Figure 1 shows the XRD patterns of cement raw meal calcined at 1300 ℃ for 2 h, with different reducing agents. The diffraction peaks of CaSO4 decreased with reductant increase. As the ratio of the added amount of carbon was 10 wt% (Fig. 1 (a)) and n(CaS)/n(CaSO4) was 3 (Fig. 1 (b)), the diffraction peaks of CaSO4 were not observed, respectively, which the main phases of samples were β-C2S and α'H-C2S. However, the sample contained a large amount of CaSO4 with a 6.7 wt% carbon. According to theoretical calculation, the amount of reduction potence provided by carbon was similar by calcium sulfide, which it was possible that calcium sulfide was more suitable as reductant than carbon.
In general, the decomposition temperature of gypsum can be effectively reduced and industrial production can be carried out in a more reasonable way. The CaS reductant was prepared at 900℃ for 1h by using activated carbon as a reductant. According to carbon amount of prepared CaS, the C/S ratio of cement prepared with CaS reductant was about 1.5. The C/S ratio of cement prepared with a 6.7 wt% carbon was about 1.6. The C/S ratio of cement prepared with a 10wt% carbon was about 3. More carbon was needed to prepare cement by directly using carbon as a reductant, probably because carbon easily reacted with air, which it increased carbon consumption.
The primary phase of belite cement is dicalcium silicate. The main chemical reactions of produced belite (2CaO·SiO2) can be simplified in the following equations:
2C + CaSO = CaS + 2CO(g) (1)
CaS + 3CaSO4 = 4CaO + 4SO2 (g) (2)
CaS + 3/2O2(g) = CaO + SO2(g) (3)
2CaO + SiO = 2CaO·SiO (4)
The specific surface areas of cement prepared by adding, 6.7 and 10 wt% carbon, and CaS (n(CaS)/n(CaSO4) = 3) were 0.580, 0.554, and 0.625 m2/g, respectively, which cement was obtained by grinding clinker at 720 rpm for 40 min using a vibration mill. The cement prepared with CaS had best grindability.
According to Chinese standard GB/T176-2017 (Methods for chemical analysis of cement), free lime was not detected in the samples, which the content of free lime was below the detectable level.
Figure 2 shows SO3 content and Loss of cement clinker obtained with different reductant. Sulfur trioxide (SO3) content of cement prepared by adding 6.7 and 10wt% carbon, and CaS (n(CaS)/n(CaSO4) = 3), were 26.28, 0.80, and 0.64 wt%, respectively. And their Losses were 0.68, 1.34 and 1.10 wt%, respectively. SO3 and Loss were all in accordance with Chinese standards GB/T175-2023 (Common portland cement, SO3 ≤ 3.5 wt% and Loss ≤ 3.0 wt% ) for cement obtained by calcination with 10 wt% and carbon CaS (n(CaS)/n(CaSO4) = 3).
Figure 3 shows the compressive strength of hydrated cement obtained with different reductant by curing for different age. The compressive strength increased with curing age increase. The strength increased rapidly before 28 d and the growth slowed down after 28 d. When a 10 wt% carbon was employed as the reducing agent, compressive strengths of the samples were 15.5, 15.6, and 15.6 MPa after carbonation curing 3, 7 and 28 d, respectively. When CaS was employed as the reducing agent, compressive strengths of the samples were 19.8, 27.2, and 34.1 MPa after carbonation curing 3, 7 and 28 d, respectively.
The strengths of samples cured by carbonized condition were much higher than samples cured by standard condition. The sample had high strength by using CaS as reductant, which strength were 2.4 and 34.1 MPa after curing 28 d without and with carbonation, respectively. And the strengths were 3.8 and 39.4 MPa after curing 90 d without and with carbonation, respectively. However, the samples had similar strength by adding 6.7 and 10 wt% carbon as reductant, which strength were 13.2 and 15.6 MPa after curing 28 d in carbonized condition, respectively.
Figure 4 shows the XRD patterns of hydrated cement for different curing ages. Belite (β-C2S and α’H-C2S) diffraction peaks were observed in all samples but the diffraction peaks intensity of carbonized samples (Fig. 5 (a2), (b2) and (c2)) were weaker than standard samples (Fig. 5 (a1), (b1) and (c1)), which CO2 can enhance the hydration activity of belite mineral.
The strong diffraction peaks of calcium carbonate were observed in carbonized samples and the peaks intensity increased with age increasing. On the other hand, the diffraction peaks of calcium carbonate were also observed in standard samples after curing 28 d, because of air containing CO2 in standard curing room. The diffraction peaks of ettringite (AFt) were observed in standard samples (Fig. 5 (a1), (b1) and (c1)) but it were not observed in carbonized samples (Fig. 5 (a2), (b2) and (c2)), probably because ettringite did not generate in carbonation environment.
The main chemical reactions of belite and CO2 can be simplified in the following Eq. (5), though the actual reaction was complexity [8]:
Ca2SiO4 + 2CO2 = 2CaCO3 + SiO2 (gel) (5)
The early strength of belite cement was enhanced by the calcium carbonate generated during carbonation curing and the C-S-H gel, thus improving its micro-mechanical properties stability in cement hydration environment [9]. Moreover, the diffraction peaks of SiO2 were observed in samples prepared with CaS reductant after curing 56 d (Fig. 5 (c2)). The strong diffraction peaks of gypsum (CaSO4·2H2O) were observed in samples prepared a 6.7wt% carbon reductant (Fig. 5 (a2)) because the residual anhydrite had some hydration activity in carbonation curing condition.
Figure 5 shows the microstructure of hydrated cement for 28 d under different curing conditions, respectively. The cement prepared with CaS reductant exhibited higher density because it had high hydration and carbonation activity (Fig. 5 (c1) and (c2)). Belite cement produced a little of needle-shaped ettringite particles during hydration process (Fig. 5 (a1), (b1) and (c1)). The ettringites overlap and were intimately associated with the surrounding calcium silicate hydrate gel, thereby enhancing the strength of the hydrated cement [10].
A lot of CaCO3 fibers were observed in carbonized curing samples (Fig. 5 (a2), (b2) and (c2)), which fibers and hydrated calcium silicate filled the loose porous structure and increased samples strength. Some short fibers were observed in the cement prepared by using a 6.7 wt% carbon as reductant (Fig. 5 (a2)), probably because residual CaSO4 hindered the fibers growth.
The strength of hydrated cement prepared with CaS reductant was the highest. The production of CaCO3 fibers and hydrated calcium silicate resulted in a filling of the loose porous structure within the overall system, leading to a more compact structure and an increase in the strength of the samples [11].
Figure 6 shows the carbon sequestration amount of cements prepared with different reductant after curing 28d. The carbon sequestration amount of cement prepared by adding 6.7 and 10wt% carbon, CaS (n(CaS)/n(CaSO4) = 3), were 7.5, 9.2, and 11.6 wt%, respectively. The cement prepared with CaS reductant had high hydration activity, as a result it had high carbonation activity.