A brief introduction to discussion 2
School of Civil Engineering Shuangxin Li*, Ian G Richardson, Rik Brydson FACULTY OF ENGINEERING
*[email protected]
Introduction A series of 20-year-old blended OPC pastes with varied GGBS content, have been characterized by thermal analysis and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, before and after partial carbonation in pure CO2 and air at a relative humidity of 72.6%. Increasing the OPC content enhances the production of CH, AFt and AFm phases, as well as increasing the Ca/Si ratios of the formed C-S-H gel. It was observed these effects from the high OPC contents had no influence on the decalcification of the C-S-H gel in the accelerated carbonation until the content increased up to 75%. In the air carbonation, there was no correlation between the content of OPC and the decalcification of the C-S-H gel. GGBS75 was observed as the most prone to carbonation in both CO2 and air. The concentration of CO2 did not influence the carbonation of GGBS25 or the decalcification of the C-S-H gel in GGBS25 at the early stage of carbonation, but GGBS75, GGBS90 and GGBS100 carbonated more slowly in air than in CO2. Keys words: decalcification degree (extent), carbonation degree (extent), decalcification rate , carbonation rate
Figure 1-Graphically presenting the decalcification degree of the CS-H in CO2 against carbonation duration (days)
Figure 2-Graphically presenting the decalcification degree of the CS-H in air against carbonation duration (days)
A brief introduction to discussion 1 TEM-EDX analysis supplied compositional information of the C-S-H gel/ Ca-modified C-S-H gel, such as average Ca/Si ratios and Al/Si ratios. All analyses were randomly taken cross the sample to ensure the veracity of the results. A T-test analysis (two-samples assuming unequal variance) was carried out on the average Ca/Si ratios, proving the ratios to be statistically significant, as the carbonation caused decalcification to the C-S-H gel. TEM-EDX analysis analyzed the areas near surface which was supported to be the most carbonated areas in a sample, and also the influence from the diffusion of CO2 into the samples on the carbonation will be negligible for the areas near the surface.
Decalcification Degree
（Ca/Si ratio before carbonation Ca/Si ratioafter carbonation ) 100% Ca/Si ratiobefore carbonation
Discussion 1 As show in figure 1, in the CO2 carbonation, the C-S-H gel of GGBS75, GGBS90 and GGBS100 have decalcified to a high degree within a duration of 1 month, with a similar decalcification rate. However, the C-S-H gel in GGBS25 clearly carbonated to the relatively low degree of 30% or lower. As show in figure 2, in the air carbonation, the C-S-H in GGBS90 and GGBS100 decalcified to the most extent, but at a different rate. The C-S-H gel in GGBS25 decalcified deeper than that in GGBS75. References Richardson, I.G. and J.G. Cabrera, The nature of C---S---H in model slag-cements. Cement and Concrete Composites, 2000. 22(4): p. 259-266 Richardson, I.G., The nature of the hydration products in hardened cement pastes. Cement and Concrete Composites, 2000. 22(2): p. 97-113 Groves, G.W. and I.G. Richardson, Microcrystalline calcium hydroxide in pozzalanic cement pastes. Cement and Concrete Research, 1994. 24(6): p. 1191-1196 G.W., G., D.I., Rodway and I.G., Richardson, The carbonation of hardened cement pastes. Advances in Cement Research, 1990. 3(11): p. 117-125 Borges, P.H.R., et al., Carbonation of CH and C-S-H in composite cement pastes containing high amounts of BFS. Cement and Concrete Research, 2010. 40(2): p. 284-292.
