Effect of Unhydrated Cement Particles on Calcination Process of Recycled Cement Clinker

Hongmei Ai[1]*, Sufeng Zhu2, Xiaoqing Liu3, Junying Bai4

1~4. School of Civil Engineering, Dalian University of Technology, Dalian 116024, China

Abstract

Utilizing all-components waste concrete without separation treatment as alternative raw materials to produce recycled cement is a significant method to recycle waste concrete in original resource level. Hardened cement paste (HCP) from waste concrete is the characteristic component of recycled cement raw materials. Hydration products and unhydrated cement particles (UCP) from HCP will carry out a series of physical and chemical changes in calcination process, which impacts the clinker calcination process and clinker mineral properties. This paper mainly studies the effect of UCP on calcination process of recycled cement clinker. The HCP with different hydration degrees were synthesized by using Portland cement P·Ⅱ52.5 to replace 0~30% of full-hydration hardened cement paste (99.3% hydration degree). Artificial waste concrete, prepared by mixing hardened cement paste, limestone and silica fume (substitute of sand) in the mixing proportion of four components concrete (C40), was used to synthesize recycled cement raw meal with UCP in different proportions. Burnability test of raw meal and petrographic analysis of clinker were the main test methods. On this basis, the study on effect of the added CaF2 dosage on burnability and microstructure of clinker with high-content UCP in its raw meal was also carried out. The results showed: recycled cement raw meal with the proportion of UCP over 5% presented poor burnability; when UCP was over 15%, the content of C2S in clinker presented an increasing tendency, and clusters of Blite, affecting the early strength of recycled cement paste, appeared in different degrees; CaF2 contributed to the improvement of burnability of recycled cement raw meal with UCP in high content, and the optimum dosage is 0.5%.

Originality

Due to the wide source of waste concrete and its different strength grades and work time, UCP from HCP in waste concrete covers a wide range of content. In high performance concrete, proportion of UCP is over 30% of total cement dosage; because of short hydration time, proportion of UCP can reach 30%~40% in waste concrete from commercial concrete mixing plants. Therefore, it is necessary to study the effect of the UCP content in HCP on calcination process of recycled cement clinker with waste concrete as its alternative raw meal. It has been reported that UCP can play a role of seed crystal in calcination process of recycled cement clinker. However, the system researches and clear conclusions on the effect mechanism of hardened cement paste in calcination process have not been formed. In this paper, artificial waste concrete were used to adjust the UCP contents in raw meal and the effect of UCP on the calcination process were studied by comparing f-CaO contents and petrographic photos of clinker produced by different raw meals. This paper contributes to explaining the calcination mechanism of recycled cement clinker produced by waste concrete.

Keywords: unhydrated cement particles; waste concrete; recycled cement; calcination


1. Introduction

Recycled cement and recycled aggregate, produced by utilizing waste concrete as raw materials, belong to the resource recovery of waste at original level. Application of these two recycled products in concrete production is beneficial for realizing the complete cyclic utilization of concrete materials (Wang Z. M., et al., 2006). Recycled aggregate has captured more attention of researchers during the last decade. However, recycled aggregate is significantly affected by mechanical strength of waste concrete and separation technology in pretreatment process, and its property is unstable and highly discrete (Cui Z. L., et al., 2012). In addition, reutilization of coarse aggregate in waste concrete is dominant method in recycled aggregate research, while the separated waste mortar is inefficiently utilized (Lu P. G., et al., 2014).

The waste concrete with limestone as coarse aggregate can provide coarse aggregate and sand as substitutes of calcareous and silicious cement raw materials, respectively. The dehydrated phase in hardened cement paste possesses the similar oxide compositions to cement raw meal. Hence, all components of waste concrete can be utilized as raw materials in the calcination process of clinker to produce recycled cement (Ai H. M., et al., 2012).

Hardened cement paste (HCP) is the characteristic component of recycled cement raw meal. It has been reported that the dehydrated phases from hydration products in HCP are beneficial for decreasing the formation temperature of clinker minerals and accelerating the clinkering reaction in the calcination process. Meanwhile, unhydrated cement particles (UCP) can act as a role of seed crystal (Meng S. S., et al., 2006; Shan J. M., et al., 2006; Zheng J. S., et al., 2012).

