Abstract

The drive towards net-zero carbon emissions within the cement and concrete industry has seen the utilisation of lower embodied carbon materials such as ground granulated blast-furnace slag (GGBS). This can however lead to overlooking the technical performance of such materials. Current EN standards restrict additions to GGBS, limiting the sulfate optimisation of the aluminate rich material and thus potentially impacting the performance of the resultant concrete. In the UK and Ireland, GGBS is generally added as a supplementary cementitious material (SCM) to the concrete mixer opposed to inter-grinding or blending in a binary or ternary cementitious system. Although practical in terms of varying the replacement rate, this can lead to under-sulfated concrete due to the dilution of calcium sulfate (CaSO4) in the cement with increasing replacement percentage of GGBS. This study aimed to assess the suitable amounts of sulfate to balance the aluminate content in concrete while using commercially available GGBS as a SCM. As the alumina content plays a pivotal role in durability against sulfate attack and mechanical strength of GGBS concrete, a slag with a high alumina content (15%) was used at varying CaSO4 additions in cement replacement values of 30% and 70%. Measurement of these performance characteristics was carried out and compared to non-sulfated specimens. It was found that the addition of CaSO4 at optimal dosages improved sulfate resistance. Durability however was hindered when the sulfate demand was not met, due to effect on the capillary pressures within the pore structure and formation of alumina-based hydration products. The impact on compressive strength was more evident at a higher replacement due to the activation effect of CaSO4. An optimal CaSO4 addition provides enough sulfate to regulate the C3A in cement with suitable volume remaining to promote C3S hydration phases for improved early age strength.

Keywords

GGBS, durability, sulfate, alumina, sustainability

DOI

10.5703/1288284318162

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Sulfate Addition to Ground Granulated Blast-furnace Slag for use as a Supplementary Cementitious Material in Concrete

The drive towards net-zero carbon emissions within the cement and concrete industry has seen the utilisation of lower embodied carbon materials such as ground granulated blast-furnace slag (GGBS). This can however lead to overlooking the technical performance of such materials. Current EN standards restrict additions to GGBS, limiting the sulfate optimisation of the aluminate rich material and thus potentially impacting the performance of the resultant concrete. In the UK and Ireland, GGBS is generally added as a supplementary cementitious material (SCM) to the concrete mixer opposed to inter-grinding or blending in a binary or ternary cementitious system. Although practical in terms of varying the replacement rate, this can lead to under-sulfated concrete due to the dilution of calcium sulfate (CaSO4) in the cement with increasing replacement percentage of GGBS. This study aimed to assess the suitable amounts of sulfate to balance the aluminate content in concrete while using commercially available GGBS as a SCM. As the alumina content plays a pivotal role in durability against sulfate attack and mechanical strength of GGBS concrete, a slag with a high alumina content (15%) was used at varying CaSO4 additions in cement replacement values of 30% and 70%. Measurement of these performance characteristics was carried out and compared to non-sulfated specimens. It was found that the addition of CaSO4 at optimal dosages improved sulfate resistance. Durability however was hindered when the sulfate demand was not met, due to effect on the capillary pressures within the pore structure and formation of alumina-based hydration products. The impact on compressive strength was more evident at a higher replacement due to the activation effect of CaSO4. An optimal CaSO4 addition provides enough sulfate to regulate the C3A in cement with suitable volume remaining to promote C3S hydration phases for improved early age strength.