CHAPTER 5. MICROSTRUCTURAL CHARACTERISTICS OF CALCIUM OXIDE-ACTIVATED
6.4. Pore Structures of CaO-CaSO 4 -GGBFS Composite Cements with Different Substitution Rate
0C 5C 10C 20C 30C 40C 50C
3 days 27.2 27.0 27.6 28.0 23.6 23.1 20.9 7 days 34.2 39.2 40.6 38.1 33.4 33.8 30.7 14 days 37.6 44.5 47.0 47.4 41.7 41.2 35.8 28 days 45.0 48.6 54.2 55.2 44.3 44.5 43.0
0 10 20 30 40 50 60
Compressive strength (MPa)
6.4.1. Results of mercury intrusion porosimetry for CaO-CaSO4-GGBFS composite cements with calcium carbonate powder
Pore size distributions and cumulative volumes of all samples at 28 days measured by MIP are presented in Figure 6- 5. As shown in Figure 6- 5, pore distribution of all samples at 28 days are briefly divided into two parts; (1) air bubbles entrapped in pastes which are larger than 100 μm, and (2) capillary pores which are smaller than 1 μm [75].
Figure 6- 5: Pore distribution and cumulative curve of CaO-CaSO4-GGBFS composite cements with different calcium carbonate powder content.
The pore size distribution is known as a better measure than other pore characteristics for evaluating the strength characteristics of hardened pastes (e.g., total pore volume or porosity). For instance, capillary pores larger than 50 nm could reduce the strength more, but pores smaller than 50 nm usually affect drying shrinkage and creep [75]. In addition, the air bubbles are relatively less detrimental to strength than capillary pores because air bubbles are generally spherical unlike capillary pores as already mentioned in chapter 5.
The total pore volume of each sample is decreased with increasing the content of calcium carbonate powder which means that the use of calcium carbonate powder into CaO-CaSO4-GGBFS composite cements could be an effective way to reduce pore volume of hardened pastes.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
0.001 0.01 0.1 1 10 100 1000
Log Differential Intrusion (mL/g)
Pore size diameter (μm)
0C_28 days 10C_28 days 20C_28 days 50C_28 days
0 0.05 0.1 0.15 0.2
0.001 0.1 10 1000
Cumulative Intrusion (mL/g)
Pore size diameter (μm) 0C_28 days 10C_28 days 20C_28 days 50C_28 days
(a)
(b)
Figure 6- 6: Pore size distribution of CaO-CaSO4-GGBFS composite cements (a) without calcium carbonate powder and (b) with 20 wt.% of calcium carbonate powder in terms of curing days.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0.001 0.01 0.1 1 10 100 1000
Log Differential Intrusion (mL/g)
Pore size diameter (μm) 0C_3 days 0C_7 days 0C_14 days 0C_28 days 0
0.05 0.1 0.15 0.2 0.25
0.001 0.1 10 1000
Cumulative Intrusion (mL/g)
Pore size diameter (μm) 0C_3 days 0C_7 days 0C_14 days 0C_28 days
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0.001 0.01 0.1 1 10 100 1000
Log Differential Intrusion (mL/g)
Pore size diameter (μm)
20C_3 days 20C_7 days 20C_14 days 20C_28 days 0
0.05 0.1 0.15 0.2 0.25
0.001 0.1 10 1000
Cumulative Intrusion (mL/g)
Pore size diameter (μm) 20C_3 days 20C_7 days 20C_14 days 20C_28 days
Pore size distribution of CaO-CaSO4-GGBFS composite cements with or without calcium carbonate powder (i.e. 0C and 20C samples) is represented in Figure 6- 6 with respect to curing days.
The modification of pore distribution with curing time is very similar regardless of the presence of calcium carbonate powder where relatively broad distributions over the range of 5 nm to 300 nm are identified at 3 days but the distributions are refined within the small range of 5 nm to 40 nm after 3 days.
The pore refinement effect of hardened pastes was more significant in the sample containing calcium carbonate powder (i.e. 20C) than the other (i.e. 0C). As shown in Figure 6- 6, total pore volume of 0C sample decreased with time but the amount is not significant while that of 20C sample is more clearly decreased with curing time [see Figure 6- 6(b)], which means that the presence of calcium carbonate could be effective to refine the pore volume of CaO-CaSO4-GGBFS composite cements with increasing curing time.
6.4.2. Relationship between pore structures and compressive strength of CaO-CaSO4-GGBFS composite cements depending on the content of calcium carbonate powder
The relative compressive strength and porosity depending on 0C sample are presented in Figure 6- 7 where values larger than 0 indicate the larger values than that of 0C sample and smaller values than 0 are the smaller than that of 0C sample. The trend lines in Figure 6- 7 were fitted with a second order polynomial function.
Figure 6- 7: Relative compressive strength and porosity at 28 days with respect to calcium carbonate powder content and trend lines of each value.
R² = 0.97
R² = 0.83 -30
-20 -10 0 10 20 30
0 10 20 30 40 50 60
Relative change of compressive strength and porosity at 28 days (%)
Calcium carbonate powder content (%) 28-day strength 28-day porosity
Increase
Decrease
The previous literature [159] reported that the strength development and porosity reduction was clearly inverse proportional but, in this study, the porosity was steadily reduced while the compressive strength decreased after 20 wt.% of calcium carbonate substitutions. The phenomena could be induced from that (1) particle size distributions of calcium carbonate powder or (2) large amount of residual calcium carbonate powder in hardened pastes. As presented in Figure 6- 1(b), the calcium carbonate powder used in this study had relatively small particle size distribution as well as quite small amount of small particles below 1 μm. The small sizes of particles and distribution could be effective to fill the pores contained in hardened matrix. The large amounts of residual calcium carbonate particles could reduce measurable hardened matrix because MIP could not identify the pore contained in calcium carbonate powder, which is similar to the case that residual GGBFS affected the pore volumes of CAS mentioned in chapter 5.
The carbon-sequestrated calcium carbonate used in this study is very useful to refine the pore structures of CaO-CaSO4-GGBFS composite cements and can improve the properties of the composite cements due to low permeability and high strength. Although the compressive strength of the composite cement decreased with above 20 wt.% of calcium carbonate powder, the strength was of a similar level of no calcium carbonate powder sample i.e. 0C up to 50 wt.% (see Figure 6- 4), which means that large amount of calcium carbonate powder could be used with CaO-CaSO4-GGBFS composite cements, indicating that significant amount of carbon dioxide could be captured in cementitious materials.
6.5. Hydration Products of CaO-CaSO4-GGBFS Composite Cements with Calcium Carbonate