Limestone powder - Binary blended binders

Chapter 2 Literature Review

2.4 Binary blended binders

2.4.3 Limestone powder

Limestone is a very strong rock and it is formed in marine environments from the precipitation of calcium carbonate (CaCO₃). There are large amounts of limestone (chalk) cliffs in the coastal regions of England. Limestone, in addition to being one of the raw material for the production of Portland cement, has been utilised within the construction industry in various ways. Limestone rocks have been used as coarse aggregate in concrete (Uysal, 2012) and as building blocks in construction projects. Crushed limestone sand has also been used, in countries such as Spain, France and Argentina, as part of the fine aggregate due to the lack of availability of natural sand (Li et al., 2009; Menadi et al., 2009; Aquino et al., 2010). Limestone powder, essentially CaCO3 ground to a particular fineness, has also been used as a filler material partially replacing the Portland cement in concrete. The standard BS EN 197-1 (2011) identifies four types of Portland-limestone cement two of which contain 6-20% limestone (CEM II/A-L and CEM II/A-LL) and the other two containing 21-35% limestone (CEM II/B-L and CEM II/B-LL). The requirements for limestone in the standard are as follows:

 CaCO₃ content ≥ 75% by mass

 Clay content ≤ 1.20g/100g

 Total Organic Carbon (TOC) ≤ 0.20% for LL limestone and ≤ 0.50% for L limestone

Addition of limestone powder can be done by either: pre-blended or factory-blended done by inter-grinding the limestone directly with the clinker in the cement production plant producing CEM II Portland-limestone cement; or by directly adding limestone powder to CEM I Portland cement within the mixer (within-mixer) when producing concrete in the batching plant. Due to the fact that limestone is weaker than clinker, after the grinding process, it ends up being finer than the ground clinker hence factory-blended Portland-limestone cements (PLCs) are generally finer than pure Portland cements and have a smaller median particle size (Higgins, 2009). Portland-limestone cements are produced at numerous plants across the UK, however these cements have not been used to any great extent within the UK compared to Europe where they accounted for more than 24% of all cements in 2000 (approximately 40 million tonnes) (Price, 2004).

There have been various studies looking at the properties and behaviour of concrete containing limestone powder. Limestone powder is currently classified as a Type I addition i.e. nearly inert which indicates that it plays no role in the hydration calcium-silicate hydrates (C-S-H) i.e. the effects are therefore not solely physical (Bonavetti et al., 2000; Bonavetti et al., 2001; Kakali et al., 2000; Yang et al., 2011). It has been generally accepted across the literature that the physical effects due to the addition of limestone powder are the dilution effect, filler effect and heterogeneous nucleation (Tsivilis et al., 1999; Tsivilis et al., 2003; Voglis et al., 2005;

Ramezanianpour et al., 2009; Kadri et al., 2010; Ingram & Daugherty, 1991).

Increase in the percentage replacement of limestone powder means a decrease in the amount of Portland cement within the concrete (dilution effect) and consequently an increase in the effective water/cement ratio hence leading to a decrease in the compressive strength of the concrete. The addition of limestone powder, due to it being finer than Portland cement, changes the initial porosity of the mix (filler effect) increasing the consistence of the concrete or in other sense

decreasing the amount of water required to maintain constant consistence. Finally the heterogeneous nucleation occurs because limestone particles act as sites of nucleation and growth for the hydrating cement increasing the surface area available for the hydrates and consequently having a catalyst effect and accelerating the rate of the hydration reaction. The impact of limestone powder on the various properties of the concrete, due to the filler effect and the heterogeneous nucleation, is very much dependent on the fineness of limestone powder used with the finer powder having a greater effect. In this section the effects of limestone powder as a partial replacement of Portland cement, on both the fresh and hardened properties of the concrete, are discussed.

2.4.3.1 Fresh properties

Limestone powder is generally finer than Portland cement hence combining these two will result in a binder which has a greater overall fineness and better particle packing than pure Portland cement. Portland-limestone cement, despite its larger surface area (SSA), has a lower paste water demand, i.e. increased consistence for the same water content, compared to pure Portland cement (Tsivilis et al., 1999;

Voglis et al., 2005; Hawkins et al., 2003). Tsivilis et al. (1999) showed that limestone replacement levels of 10, 20 and 35% reduced the mix water demand from 26% for the reference Portland cement mix to 25.4%, 23.5% and 22.9%

respectively. Other studies also noticed a similar trend (Lv et al., 2011; Wang et al., 2011; Yang et al., 2011). Ramezanianpour et al. (2009) showed that a concrete mix, water/binder ratio of 0.37, with 10% limestone powder replacement level has a slump of 90mm compared to 80mm for the reference Portland cement mix. Wang et al. (2011) showed that the reduction in paste water demand was greater as the fineness of the limestone powder increased. For a 10% replacement level, limestone powders having SSA of 416, 841 and 1243m²/kg gave slump values of 180, 185 and 195 respectively compared to a slump of 170mm for the pure Portland cement mix. However the results of Gesoğlu et al. (2012) seem to contradict this as the slump flow diameter of the SCC mixes with 10 and 20% limestone powder replacement was lower (650 and 660mm respectively) than that obtained for the Portland cement control mix (700mm).

