When concrete is exposed to sulphate, the penetration of aggressive agents and it’s reaction with the cement matrix can cause concrete deterioration. The three main reactions involved are:

1. Ettringite formation - conversion of hydrated calcium aluminate to calcium sulphoaluminate
2. Gypsum formation - conversion of the calcium hydroxide to calcium sulphate
3. Decalcification - decomposition of the hydrated calcium silicates

Each of these chemical reactions can lead to expansion, cracking and the loss of strength and elastic properties of concrete. The form and extent of the damage to concrete will vary depending on the:

1. Sulphate concentration
2. Type of cations (sodium or magnesium) in the sulphate solution
3. pH of the solution
4. 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:

Additional protective measures are required for environments more severe than those listed in NZS 3101:2006, or in more highly acidic and permeable soils with a pH below 3.5 – in order to isolate the concrete from direct contact with the aggressive ground conditions.

Including Microsilica SF or EverPlus Fly Ash at the specified replacement levels complies to the binder requirements of NZS 3101:2006.

Alternatively, using a ternary blend of EverSure Type GP, Microsilica SF and EverPlus Fly Ash from our product range will offer the highest level of resistance against sulphate attack.

ASR is best pictured as a two-stage process:

1. Alkali hydroxides mainly from cement and siliceous components found within aggregate particles, will react in the presence of moisture to form an alkali–silica gel. This gel absorbs further moisture, swelling in the process and imposing expansive stresses on the concrete.
2. In view of the relatively low tensile strength of the concrete matrix, these expanding reaction sites generate radiating micro cracks. As the gel absorbs more moisture, the reaction product can migrate into the micro cracks it had previously produced.

It’s usual to distribute reactive aggregate particles throughout the concrete, in order to link the micro cracking from individual reaction sites into a network of cracking – meaning that the physical and mechanical properties of the concrete material can be impaired.

The CCANZ document TR3 – Alkali Silica Reaction provides 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, including of a minimum 8% Microsilica SF will reduce the associated associated when reactive aggregates are used.

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

• Reduction in the available alkali.
• Pozzolanic reaction binds alkalis, meaning they’re unable to take part in ASR.
• Calcium hydroxide consumption, due to the pozzolanic reaction reducing the concrete’s pH that is required for ASR to take place.
• Concrete will be stronger and more durable; able to resist ingress of moisture, diffusion of alkalis to the reactive aggregates, and any other expansive forces.

Eventually all Portland cement concretes will break down under organic acid attack

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

The acid will continually lower the pH of concrete. At pH below 7, the calcium hydroxide followed by the calcium silicate hydrate phases are readily decomposed.

The process can be slowed with the inclusion of Microsilica SF and EverPlus Fly Ash. Alternatively, a ternary blend of EverSure Type GP, Microsilica SF and EverPlus Fly Ash from our product solution range will offer the highest level of acid resistance.