Chemical Resistance

A chemical attack involves dissolution of substances or chemical reactions between substances and components of the concrete. Reaction products might cause problems, due to dissolution or expansion. The primary transport mechanisms involved are permeability, capillary absorption and diffusion.

The types can be categorised as per below: 

Sulphate Attack

This is external sources contained within ground water, sea water, soil or internal sulphate attack (delayed ettringite formation). 

The deterioration of concrete exposed to sulphate is the result of the penetration of aggressive agents into the concrete and their chemical reaction with the cement matrix. The three main reactions involved are: 

  • Ettringite formation – conversion of hydrated calcium aluminate to calcium sulphoaluminate,
  • Gypsum formation – conversion of the calcium hydroxide to calcium sulphate, and
  • Decalcification – decomposition of the hydrated calcium silicates.

These chemical reactions can lead to expansion and cracking and/or the loss of strength and elastic properties of concrete. The form and extent of damage to concrete will depend on the sulphate concentration, the type of cations (eg sodium or magnesium) in the sulphate solution, the pH of the solution and the microstructure of the hardened cement.

NZS 3101:2006 has prescriptive requirements in the durability section for concrete to be used in this environment for a 50 year design life.  The exposure class is determined by the severity of the application.  These are shown in the table below:

Exposure Class

Cement Binder Type

Minimum
Binder Content (Kg/m3)

Maximum
W/C Ratio

Minimum Concrete Cover

Minimum Curing period (Days)

 

XA1 – slightly aggressive

 

EverSure

 

 

340

 

0.50

 

50

 

3

 

XA2 – moderately aggressive

 

EverSure and 8%
Microsilica 600
or 30% EverPlus

 

370

 

0.45

 

50

 

7

 

XA3 – highly aggressive

 

EverSure and 8% Microsilica 600
or 30% EverPlus

 

400

 

0.40

 

55

 

7

For environments more severe than those listed in NZS 3101:2006 or in highly acidic and permeable soils where pH is below 3.5, additional protective measures are required to isolate the concrete from direct contact with the aggressive ground condition. 

The inclusion of Microsilica 600 or EverPlus Class C Fly Ash at the specified replacement levels deems compliance to the binder requirements of NZS 3101:2006.  Alternatively the use of a ternary blend of Microsilica 600 and EverPlus Class C Fly Ash from our product solution range will offer the highest level of sulphate resistance.

Alkali–Silica Reaction (ASR)

ASR is best pictured as a two-stage process. Initially, alkali hydroxides, mainly from cement, react with susceptible siliceous components within aggregate particles, in the presence of moisture, to form an alkali–silica gel. Subsequently, this gel can absorb further moisture, swelling in the process and imposing expansive stresses on the concrete.

In view of the relatively low tensile strength of the concrete matrix, these expanding reaction sites generate radiating micro cracks.   As the gel absorbs further moisture the reaction product becomes able to migrate into the micro cracks it had previously produced.

It is usual that the reactive aggregate particles are distributed throughout the concrete so that the micro cracking from individual reaction sites can become linked into a network of cracking that can impair the physical and mechanical properties of the concrete material. 

The CCANZ document TR3 – Alkali Silica Reaction is a comprehensive document providing guidance and notes on how to minimise the risk of ASR.  The durability section in NZS 3101:2006 – Concrete Structures specifies the use of TR3.

To minimise the risk of ASR it is widely accepted that the inclusion of a minimum 8% Microsilica 600 will reduce expansion associated with ASR when reactive aggregates are used.

The mechanisms of including 8% Microsilica 600 in concrete are:

  • calcium hydroxide consumption due to the pozzolanic reaction which reduces the concrete’s pH that is required for ASR to take place
  • pozzolanic reaction binds alkalis so they are unable to take part in ASR
  • reduction in the available alkali
  • concrete will be stronger and more durable; able to resist ingress of moisture, diffusion of alkalis to the reactive aggregates and resist any expansive forces.

Acid Resistance

Eventually all Portland cement concretes will break down under organic acid attack.  The acid continually lowers the pH of the concrete and at values below 7 the calcium hydroxide and then the calcium silicate hydrate phases are readily decomposed. 

Two factors determine the rate of attack - the concentration and the strength of the acid and the rate at which fresh acid is supplied to the concretes surface.

The process can be slowed with the inclusion of Microsilica 600 and/or EverPlus Class C Fly Ash. Alternatively the use of a ternary blend of Microsilica 600 and EverPlus Class C Fly Ash from our product solution range will offer the highest level of acid resistance.