As one of the most utilised materials in the world, concrete is a vital ingredient in the construction of many structures. Quality of the materials used plays a vital role in determining strength and durability of the end result. Cement, in particular, as a glue that binds other materials and the main ingredient in concrete, needs to offer the stability and resilience. Reports of building collapses and structural failures from around the world confirm the importance of quality materials for a quality build.
In recent times, cement production has evolved to include more sustainable methods of production in the making of what has come to be known as composite cements. Typically, these make use of by-products from other industries such as Ground Granulated Blast Furnace Slag (GGBFS) from the steel manufacturing industry, Pulverised Fly Ash (PFA) from coal-powered stations and Silica fume, a by-product of the silicon and ferrosilicon alloy production and limestone.
The use of mineral components in cement not only lowers the carbon footprint but also offers significant technical advantages over traditional cements. Generally, composite cements are known to create more durable concrete compared to pure Portland cement. The use of Fly Ash, Slag and limestone in this mixture plays a huge role in improving durability.
Fly Ash particles are spherical in shape and finer than cement particles. In the concrete mix, these particles find themselves between the cement particles, displacing water that would normally occupy this space. As a result, they act as ball bearings, allowing the other materials in the mix to pass more easily across each other without the need to use too much water. The resultant mix is far more cohesive, bleeds less and gives an improved finish. This mix is also less permeable, preventing water and sulphate penetration which may subsequently weaken the concrete. Where substantial concrete structures are cast using pure cement, the concrete tends to generate heat which cannot escape easily. Fly Ash has a low heat of hydration and therefore prevents this thermal cracking in concrete.
Any steel reinforcement exposed to water or chemicals is prone to chloride ingress, leading to corrosion. Slag is known to capture these chloride ions that cause this corrosion to steel reinforcement, limiting chemical attack.
On the other hand, limestone is softer than the manufactured Portland cement clinker and, in the grinding process, grinds preferentially, producing very small particle sizes of limestone. Again these particles displace water between the cement particles and provide many more nucleation sites for the cement gel to form on, thereby improving durability.
The finer particles in GGBFS, Fly Ash and limestone give composite cements its reduced permeability properties. This mix prevents water and sulphate penetration which may subsequently weaken the concrete. Because water tends to cause corrosion, this reduced water penetration leads to increased corrosion resistance in the concrete.
All these factors combine to make concrete more durable. The durability of concrete is dependent on the strength of the concrete chosen for the mix and the pore structure of that particular mix.
Due to its less refined pore structure, the traditional pure Portland cement lends itself to attack by sulphates which infiltrate the concrete, while thermal cracking as a result of heat generated is a real possibility. These factors can lead to rapid deterioration of the concrete.