In this paper, low-heat Portland cement (LHC) clinkers were prepared by calcining raw materials at 1350°C for 2.0 hours, 1400°C for 1.0 hour, 1400°C for 1.5 hours, 1400°C for 2.0 hours, 1450°C for 1.0 hour, and 1450°C for 2.0 hours. The clinkers were ground with gypsum to produce LHC. The particle size of periclase was analysed by BSEM. Expansion of LHC pastes due to hydration of periclase was measured. The hydration degree of periclase in LHC pastes was quantitatively determined by XRD internal standard method and BSEM. The results showed that the particle size of periclase was larger when clinkers were calcined at higher temperatures or for longer time. Smaller periclase (2.60
Low-heat Portland cement (LHC), namely, high belite cement [
Like conventional Portland cement, MgO as a minor component exists in LHC. MgO as an expansive additive is often used in dam construction to compensate for the slight natural shrinkage of PC during hydration, which can continue for months or years in service [
Chen et al. [
Periclase in Portland cement clinkers may cause unsound issues to concrete, and its content is often limited in many specifications of cements [
In order to achieve the compensation effect on the temperature shrinkage of hydraulic concrete by using the delayed expansion of periclase in the LHC cement, LHC clinker was prepared under different calcination conditions, and the influence of crystal size of periclase on the hydration and expansion of the self-prepared low-heat cement was studied in this paper.
The low-heat Portland cement (LHC) clinkers used were prepared in the laboratory by calcining raw materials of limestone, silica, dolomite, aluminum ore, and copper slag that came from Sichuan Jiahua Special Cement Company. The gypsum used was from Huaneng Power Plant in Nanjing. Table
Chemical compositions of raw materials (by weight, %).
Materials | LOI |
SiO2 | Al2O3 | Fe2O3 | CaO | MgO | K2O | Na2O | SO3 | Ratio |
---|---|---|---|---|---|---|---|---|---|---|
Limestone | 41.76 | 4.03 | 0.72 | 0.50 | 50.29 | 1.65 | 0.10 | 0.02 | 0.04 | 67.04 |
Silica | 1.66 | 87.16 | 2.41 | 5.02 | 1.59 | 1.52 | 0.12 | 0.09 | 0.07 | 10.23 |
Dolomite | 43.00 | 5.04 | 1.11 | 0.66 | 29.60 | 19.80 | 0.20 | 0.08 | 0.03 | 15.09 |
Aluminum ore | 14.72 | 30.48 | 28.36 | 21.64 | 1.86 | 1.46 | 0.73 | 0.24 | 0.00 | 5.77 |
Copper slag | 0.00 | 26.58 | 6.57 | 45.80 | 12.37 | 5.78 | 0.24 | 0.08 | 0.06 | 1.87 |
Gypsum | 6.36 | 2.88 | 0.32 | 0.25 | 33.14 | 0.33 | — | — | 42.54 | — |
X-ray diffraction (XRD) pattern of gypsum.
Theoretic mineral composition of LHC clinkers (%).
C3S | C2S | C3A | C4AF | KH | SM | IM | |
---|---|---|---|---|---|---|---|
Clinker | 34.99 | 40.01 | 3.01 | 15.00 | 0.80 | 2.64 | 0.87 |
The obtained LHC clinkers were ground into powders with less than 10% sieve residue. 95% clinker powders and 5% gypsum were put into the mixing bucket, and the mixture was blended with 12 hours to obtain LHC.
The cement pastes were prepared according to the JC/T 313-2009 (Chinese Standard). The fresh pastes were cast into a mould of 20 mm × 20 mm × 80 mm. The w/c ratio was set at 0.27. The hydration samples were made by casting the pastes into the mould of 20 mm × 20 mm × 20 mm while molding expansion specimens. The pastes with the mould were cured in a moist environment (98% relative humidity) at 20 ± 1°C for 24 ± 2 h, and then demoulded to measure the initial length
The XRD (Smart Lab, Rigaku, Tokyo, Japan) internal standard method [
The ovendry paste samples were ground into a powder through an 80
The median sizes of periclase in the LHC clinkers were got by the statistical methods based on the images of FE-SEM (Nava NanoSEM 450, FEI, Oregon State, USA). The FE-SEM was also used to analyse the hydration process of the periclase in cement paste. For BSEM image analysis, the dried slice samples of the cement clinker and the hydrated cement pastes were impregnated with epoxy resin in order to block in the hole. After the epoxy hardened, the samples were polished with 240, 400, 600, 2400, and 4000 grit abrasive paper and then polished using polishing liquid to the mirror surface by Automatic Grinder Polisher (EcoMet 250, Buehler, Illinois, USA).