2.4.3.2 Early age properties

As mentioned above, with the addition of limestone powder, due to the heterogeneous nucleation effect, the rate of hydration reaction is increased resulting in the reduction in both the initial and final setting time of the mix. Mounanga et al.

(2011) mentions that, with a limestone replacement of 25% and 50%, the final setting time reduces from 537min to 421min and 413min respectively. Other studies also show similar trends (Tsivilis et al., 1999; Rahhal et al., 2012; Lv et al., 2011). The reduction in the initial and final setting time is greater with increased percentage replacement of limestone powder and also with increased specific surface area (fineness) of the limestone powder (Lv et al., 2011).

As for the heat of hydration, the maximum intensity of heat output decreases with the increase in limestone powder content since the cement content available to hydrate is reduced. With increased limestone powder content the amount of hydrates formed is reduced leading to less heat output (Figure 2-7). It is worth mentioning that the time taken to reach maximum heat flow reduces with increase in limestone powder which could be attributed to the catalyst effect of limestone powder accelerating the rate of the cement hydration reaction. Referring to Figure 2-7, a limestone powder replacement of 25% and 50%, the time to reach maximum heat output reduced from 498min to 484min and 391min respectively (Mounanga et al., 2011).

Figure 2-7 Hydration heat output rate for limestone powder concrete (Mounanga et al., 2011)

2.4.3.3 Strength

As mentioned in Section 2.4.3, the heterogeneous nucleation effect, due to the addition of limestone powder, increases the rate of hydration reaction resulting in the concrete having a slightly higher compressive strength at early ages (up to 7 days) (Tsivilis et al., 1999; Ghrici et al., 2007; Ramezanianpour et al., 2009). Due to the dilution effect, the long-term strength of the concrete incorporating limestone powder as a partial replacement of Portland cement will still be lower relative to concrete where Portland cement is used as the sole binder. The results from Tsivilis et al. (1999) show that the 7-day compressive strength of the concrete mix, up to 20% limestone powder replacement (by inter-grinding), was similar or higher than that of the reference Portland cement mix. The strength of the Portland-limestone concrete, at 28 days, was lower than the reference Portland cement mix except for the mix containing 5% limestone powder replacement where the strength was about 2MPa higher. Ghrici et al. (2007) and Ramezanianpour et al. (2009) also show similar results in their studies i.e. slightly higher early strength with a drop in strength at later ages for Portland-limestone concrete. Tsivilis et al. (1999) also showed that by inter-grinding the clinker and limestone for a longer period of time produces a finer product resulting in an increased rate of hydration reaction. Taking the same mix with 20% limestone powder replacement, the mix where the clinker and limestone had been inter-ground for 38 minutes had 28-day strength of 44MPa whereas in the mix where the inter-grinding time was 60 minutes, the strength was 47MPa at the same age. Wang et al. (2011) also looked at the effect of limestone fineness on the compressive strength of concrete and concluded that for the same replacement level (up to 15%), using a finer limestone powder leads to improvement in the compressive strength of the concrete both at 7 and 28 days.

Considering the 28-day strength, the reference Portland cement mix obtained a strength of 77.2MPa, and the mixes with 15% replacement incorporating limestone powder with fineness (SSA) of 416, 841 and 1243m²/kg obtained compressive strengths of 74.5, 77.3 and 79.2MPa respectively. Lv et al. (2011) conducted similar tests with a higher range of limestone replacement levels (up to 35%) and concluded that the increase in fineness of the limestone powder is favourable for the development of compressive strength up to 20% replacement level. Hence it can be

said that in terms of the powder properties, the fineness or particle size distribution of limestone powder is a key property affecting the performance of the Portland-limestone cement. However it is important to note that for a particular replacement level, it might not necessarily be the case that a higher fineness of limestone powder would increase the compressive strength hence it is necessary to obtain the optimum particle size distribution which would yield the most favourable results.

