enable an impermeable form for concrete as much as possible.

What is Permeability in Cement?

Permeability is a term that defines the potential of movement of liquids and gases within the cemented systems. Especially in environments where the structure may be exposed to chemical attacks, it is highly important for the concrete to have minimum permeability, even for it to be impermeable. Thus, impermeability is a precondition for the structure to be durable.

Parameters used to determine permeability in concrete are void structure, void amount, connections of voids with each other, their forms and distribution.

  • Voided structure is not always an indication of permeability. The amount and concentration of the connections between voids have a direct impact on permeability.
  • Connected and unconnected voids can be seen in the below figure. High amounts of connected voids within the structure would make the system more permeable.
cement-properties-permeability

Figure 1. Types of Pores in Cemented Systems

  • According to studies, structures with voids bigger than 500A° may be held responsible for the strength and durability of the concrete. Another study conducted by T.C Powers indicates that structures bigger than 1mm called capillary break cause permeability.
  • Voided structure formation and permeability decreases with the decrease of water/cement rate and a more concentrated cement matrix can be obtained.
  • In addition to this, placement ease, homogeneous aggregate gradation and long curing times also contribute to the impermeable concrete design.
  • Permeability is observed to decrease as the age of concrete increases.

What are the Deformations that can be observed in Highly Permeable Concretes?

Certain deformations are observed in highly permeable concretes depending on the various chemical attacks they are exposed to. These deformations are;

  • Sulphate Attacks
  • Alkali-Silica Reactions
  • Efflorescence
  • Carbonation
  • Freeze-Thaw Effect
  • Chlorine Attacks
  • And Related Cracks and Corrosions.

All deformations would decrease the useful life of the structure and cause decrease in strengths.

What are the Potential Causes of Concrete Permeability?

Main factors that have an impact on the permeability of the concrete include the following application errors.

  • High Water/Cement Rate
  • Curing conditions (Especially the first 7 days)
  • Insufficient Curing Time
  • Wrong Aggregate Usage
  • Additive Choice

Providing optimum conditions and working with proper raw materials when designing concrete are quite effective in preventing permeability problems.

What is the Mineral Additive Effect in Permeability?

Materials used to improve certain properties of the concrete or provide special qualities  for the concrete which are stored in silos in powder form like cement such as blast furnace slag, fly ash, silica fume, truss etc. are called ‘Mineral Additives’.

Mineral additives have properties that increase late compressive strengths of the concrete and decrease permeability. Mineral additives create more solid structures by reacting with the components that emerge as a result of cement and water reaction.

cement-properties-what-is-permeability

Figure 2. Mineral Additive Types

As a result of hydration reaction between cement and water, C-S-H gel, Ca(OH)2, ettringite, monosulphohydrate and various hydrates emerge that provide strength for the concrete.

Mineral additives enter into a second reaction with Ca(OH)2 to create more C-S-H. Decrease in the amount of Ca(OH)2 and increase in the amount of C-S-H are positive in terms of strength, durability and aggregate-cement paste adherence.

Mineral additives require the products of previous reaction due to their pozzolanic nature. Therefore, hydration reaction lasts longer in the concrete and a more durable structure with less voids is formed in older ages.

What is the Effect of Blast Furnace Slag Usage?

Industrial byproducts such as Blast furnace slag and Silica fume are used increasingly as mineral additives in cement and concrete. When the effects of slag use on cement parameters are analyzed, they are observed to increase concrete impermeability.

Figure 3. Fresh and Hardened Concrete Properties

These mineral additives not only provide economic and environmental benefits, they also improve concrete durability by decreasing concrete permeability through proper additive choice, mixture rates and curing techniques.

Below figure shows the structures that form during Portland cemented concrete hydration (above) and hydration products of concrete manufactured using blast furnace slag (below). Hydration products are observed to increase with the effect of slag.

cement-properties-how-to-prevent-permeability-2

Figure 4. Hydration Products of Portland Cemented Concrete and Concrete with Mineral Additives

What is the Effect of Wet Curing Time?

Proper curing time is known to decrease permeability and increase strength. The time required for cement hydration to reach its full maturity mostly depends on water/cement rate and wet curing time.

The time required for blocking the connections between capillary breaks in cement pastes stored in the same curing conditions increases depending on water/cement rate. For example, the necessary maturity time for blocking capillary breaks at a water/cement rate of 0.40 is 3 days while this time goes up to 1 year when water/cement rate reaches 0.70.

 What is the Effect of Water/Cement Rate?

  • When water/cement rate goes from 0.4 to 0.7, the strength is observed to decrease by 60%, and when it goes from 0.6 to 0.7, the system becomes 6 times more permeable.
  • Water/cement rate should be adjusted considering strength, permeability, and workability.

Why Should Permeability Be Prevented?

One of the most important properties in cemented systems is permeability. As a factor that has a direct effect on the useful life of the structure, it should be noted that keeping permeability at low levels would enable building stronger, more durable structures against chemical attacks which could be used for a long time.

In this article, we will introduce the flux product produced at Çimsa to be used to increase efficiency in the Iron- steel …

By nature, concrete exhibits low tensile strength, brittleness, unstable crack propagation, low breaking strength, and inadequate ductility. These shortcomings are generally eliminated …