Due to the wide source of waste concrete and its various strength grades and work time, UCP in HCP covers a wide range of content. In high performance concrete, proportion of UCP is over 30% of total cement dosage. Moreover, for the reason of short hydration time, proportion of UCP can reaches 30%~40% in waste concrete from commercial concrete mixing plants. (Yang L., et al., 2004). Hence, this paper attempts to explore the effect of the HCP with UCP in different proportions (different hydration degree) on calcination process of recycled cement clinker with waste concrete as its alternative raw material. On this basis, the study on effect of the added CaF2 dosage on burnability and microstructure of clinker with high-content UCP in its raw meal is also carried out.

2. Experimental

2.1. Raw Materials

HCP was artificially prepared from cement paste specimen with water/powder ratio 0.4. Cement is P.Ⅱ52.5 cement from Dalian Onoda Cement Co., Ltd (DOCC) in Liaoning province, China. Paste specimens were prisms of 40×40×160 mm and firstly cured in standard environment with RH 95%±5% and 20℃±2℃ for 1day. After demolding, the specimens were crushed into small blocks and cured in a consistent temperature and moist heat condition for 1 year, which were kept in a covered container filled with water and placed in the oven at 60℃. Hydration degree of artificial HCP was measured at 99.3% on the basis of the quantity of chemically bound water, which is deemed as complete hydration. The HCP with different hydration degree were prepared by blending fully-hydrated HCP with P·Ⅱ52.5 cement in different proportion (0%, 5%, 10%, 15%, 20%, 25% and 30%), marked N1~N7.

HCP, limestone and silica fume were ground by a ball mill to a fineness of 5% residue on a 75μm sieve, respectively. Silica fume was chosen as the substitute of sand to exclude the complicated influence of high crystallinity SiO2 on the calcination of recycled cement clinker. These materials were blended to prepare the artificial waste concrete according to the mix ratio shown in table1, which was modified from concrete mix ratio of four-components C40 concrete on the basis of unaltered SiO2 content in silica fume and sand. Limestone and silica fume are sourced from cement industrial materials from DOCC.

Correction materials Al2O3 and Fe2O3 and mineralizer CaF2 are analytical reagent.The chemical compositions of raw materials by XRF are shown in table 2.

Tab. 1 Concrete mix proportion /%

C40 concrete / Compositions / Cement / Limestone / Sand
Content / 16.8 / 46.4 / 28.4
Artifical waste concrete / Compositions / HCP / Limestone / Silica Fume
Content / 21.6 / 48.4 / 30

Tab. 2 Chemical compositions of raw materials /%

Compositions / SiO2 / CaO / Al2O3 / Fe2O3 / MgO / SO3 / K2O / Na2O / Loss
HCP / 15.18 / 57.84 / 2.46 / 3.71 / -- / 1.81 / 0.25 / -- / 16.83
Waste Concrete / 32.51 / 39.21 / 0.71 / 0.95 / 0.10 / 0.49 / -- / -- / 25.06
Limestone / 0.95 / 55.05 / 0.31 / 0.29 / -- / 0.09 / 0.12 / -- / 42.99
Silica Fume / 95.95 / 0.24 / 0.09 / 0.04 / 0.34 / 0.17 / 0.96 / 0.08 / 2.03

2.2. Experimental Process

The raw meal modulus of recycled cement are LFS=0.9 ± 0.02, SM=2.5 ± 0.02, IM=1.6 ± 0.02. Raw materials were ground into a fineness of 5% residue on a 75μm sieve, respectively, and blended uniformly according to the mix proportion shown in table 3. Raw meal was formed into a cylindrical specimen of Φ50×8 mm under a pressure of 50kN. After drying in an oven at 105℃ for 24h, the specimens were calcinated in an electric furnace at 950℃for 30min firstly, and afterwards were removed out quickly and kept in another furnace at 1400℃ for 30min. Finally, the calcined specimens were removed from the furnace and cooled down rapidly to room temperature by mechanical fan for the next test.

Tab. 3 Mix proportion of recycled cement raw materials /%

Compositions / Waste concrete / Limestone / Al2O3 / Fe2O3
Content / 44.95 / 50.05 / 3.24 / 1.75

The raw meal specimens with 30% UCP in HCP (N7) was added with 0.5%, 1.0% and 1.5% mineralizer CaF2 and marked M1~M3, respectively. The specimens were calcinated in the same calcination process at the identical modulus. The calcined specimens were ground into a fineness below 75μm. The powders were used in ethylene glycol method to determine the content of free lime. The blocky calcined specimens, after surface polishing and eroding with 1% alcohol nitric acid solution for 20~40s, were used to petrographically analysed on a XJZ-6A metalloscope (originalmagnification×640).

