SCMs reduce concrete emissions

This Issue This is a part of the Concrete, steel and timber feature

By - , Build 189

New research shows that using natural supplementary cementitious materials (SCMs) with Portland cement can maintain the strength and durability of concrete while reducing carbon emissions. Changes to cement standards may be needed, however, before SCM use can grow.

Figure 2: Strength development of higher-strength concrete mixes (water to binder ratio = 0.45).
Figure 1: 28-day compressive strength versus bound water content of cementitious pastes.
Figure 3: Embodied carbon dioxide comparisons for 50 MPa concrete at 28 days.

NEW ZEALAND has seen a 15% reduction in carbon emissions from Portland cement production since 2005. This is despite concrete production increasing 13% during the period and was due to better process efficiencies at cement factories and replacing less-efficient production facilities.

These developments, combined with better use of supplementary cementitious materials (SCMs) could help reduce New Zealand carbon emissions from cement manufacture by a further 15% before 2030.

What are SCMs?

SCMs are sourced from either industrial waste such as fly ash, blast-furnace slag and silica fume or from natural materials beneficiated into pozzolans such as pumicite, micro-silica, diatomite and calcined clays like metakaolin. They have diverse chemical and physical properties affecting their reactivity and interaction with Portland cement.

Possible New Zealand natural pozzolans and clay minerals include pumice, amorphous silica, tuff and ignimbrite, kaolinite and diatomaceous earth. Research into the potential of New Zealand natural pozzolans undertaken 60 years ago showed that reactivity of natural pozzolans improved with increasing fineness and glass content.

Characterisation of SCMs

Recent research compared the reactivity of natural versus commonly used industrial SCMs such as fly ash and slag. This experimental work investigated a range of binders including:

  • Portland cement from New Zealand for control concrete mixes – denoted PC
  • two types of fly ash (ASTM Class F and Class C) – denoted FAF and FAC respectively
  • perlite and natural pozzolana from New Zealand – denoted Perl and Pozz
  • respectively calcined clay using moderate grade kaolinite (55%) from New Zealand – denoted CC.

Classification of SCMs

Before new SCMs could be used in concrete, reactivity assessment was undertaken using cement paste and mortar trials. Classification of SCMs was done using local and international standards with mortar or paste results compared with 28-day strength of concrete mixes using the same materials.

Current classification findings using relative strength where mortar mixes were cast at constant consistence but having variable water to binder ratios had almost no correlation with concrete strengths (R2=0.003). Bound water of paste samples based on thermo-gravimetric analysis showed good correlation with concrete strengths (R2=0.94) (see Figure 1).

Figure 1: 28-day compressive strength versus bound water content of cementitious pastes.

Strength performance of SCM concrete

Strength development of concrete mixes was assessed for a higher and lower strength series. Higher-strength concrete mixes had a total binder content of 350 kg/m³ – the prescriptive minimum cementitious content from some exposure classes when using NZS 3101:2006 Concrete structures standard. Figure 2 shows the strength development of control and SCM concrete mixes with comparable longer-term strengths for some SCM concrete mixes.

From strength data in trials of high and low strength series, it was possible to compare the performance for a reference strength of 50 MPa. Cementitious contents can then be compared and the embodied carbon for each concrete mix type calculated based on international data.

Figure 3 shows the embodied carbon for each binder combination, with all SCMs except perlite reducing embodied carbon dioxide by 15–20% when used at 30% replacement.

Figure 2: Strength development of higher-strength concrete mixes (water to binder ratio = 0.45).
Figure 3: Embodied carbon dioxide comparisons for 50 MPa concrete at 28 days.

Durability performance

The durability performance of concrete was assessed across a wide range of properties including carbonation or chloride resistance and mitigation of alkali silica reaction. The research showed that SCMs may have variable performance in concrete when durability properties are assessed although there was a general improvement in most cases.

More-reactive SCMs such as fly ash, natural pozzolan and calcined clay consistently lowered porosity and permeability. All SCM concrete showed a beneficial effect on reducing expansion associated with alkali silica reaction.

Research conclusions

The adoption of natural pozzolans in concrete construction may be challenging using a cement classification system designed for industrial SCMs such as fly ash or slag.

This research showed that rapid and reliable classification can be done using methods such as isothermal calorimetry or thermal gravimetric analysis. Cement standards should be updated to allow these alternate classification systems to be adopted in New Zealand.

In some cases, replacement of 30% Portland cement with SCMs in concrete achieved reasonable strengths and superior durability. Durability performance was varied however. Some properties of SCM concrete such as carbonation resistance had poorer durability than PC concrete.

The chloride resistance of SCM concrete improved significantly with age while most SCMs reduced expansion from alkali silica reaction. Concrete containing reasonably reactive SCMs were also found to have lower embodied carbon dioxide than control concrete when compared at equivalent strength.

 

Note

This work was funded by the Building Research Levy and Concrete NZ with experimental work carried out at the University of Canterbury. See the full report ER66 Removing the barriers to the use of significant levels of SCMs in concrete production in NZ.

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Figure 2: Strength development of higher-strength concrete mixes (water to binder ratio = 0.45).
Figure 1: 28-day compressive strength versus bound water content of cementitious pastes.
Figure 3: Embodied carbon dioxide comparisons for 50 MPa concrete at 28 days.

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