Figure 3-The ignited weight percentages of Ca equivalent to the total carbonates formed in the CO2-carbonated samples against the carbonation time
Figure 4-The ignited weight percentages of Ca equivalent to the total carbonates formed in the air-carbonated samples against the carbonation time
A brief introduction to discussion 2 The formation of carbonates was followed by the differential thermal analysis (DTA) using first derivative, combined with the thermogravimetry. The ignited amounts of formed carbonates in the system were estimated by the tangent method. The resulted contents of carbonates have been converted to the equivalent ignited weight percentages of Ca. Discussion 2 As shown in figure 3, in accelerated carbonation, within a same duration of 2 months, GGBS75 carbonated the. GGBS90 and GGBS100 have been carbonated to a similar extent as each other. Despite having the second largest degree of carbonation, between 2 days to 1.5 months the GGBS25 paste showed no increase in carbonation, which was not observed for other samples. As shown by Figure 4, GGBS75, GGBS90 and GGBS100 carbonated more gradually in air than in CO2.
During the 2 months exposure, GGBS75 carbonated to the greatest extent, followed by GGBS25. GGBS90 and GGBS100 are shown to be less prone to carbonation by air. It can be observed from figures 3 and 4 that GGBS25 carbonated with a same approach in both CO2 and air, since the two line representing GGBS25 in these figures are follow the same trend. Conclusion • In the accelerated carbonation, the content of OPC did not influence the decalcification rate of the C-S-H gel until when it increased to 75%. •In the air carbonation, the C-S-H gel in GGBS75 decalcified most slowly, suggesting there is no correlation between the content of OPC and the decalcification of the C-S-H gel. • Within a same duration, the C-S-H gel in GGBS25 decalcified in air to a similar degree as in CO2, thus the concentration of CO2 is not a significant factor affecting the decalcification of the C-S-H in GGBS25. •The C-S-H gel in GGBS75, GGBS90 and GGBS100 decalcified more slowly in air than in CO2, thus the concentration of CO2 did affect the C-S-H gel in the low content OPC pastes. •In neither CO2 carbonation or air carbonation, there was no clear correlation between the carbonation rate and the content of OPC. •Regarding with the carbonation of whole sample, the concentration of CO2 did affect the carbonation rate of GGBS75, GGBS90 and GGBS100. Increasing the concentration enhanced the carbonation of them. But it did not affect the carbonation of GGBS25. •75% GGBS and 25% OPC was observed to be the most likely carbonated constituents here.
Institute for Resilient Infrastructure
*[email protected]
Introduction A series of 20-year-old blended OPC pastes with varied GGBS content, have been characterized by thermal analysis and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, before and after partial carbonation in pure CO2 and air at a relative humidity of 72.6%. Increasing the OPC content enhances the production of CH, AFt and AFm phases, as well as increasing the Ca/Si ratios of the formed C-S-H gel. It was observed these effects from the high OPC contents had no influence on the decalcification of the C-S-H gel in the accelerated carbonation until the content increased up to 75%. In the air carbonation, there was no correlation between the content of OPC and the decalcification of the C-S-H gel. GGBS75 was observed as the most prone to carbonation in both CO2 and air. The concentration of CO2 did not influence the carbonation of GGBS25 or the decalcification of the C-S-H gel in GGBS25 at the early stage of carbonation, but GGBS75, GGBS90 and GGBS100 carbonated more slowly in air than in CO2. Keys words: decalcification degree (extent), carbonation degree (extent), decalcification rate , carbonation rate
Figure 1-Graphically presenting the decalcification degree of the CS-H in CO2 against carbonation duration (days)
Figure 2-Graphically presenting the decalcification degree of the CS-H in air against carbonation duration (days)
A brief introduction to discussion 1 TEM-EDX analysis supplied compositional information of the C-S-H gel/ Ca-modified C-S-H gel, such as average Ca/Si ratios and Al/Si ratios. All analyses were randomly taken cross the sample to ensure the veracity of the results. A T-test analysis (two-samples assuming unequal variance) was carried out on the average Ca/Si ratios, proving the ratios to be statistically significant, as the carbonation caused decalcification to the C-S-H gel. TEM-EDX analysis analyzed the areas near surface which was supported to be the most carbonated areas in a sample, and also the influence from the diffusion of CO2 into the samples on the carbonation will be negligible for the areas near the surface.