3. Results and Discussion

3.1. Burnability

The f-CaO content of recycled cement clinker with different content of UCP in raw meal is illustrated in figure1. f-CaO content in clinker increases with UCP content rising. As UCP content exceeds 5%, f-CaO contents are all beyond the standard value of 1.5%, which is indicative of poor burnability of raw meal.

UCP, acting as seed crystal in the calcination process of clinker, is beneficial for accelerating solid phase reaction, promoting the C3S nucleation rate under lowsupersaturationcondition and improving clinker mineral crystal growth. Furthermore, UCP is the crucial reason for the lower calcination temperature of recycled cement than that of traditional Portland cement (Ai H. M., et al., 2011). However, excessive UCP leads to the increasing energy consumption of clinker calcination and the insufficient clinkering reaction in identical heating and heat preservation condition. Meanwhile, the low content of C3S in clinker negatively impacts the early strength of recycled cement.

Fig.1 Free lime content in recylced cement clinker
with different content of UCP in raw meal

HCP and the high crystallinity SiO2 sourced from sand in unseparated waste concrete are two distinctive components of recycled cement raw meal from the traditional raw meal. It has been reported, in case of the proportion of waste concrete increasing, the firm and steady Si-O structure and difficultly-decomposed silicon-oxygentetrahedron in sand lead to raw meal burnability deteriorating, clinker quality declining and energy consumption growing (Meng S. S., et al., 2006; Zheng F. Y., 2007). Mineralizer are utilized by cement production enterprises to promote depolymerizing activity of SiO2.

CaF2, a widely-used mineralizer, can provide F- to break crystallattice of raw materials, promote reaction activity of raw meal, accelerate calciumcarbonate decomposition and significantly reduce the liquid phase appearance temperature. In addition, CaF2 is beneficial for decreasing the liquidviscosity, promoting moleculardiffusion in liquid phase and accelerating the formation of alite.

In this paper, CaF is added into the raw meal with high content of UCP to improve the poor burnablity. As shown in figure 2, CaF2 significantly improve the burnablity. The f-CaO content of clinker remarkably declines with CaF2 dosage increasing. The raw meal, added 0.5% CaF2, reaches the requirement of burnability with f-CaO content at 0.75%.

Fig.2 Free lime content in the recylced cement clinker with different
content of CaF2 and high content of UCP in raw meal

3.2 Petrographic analysis

Petrographic microstructure of the recylced cement clinker with different content of UCP in raw meal are exhibited in figure 3. The increasing content of UCP impacts little on the formation of alite, which presents well-crystalline microstructure with clear and sharp crystal boundaries and fine and symmetrical crystal grains. In the pictures of specimens with UCP content beyond 15%, more detectable blite appears and accumulates, which is apotentialnegativefactor on early strength of recycled cement.

N2 (5%) / N3 (10%)
N4 (15%) / N5 (20%)
N6 (25%) / N7 (30%)
Fig.3 Petrographic microstructure of the clinker with different UCP content in raw meal

The effect of CaF2 on petrographic microstructure of recycled cement clinker with high content of UCP in raw meal, is shown in figure 4. As the dosage of CaF2 growing, coarse alite crystal grain and increasing liquid phase are detected. In the petrographic pirture of M1, the dosage of CaF2 at 0.5%, alite is well crystalline with an appropriate length diameter ratio. Meanwhile, the liquid phase amount is close to the blank sample N7. And in the specimen with 1% additive CaF2, larger crystal size of alite and obvious solid solution phonemenon are found. When CaF2 dosage reaches 1.5%, substantial liquid phase and bulky alite crystal grain appear in the microstructure. Petrographic analysis suggests that the optimum dosage of CaF2 for clinker mineral growth is 0.5%

N7 (0%) / M1 (0.5%)
M2 (1.0%) / M3 (1.5%)
Fig.4 Petrographic microstructure of the recylced cement clinker with different content of CaF2
and high content of UCP in raw meal

4. Conclusions