Figure
BSEM images of LHC clinkers calcined at 1350°C for 2.0 hours (a), 1400°C for 1.0 hour (b), 1400°C for 1.5 hours (c), 1400°C for 2.0 hours (d), 1450°C for 1.0 hour (e), and 1450°C for 2.0 hours (f) (the deep color particles represent periclase).
The particle size and the content of periclase in LHC clinkers calcined at different conditions.
Calcination conditions | 1.0 hour | 2.0 hours | 1400°C | |||||
---|---|---|---|---|---|---|---|---|
1400°C | 1450°C | 1350°C | 1400°C | 1450°C | 1.0 hour | 1.5 hours | 2.0 hours | |
Particle size of periclase ( |
2.60 | 3.92 | 2.94 | 3.00 | 4.00 | 2.60 | 2.82 | 3.00 |
Content of periclase (wt./%) | 3.91 | 3.72 | 4.13 | 3.65 | 3.56 | 3.91 | 3.81 | 3.65 |
Figure
The expansion of LHC pastes prepared at different calcination conditions: (a) 20°C, (b) 30°C, (c) 40°C, and (d) 80°C.
Figure
The XRD patterns of cement pastes cured for 240 days at 20°C and 80°C, respectively. (a) The cement pastes with 2.60, 3.92, and 4.00
The XRD internal standard method was used to determine the content of periclase in cement pastes. The hydration degrees of periclase are shown in Table
Hydration degree of periclase in LHC cement pastes cured for 7, 28, 60, 90, 180, and 240 d (%).
Particle size of periclase ( |
Curing temperature, |
The hydration degree of periclase (%) | ||||||
---|---|---|---|---|---|---|---|---|
0 d | 7 d | 28 d | 60 d | 90 d | 180 d | 240 d | ||
2.60 | 20 | 0.00 | 0.27 | 3.77 | 4.85 | 16.71 | 29.38 | 30.73 |
3.92 | 20 | 0.00 | 6.80 | 11.05 | 11.33 | 15.86 | 19.55 | 24.36 |
4.00 | 20 | 0.00 | 6.69 | 11.63 | 12.79 | 16.28 | 25.29 | 28.20 |
4.00 | 40 | 0.00 | 8.43 | 25.87 | 36.63 | 39.24 | 45.93 | 45.93 |
4.00 | 80 | 0.00 | 32.56 | 48.55 | 61.92 | 63.66 | 66.57 | 67.15 |
Combining Figure
BSEM images of cement paste with 4.00
BSEM images of cement paste with 4.00
From Figure
There was obvious production of Mg(OH)2 around periclase in cement paste cured at 40°C for 240 days, which can be seen from Figure
According to the aforementioned phenomenon, it can be concluded that the periclase hydrated slowly at 20°C. The periclase will hydrate quickly to produce more brucite with the increasing of curing ages and temperature.
In order to study the effect of the particle size of periclase on the expansion and hydration of low-heat cement, the expansion of cement paste, hydration degree of periclase, and particle size of periclase were evaluated by testing the length of specimens, XRD internal standard method, and BSEM, respectively. The main conclusions in this paper can be drawn as follows: The particle size of periclase was bigger with the holding time increasing at the same calcination temperature. When the residence time was kept consistent, the higher the calcination temperature of the cement linker was, the greater the size of the periclase particle was. And the particle size of periclase in the LHC clinker calcined at 1450°C for 2 h was the largest (4.00 The cement paste has greater expansion than others when the particle size of the periclase in LHC clinker became smaller. The expansion of cement paste with the 4.00 The hydration speed of periclase in cement paste cured at 20°C for 240 days was higher with the smaller particle size of the periclase. And periclase needed much more time to hydrate when cement paste was cured at 20°C because of the slow hydration speed. The speed of periclase hydration was faster under 80°C than that at 20°C and 40°C, and the hydration degree of periclase with the 4.00 The periclase hydrated a lot in the 80°C water-curing condition for 240 days. And the small-size periclase had hydrated completely, and a large amount of dense brucite was generated around the large particles of the periclase when the curing temperature was 80°C.
All data generated or analysed during this study are included in this published article. And readers can access all data used to support conclusions of the current study from the corresponding author upon request.
The authors declare no conflicts of interest.
Man Yan and Chen Wang designed and conducted the experimental program. Min Deng provided and designed the project. Zhiyang Chen helped to do experiment and gave many writing suggestions. All authors contributed to the analysis and conclusion.
This work was supported by the State Key Laboratory of Materials-Oriented Chemical Engineering of Nanjing Tech University and granted financial resource from the National Key Research and Development Plan of China (No. 2016YFB0303601).