Decalcification Degree
（Ca/Si ratio before carbonation Ca/Si ratioafter carbonation ) 100% Ca/Si ratiobefore carbonation
Discussion 1 As show in figure 1, in the CO2 carbonation, the C-S-H gel of GGBS75, GGBS90 and GGBS100 have decalcified to a high degree within a duration of 1 month, with a similar decalcification rate. However, the C-S-H gel in GGBS25 clearly carbonated to the relatively low degree of 30% or lower. As show in figure 2, in the air carbonation, the C-S-H in GGBS90 and GGBS100 decalcified to the most extent, but at a different rate. The C-S-H gel in GGBS25 decalcified deeper than that in GGBS75. References Richardson, I.G. and J.G. Cabrera, The nature of C---S---H in model slag-cements. Cement and Concrete Composites, 2000. 22(4): p. 259-266 Richardson, I.G., The nature of the hydration products in hardened cement pastes. Cement and Concrete Composites, 2000. 22(2): p. 97-113 Groves, G.W. and I.G. Richardson, Microcrystalline calcium hydroxide in pozzalanic cement pastes. Cement and Concrete Research, 1994. 24(6): p. 1191-1196 G.W., G., D.I., Rodway and I.G., Richardson, The carbonation of hardened cement pastes. Advances in Cement Research, 1990. 3(11): p. 117-125 Borges, P.H.R., et al., Carbonation of CH and C-S-H in composite cement pastes containing high amounts of BFS. Cement and Concrete Research, 2010. 40(2): p. 284-292.
Figure 3-The ignited weight percentages of Ca equivalent to the total carbonates formed in the CO2-carbonated samples against the carbonation time
Figure 4-The ignited weight percentages of Ca equivalent to the total carbonates formed in the air-carbonated samples against the carbonation time
A brief introduction to discussion 2 The formation of carbonates was followed by the differential thermal analysis (DTA) using first derivative, combined with the thermogravimetry. The ignited amounts of formed carbonates in the system were estimated by the tangent method. The resulted contents of carbonates have been converted to the equivalent ignited weight percentages of Ca. Discussion 2 As shown in figure 3, in accelerated carbonation, within a same duration of 2 months, GGBS75 carbonated the. GGBS90 and GGBS100 have been carbonated to a similar extent as each other. Despite having the second largest degree of carbonation, between 2 days to 1.5 months the GGBS25 paste showed no increase in carbonation, which was not observed for other samples. As shown by Figure 4, GGBS75, GGBS90 and GGBS100 carbonated more gradually in air than in CO2.
During the 2 months exposure, GGBS75 carbonated to the greatest extent, followed by GGBS25. GGBS90 and GGBS100 are shown to be less prone to carbonation by air. It can be observed from figures 3 and 4 that GGBS25 carbonated with a same approach in both CO2 and air, since the two line representing GGBS25 in these figures are follow the same trend. Conclusion • In the accelerated carbonation, the content of OPC did not influence the decalcification rate of the C-S-H gel until when it increased to 75%. •In the air carbonation, the C-S-H gel in GGBS75 decalcified most slowly, suggesting there is no correlation between the content of OPC and the decalcification of the C-S-H gel. • Within a same duration, the C-S-H gel in GGBS25 decalcified in air to a similar degree as in CO2, thus the concentration of CO2 is not a significant factor affecting the decalcification of the C-S-H in GGBS25. •The C-S-H gel in GGBS75, GGBS90 and GGBS100 decalcified more slowly in air than in CO2, thus the concentration of CO2 did affect the C-S-H gel in the low content OPC pastes. •In neither CO2 carbonation or air carbonation, there was no clear correlation between the carbonation rate and the content of OPC. •Regarding with the carbonation of whole sample, the concentration of CO2 did affect the carbonation rate of GGBS75, GGBS90 and GGBS100. Increasing the concentration enhanced the carbonation of them. But it did not affect the carbonation of GGBS25. •75% GGBS and 25% OPC was observed to be the most likely carbonated constituents here.
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