by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 35 two mixes (25 MPa and 32 MPa) have 25% of the Portland cement replaced by fly ash. e second two mixes (25 MPa and 32 MPa) have 40% of the Portland cement replaced by GGBFS. ese percentages are chosen because they are commonly applied in construction projects in Australia. It is also noted that large cement replacements in lower grade concretes such as these are not desired by the construction industry due to construction process being slowed down by slow early strength development of these concretes. e 25 MPa and 32 MPa concretes are commonly used standard strengths defined in Australian Standards (AS1379). Figure 3 (Flower et al, 2005) shows the results of this analysis. Portland cement is the dominant source of emissions in all of the concretes, blended or otherwise. e fly ash blended concretes show reduced CO2 emissions (13-15%), but it is the GGBFS blended concretes that show more substantial reductions (22%). is is because high percentage of GGBFS can be included in a blended mix without changing the engineering properties of the concrete, due to its natural cementitious properties. So while GGBFS has a higher material emission factor than fly ash, it can replace more cement, which leads to lower total emissions. In recent years, high performance concrete has become popular in construction due to its superior mechanical properties, such as high elastic modulus and high compressive and tensile strengths, and superior durability in a variety of environments. e high performance concrete utilises about twice the amount of cement to produce the same volume of concrete as conventional concrete. Widespread use of high performance concrete would significantly increase the greenhouse gas emission due to concrete construction. High strength concrete is usually used in construction to reduce the size of compression members (eg columns), thereby reducing the amount of concrete used. However, often in precast or prestressed concrete constructions, high strength concrete is used for obtaining high early strengths, which may not be necessary for service conditions. e high strength concrete is commonly produced by reducing the water/cement ratio to around 0.35 to 0.3, but in some instances as low as 0.2. At these levels of water/cement ratios a large part of the cement used is left unhydrated, effectively using the cement as a filler material. Considering the environmental and other costs of cements, this is a waste of resources. Research needs to be focused in this area to make high performance concrete without the wastages of Portland cement. 2.0 BLENDED CEMENTS Many research studies have demonstrated that high strength, high performance concrete can be made with slag blended cements containing slag replacement of Portland cements of 30%, 50% and 70% levels without any loss of the advantages the high performance concrete offers (Sioulas & Sanjayan, 1997). Some of the advantages of using slag-blended cement in high performance are summarised as below: 1. Using slag replacements, such as a 50% replacement of Portland cement, high performance concrete can be made with the same level of consumption of Portland cement as in the conventional concrete. A higher percentage replacement, such as 70%, will even reduce the Portland cement usage in high performance concrete. 2. Due to the high levels of Portland cement usage in high performance concrete, high levels of heat are generated by hydration in high performance concretes increasing the risk of thermal cracking. With the use of slag replacements, the risk of thermal cracking can be reduced (Sioulas & Sanjayan 2000a). 3. High performance concretes made with slag-blended cements offers superior durability, especially in marine applications. However, it should be noted that the use of slag-blended cements also comes with some limitations. Since the hydration of slag relies on the release of Ca(OH)2 from the hydration of Portland cement, the rate of strength development of concretes made with blended cements are very low (Wainwright, 1986). Also, as the hydration of slag occurs over an extended period of time, the extended curing period is essential to obtain the desirable properties. While the standard test cylinder specimens are kept in 100% moist conditions, the insitu concretes do not receive the same treatment. As a result, there exists a significant difference between the insitu strength properties of concrete made with slag blended Figure 3. CO2 Emissions generated by SCM blended cement concretes. Figure 2. CO2 Emissions generated by concretes. CO2 Emissions Breakdown 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 25MPa 32MPa 40MPa 50MPa t CO2-e/m3 Cement Coarse Aggregates Fine Aggregates Concrete Batching Construction Activities Concrete Transport CO2 Emissions Breakdown 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 25MPa (FA) 32MPa (FA) 25MPa (GGBFS) 32MPa (GGBFS) 25MPa (GP) 32MPa (GP) t CO2-e/m3 Cement Fly Ash GGBFS Coarse Aggregates Fine Aggregates Concrete Batching Construction Activities Concrete Transport