Ghrici et al. (2007) showed that the flexural strength of the concrete decreases with an increase in limestone replacement i.e. a concrete with no limestone replacement and one with 20% replacement, at 28 days, had flexural strengths of 8.9 and 7.6 MPa respectively, which is expected as the flexural strength of concrete is proportional to the compressive strength. In their study, the ratio between flexural strength and compressive strength remained at about 0.18- 0.19 for all mixes tested.

2.4.3.4 Durability properties

According to Pipilikaki & Beazi-Katsioti (2009), the use of limestone powder in blended cements changes the pore structure of the hardened cement paste. When the maximum allowable amount (i.e. 35%) is used in blended cements, the capillary pore size increases from 20nm to 40nm however it reduces the threshold diameter, the largest pore size which is structurally continuous, of the paste and provides a more uniform pore size distribution. The reduction in the threshold diameter makes it more difficult for chemicals to enter the hardened cement paste, and the increase in capillary pore size and the more uniform pore distribution allows the chemicals to travel around easier within the hardened cement (Pipilikaki & Beazi-Katsioti, 2009). Furthermore, the resistance to freezing and thawing and salt decay increases which could be attributed to the reduced threshold diameter of the paste better preventing chemicals entering the hardened cement.

Ramezanianpour et al. (2009) studied the effect of limestone powder replacement on the chloride ion penetration resistance of concrete. They conducted the rapid chloride penetration (RCP) test in accordance with ASTM C1202 (2007), on various Portland-limestone concrete mixes with different levels of limestone

powder replacement. The results are illustrated in Figure 2-8. It can be noticed that the charge passed, at 10% limestone powder replacement, was lower than the reference mix but increases at higher replacement levels. This is also mirrored in Uysal et al. (2012) where they tested SCC mixes with water/binder ratio 0.33;

however Gesoğlu et al. (2012) tested SCC mixes with water/binder ratio of 0.35 and obtained a reduction in the charge passed for replacement levels of 5, 10 and 20%.

Also the charge passed is reduced with longer curing period and also by reducing the water/binder ratio, both of which result in the formation of a denser matrix improving the chloride penetration resistance of the concrete.

Figure 2-8 Effect of limestone replacement on RCP test results (Ramezanianpour et al., 2009)

Bonavetti et al. (2000) reported that, when immersed in 3% NaCl solution for 45 days, concrete mixes, cured under water at 20°C, containing 10 and 20% limestone replacement resulted in an increase of the chloride penetration depth from 7mm to 10mm and 15mm respectively. They also conducted tests on another set of mixes, cured in air, where the results show that, for all mixes, the chloride penetration depth is higher compared to water-cured mixes which could be due to the formation of a less dense matrix as lack of availability of water did not allow full hydration to take place. The inclusion of 10 and 20% limestone powder in this case resulted in a reduction in penetration depth from 30mm to 17 and 20mm respectively indicating

that the chloride penetration depth is also influenced by the curing regime (Bonavetti et al., 2000). Ghrici et al. (2007) also showed that the penetration of chloride ions increases in concrete containing 15% limestone powder replacement.

Dhir et al. (2007) used an accelerated test method to obtain the chloride diffusion coefficients for concrete mixes with different limestone replacement levels and water/cement ratios. The accelerated test method, explained in Dhir et al. (1990), involved placing concrete slices (100mm in diameter and 25mm thick) in a two-compartment cell containing a chloride solution, applying a small potential difference (7.5 Volts) across to accelerate the transport of chloride ions through the concrete and finally calculating the chloride diffusion coefficient using Fick’s First Law of Diffusion. The results of the tests are illustrated in Figure 2-9.

Figure 2-9 Relationship between chloride diffusion and (a) w/c ratio and (b) 28-day cube strength of concrete with limestone powder addition (Dhir et al.,

2007)

As it can be noticed on Figure 2-9 (a), the chloride diffusion coefficient decreases as the water/cement ratio of the concrete decreases which is in agreement with the conclusion of Ramezanianpour et al. (2009), that concrete mixes with lower water/cement ratio have better chloride penetration resistance. Dhir at al. (2007) also shows that the addition of limestone powder seems to decrease the chloride resistance of the concrete, as illustrated in Figure 2-9 (a). The differences in the chloride diffusion for the mix with 15% limestone replacement (LS15) compared to the reference mix without limestone powder (PC) were relatively small and seem to

decrease with lower water/cement ratios. The general consensus within the literature seems to be the chloride resistance of the concrete is worsened with the addition of limestone powder. Accounting for the fact that the addition of limestone powder leads to a decrease in compressive strength, Dhir et al. (2007) plotted the chloride diffusion coefficient against the cube compressive strength of the mix, illustrated in Figure 2-9 (b), where, for a given 28-day strength, there seems to be little or no difference between the concrete mixes with and without limestone powder. Hence it could be said that two mixes with similar 28-day compressive strength, with or without limestone powder, would have similar or comparable chloride resistance.

