A recent study has indicated benefits in using GGBS cement in making concrete for farm use. Traditionally, most of the concrete used here is made using ordinary portland cement (OPC).

A cubic metre of 40N concrete purchased for making a silage slab will have 350kg of such cement. However, in recent years interest has grown in including an alternative cement type, GGBS, in the mix. It is now supplied on to the Irish market by a number of firms, the best known of which is Ecocem.

GGBS stands for ground, granulated, blast furnace slag. It is a byproduct of the iron smelting industry. Like portland cement, blast furnace slag originates from limestone, so the two cement types have much in common. However, there are differences. There is research showing that including GGBS cement in concrete makes it more resistant to the damaging effects of acids and salts. This makes it of very direct relevance to farmers.

Many livestock farmers are very aware of the dramatic corrosion eventually caused to concrete slabs by silage acid. A number of concrete manufacturers now include GGBS cement, on request, in their concretes, typically reducing the OPC content and replacing it one-for-one with GGBS material. The total cement content and water to cement ratio will be unchanged.

The Department of Agriculture specification for farm concrete does not include GGBS cement. However, its use has been considered for the updated specification, which is now being drawn up by an expert panel, under the leadership of the Department. It will look at all research information, Irish and international, on a range of factors affecting concrete durability.

Previous revisions to the specifications saw a switch to 40N concrete for any slab, tank, channel or other structure coming into contact with silage acid or slurry effluent. This has been successful in making slabs more resistant to silage acid. But the Department is looking to see if further improvements can be made that will benefit farmers.

Wexford

This most recent study was carried out by engineer Ali Salih, technical manager at Casey Concrete in Co Wexford, as preparation for a PhD. He did the study in conjunction with Pawel Luckowskp from Warsaw University of Technology and John Reddy of Ecocem.

Ali said Casey Concrete is busy, putting out 300-400m3 of concrete per day, much of it to farmer customers. The firm has 30 trucks on the road, all of which is good to hear.

The aim of this study was to investigate the influence of including GGBS cement on the strength development and durability of concrete placed in aggressive environments.

Different mortar blocks were prepared using OPC and GGBS. Inclusion rates of GGBS cement were 0%, 30%, 50% and 70%.

The blocks were cured for 28 days and then were immersed in acid and salt solutions. Five 28-day cycles of effluent exposure were repeated over a six-month period.

In order to determine the rate of deterioration, following each cycle, the mortars were tested for water absorption, compressive strength and mass loss.

Results

Visual inspection following exposure to silage effluent: Samples incorporating GGBS visually fared better on exposure to silage effluent. Visual degradation was obvious on 100% OPC mortars, with corners and edges invariably disintegrating when pressurised. Samples containing 30% as well as those including 50% GGBS remained largely unaltered, except for slight rounding of corners of the 30% GGBS samples.

Loss of compressive strength following exposure to silage effluent: GGBS mixes showed a greater strength than the 100% OPC mix. The percentage loss in strength of the 100% OPC samples was over twice as much as that of the 50% GGBS samples (see Figure 1).

Mass loss (g) on exposure to silage effluent: The 100% OPC mortars lost mass at a faster rate than those containing GGBS. The 50% GGBS samples showed both the lowest mass loss and the slowest rate of loss (see Figure 2).

Water Absorption of OPC and GGBS samples: The OPC mortars initially absorbed more water than the GGBS samples. All samples showed a progressive increase in water absorption. The amount of water absorbed by the GGBS samples is lower than that absorbed by the OPC samples.

Conclusion: OPC composites that include GGBS are more durable than those made with OPC alone in aggressive environments under the action of acids and salts such as those produced by silage, says Ali Salih in his report.

Durability increased with increasing amounts of GGBS. GGBS mixes showed the smallest progressive rise in water absorption and the smallest mass and compressive strength loss as a result of silage immersion and salt cycling.

A more durable concrete mix for agricultural use in silos can be specified by incorporating GGBS as a partial substitute for OPC.

GGBS concrete has better water permeability characteristics as well as improved resistance to corrosion and sulphate attack. As a result, the service life of a structure is enhanced and the maintenance cost reduced.

Farmyard environment

The farm environment is very demanding on concrete, Ali Salih says. Exposure to silage effluent, slurry and cleaning fluids along with mechanical abrasion from farm vehicles often results in significant damage to concrete, requiring concrete to be repaired or replaced early in its service life.

The most common aggressive agents on the farm are acids and sulphates. Lactic and acetic acid are present where silage is stored, sulphates are present in various effluents and manures. The greatest amount of degradation tends to occur where there is a combination of both chemical attack and physical abrasion, such as silage pit aprons where tractors are handling silage, where animals are trafficking slabs, or where power-hosing washes acids on to concrete floors.

Concrete made with GGBS has greater long-term strength than concrete made with OPC, due to the denser cement matrix of GGBS concrete.

The same 28-day strength will be achieved in concrete with 50% GGBS as concrete made with 100% OPC.

However, beyond 28 days, GGBS concrete continues to hydrate and gain in strength more than concrete made only with OPC.

Concrete made with GGBS cement has a lower porosity and permeability than concrete made with OPC. Pore sizes are smaller and less numerous. This reduces the rate of penetration of acids into the concrete, giving enhanced protection from acids, erosion and abrasion damage.

For most farm applications, the most durable concrete will be obtained using a 50:50 blend of GGBS and OPC. In the most aggressive environments, the use of 60-70% GGBS might be required.