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Concrete In Australia : March 2012
30 Concrete in Australia Vol 38 No 1 FEATURE: CEMENT TECHNOLOGIES limestone results in less workable concrete than finer limestone (Moir & Kelham, 1997). Decreasing the average particle size of the limestone, in the 3 μm to 0.7 μm size range, results in improved flow properties (Nehdi et al, 1998). 3.3.2 Bleed Bleed rate is a function of the surface area of the cement and as such increasing the fineness of the cement or limestone will result in decreased bleed rates. 3.3.3 Set time e general literature consensus is that limestone fineness influences set times. As the combined limestone clinker product is ground finer the setting time decreases (Hooton et al, 2007). e authors experience is that set times of production Portland-limestone blends are impacted by up to 5% limestone with constant surface index (increased limestone content effectively coarsens the Portland clinker component) whereas maintaining constant 45 μm residue reduces the impact of increased limestone addition. is is consistent with the findings of Hooton et al (2007) and Tsivilis et al (1999) who observed limestone impact on initial and final set decreases as fineness increases. 3.3.4 Strength Work by Hawkins et al (2003) in laboratory milled cement showed equivalent strength can be achieved with Portland- limestone cement with up to 8% Portland clinker replacement in concretes when a constant +45 μm is maintained. Full-scale production trials in Canada have also shown by increasing surface area an equivalent strength can be achieved in Limestone addition (%) Surface area AS2350.8 (m2/kg) +45μm -- Laser particle size analysis. (%) Cement initial set time AS2350.4 (minutes) Cement nal set time AS2350.4 (minutes) Concrete initial set time AS1012.18 (minutes) Concrete nal set time AS1012.18 (minutes) 5 405 6 115 140 325 425 7.5 420 6 115 145 345 460 10 445 6 125 160 360 470 12.5 465 6 120 190 330 430 Table 1. Laboratory mix designs per m3 of concrete nominal N32 grade. Figure 4. Relationship between surface area and bleed rate at CCAA trials site 3. Figure 3. Strength development of mortars using Portland-limestone cements at constant surface area (left) or constant +45 um (after Hawkins et al, 2003).