Looking at the effect of limestone powder on the sorptivity coefficient of the concrete, Ramezanianpour et al. (2009) showed that the sorptivity coefficient of the concrete increases with increasing limestone replacement, decreases with the reduction in water/binder ratio (Figure 2-10) and with longer curing periods similar to the trends observed with the chloride resistance of concrete. At water/binder ratios of 0.45 and 0.55, the sorptivity coefficients increased with increasing limestone powder replacement however at lower water/binder ratio (0.37), the mix with 10% limestone replacement had the same sorptivity coefficient as the reference PC mix.

Figure 2-10 Sorptivity coefficients (28 days) for concrete with different limestone powder replacement levels (Ramezanianpour et al., 2009)

Ghrici et al. (2007) observed that the incorporation of 15% limestone powder as a partial cement replacement, at both water/binder ratio of 0.6 and 0.4, decreased the sorptivity coefficients at 28 and 90 days, contradicting the trend observed by Ramezanianpour et al. (2009). This was mirrored by Tsivilis et al. (2003) where increasing limestone powder replacement (water/binder ratio 0.7) lowered the sorptivity coefficient of the mix from 0.237 for the control PC mix to 0.220mm/min^0.5 for 20% replacement level.

Dhir et al. (2007) studied the effect of limestone powder addition on the carbonation of the concrete using two test environments; in the first test environment (CEN environment), the specimens were exposed to 0.035% CO2 at 20°C and 65% relative humidity for 365 days and in the second the specimens were stored in an enriched 4% CO2 environment (accelerated environment) at 20°C and 65% relative humidity for 20 weeks. They observed that the addition of limestone powder led to an increase in the carbonation depth in both cases. Their result is illustrated in Figure 2-11. The carbonation resistance, as with the chloride resistance, increased with lower water/binder ratios and longer curing periods. Dhir at al. (2007) also plotted the carbonation depth versus the 28-day cube compressive strength for all the mixes tested, shown in Figure 2-11 (b), and as with the chloride

diffusion coefficient test results (Figure 2-9), this indicated that concrete mixes produced with the same 28-day compressive strength would have similar or comparable carbonation resistance regardless of whether the mix contains any additions. Collepardi et al. (2004) also showed that substitution of 15 or 25% of the Portland cement by limestone powder, fly ash or slag led to an increase in carbonation rate when concrete was compared at equal water/binder ratio, but that the rate was comparable for concretes of equal strengths. Hence the conclusion from these studies is that for a given degree of moist curing and exposure conditions, the rate of carbonation is a function of the concrete strength and seems to be independent of the type of binder used.

Figure 2-11 Comparison of carbonation resistance at equivalent (a) w/c ratio (CEN environment) and (b) 28-day cube strength (accelerated environment)

(Dhir et al., 2007)

The paper by Tsivilis et al. (2002) reported that the addition of limestone powder shows no carbonation after exposure times of 9 and 12 months, compared to a carbonation depth of 3mm and 5mm in Portland cement concrete respectively, contradictory to the findings of Dhir et al. (2007) and Collepardi et al. (2004). Also mentioned in the paper is that there is a clear decrease of corrosion in limestone concrete specimens and also the mass loss of rebars is less than that of Portland cement concrete after a 12 months of immersion in a 3% by weight NaCl solution.

The mass loss of rebars decreased from 1.1 mg/cm² for the PC mix down to 0.35 mg/cm² for mix with 20% limestone replacement after 12 months immersion in the

NaCl solution. Ramezanianpour et al. (2009) mentioned that the corrosion-resisting performance of the concrete is influenced by its electrical resistivity, the higher the electrical resistivity the better the corrosion resistance of the concrete. It went on to show that the electrical resistivity of concrete decreases due to the addition of limestone powder hence the corrosion resistance of the concrete is reduced, mirroring the conclusions of Dhir et al. (2007) and Collepardi et al. (2004).

Hobbs (1983) looked at the effect of limestone powder on alkali-silica reaction (ASR) in mortars and reported that the use of 5% limestone powder, in mortar bars made with high-alkali cement, slightly extended the time to cracking but did not eliminate it. Thomas et al. (2010) conducted three different tests, two accelerated tests, one on mortar bars and the other on concrete prisms (AMBT and ACPT respectively), and one long-term test on concrete prisms (CPT). Two sets of mixes were tested, one made with Portland cement and the other with Portland-limestone cement, to study the effect of limestone powder addition, if any, on ASR. The expansion results are illustrated in Figure 2-12. It can be seen that there is no consistent difference in the expansion values of specimens made with Portland

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