The role of various powders including glass powder (GP), limestone powder (LP), and steel slag powder (SSP) during the hydration process of cement-based materials was investigated by using X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), and strength tests. GP has adverse impact on early strength, but the pozzolanic reaction at later stage enhances the strength development greatly. LP can significantly improve early strength. SSP has a good contribution to the early and later strength of the paste when its content is less than 15%. GP has little effect on the kind of hydration products but relatively large effects on the quantity. Calcium hydroxide (CH) content of GP paste decreases over curing age gradually, which is different from pure cement paste because its pozzolanic activity consumes more CH than that generated from the cement hydration. SSP and LP mainly play a role of filling effect at early stage. Nucleating effect of LP also promotes the early hydration of cement. The hydration of LP occurs at later stage and forms the calcium carboaluminate hydrates. The hydration of SSP is relatively slow, which generates CH at later stage and is effective in the strength development.
Mineral admixture has become an indispensable component in concrete because it generally has some pozzolanic reactivity during the hydration process. Mineral admixture can replace cement and reduce cost of concrete; moreover, it can improve the workability of fresh concrete or some properties of hardened concrete. Therefore, the use of mineral admixture in concrete brings significant benefits.
Glass powder (GP) is ground waste glass which mainly comes from industrial waste glass (such as flat glass and glass fiber) and household waste glass (such as glass containers and light bulbs) [
The main component of limestone powder (LP) is calcite (CaCO3), which is a cheap and easily available material. At present, LP is mainly used to replace partial fine aggregate or as mineral admixture in the concrete. As a mineral admixture, LP is observed to have hydration reactivity when it is ground to a certain degree of fineness and it needs to have a certain environment and enough hydration time [
Steel slag powder (SSP) is the slag exhausted from converter, electric furnace, and refining furnace, whose main components are silicate and ferrite. In developed countries, the majority of SSP is used as aggregate in concrete [
Recently, little existing literatures focused on investigation of the hydration properties and process of the three various powders. As mineral admixtures, the hydration process and strength development of cement-based materials containing the three powders are relatively different. In this paper, GP, LP, and SSP will be used as mineral admixtures to study the role during the hydration process of cement-based materials.
The ordinary Portland cement PO 42.5 (POC) complied with the Chinese standard GB175-2007 is used in this test. The chemical compositions of GP, LP, SSP, and POC determined by X-ray fluorescence (XRF) are listed in Table
Chemical compositions of raw materials/mass, %.
Compositions | SiO2 | Al2O3 | CaO | Fe2O3 | MgO | SO3 | K2O | Na2O | P2O5 | loss |
---|---|---|---|---|---|---|---|---|---|---|
Cement | 21.25 | 2.91 | 63.09 | 3.24 | 0.68 | 3.36 | 1.12 | 0.31 | 0.17 | 3.52 |
GP | 55.75 | 10.64 | 6.60 | 0.28 | 1.01 | 0.27 | 0.54 | 9.92 | 0.03 | 11.9 |
LP | 1.79 | 0.56 | 54.69 | 0.35 | 0.40 | 0.03 | — | 0.06 | — | 41.93 |
SSP | 14.11 | 3.51 | 42.39 | 17.45 | 6.60 | 0.50 | 0.13 | 0.22 | 1.63 | 8.72 |
40 mm
Mix proportions of the cement pastes/g.
Samples | Cement | GP | LP | SSP | Water |
---|---|---|---|---|---|
PC | 600 | 0 | 0 | 0 | 240 |
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GP-15 | 510 | 90 | 0 | 0 | 240 |
GP-30 | 420 | 180 | 0 | 0 | 240 |
GP-45 | 330 | 270 | 0 | 0 | 240 |
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LP-15 | 510 | 0 | 90 | 0 | 240 |
LP-30 | 420 | 0 | 180 | 0 | 240 |
LP-45 | 330 | 0 | 270 | 0 | 240 |
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SSP-15 | 510 | 0 | 0 | 90 | 240 |
SSP-30 | 420 | 0 | 0 | 180 | 240 |
SSP-45 | 330 | 0 | 0 | 270 | 240 |
Figures
Effect of GP content on the compressive strength of the paste.
Effect of LP content on the compressive strength of the paste.
Effect of SSP content on the compressive strength of the paste.
Different from GP group, the strength of LP group at 3 and 7 days first increases and then decreases along with the increase of LP content. When the content is 15%, the paste strength increases obviously.
When the content increases to 30%, the paste strength decreases and is lower than that of PC sample. The strength at 28 and 90 days is all less than that of PC sample. Moreover, the more the LP content is, the more the strength decreases. The reason is that a proper content of LP provides the nucleation site [
The strength of SSP group increases along with the curing age. 15% SSP content has little reduction effect on the strength and the strength at 7 days even is higher than that of PC sample. When the content is 30% or higher than 30%, the strength decreases rapidly, which suggests that large amount of SSP is disadvantageous to the strength.
In order to analyze the activity of the three powders, the activity index [
Although the cement content of the paste containing various powders decreases correspondingly compared to PC group,
Thus a relative index is obtained, which is the contribution rate of hydration activity (
The compressive strength contribution rate of the mineral mixtures’ hydration activity.
Samples | 3 days | 7 days | 28 days | 90 days |
---|---|---|---|---|
GP-15 | −0.25 | −0.38 | −0.26 | −0.10 |
GP-30 | −0.31 | −0.30 | −0.36 | 0.04 |
GP-45 | −0.12 | −0.09 | −0.15 | 0.06 |
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LP-15 | 0.26 | 0.25 | −0.06 | 0.01 |
LP-30 | 0.13 | 0.14 | 0.04 | 0.11 |
LP-45 | 0.08 | −0.13 | −0.20 | 0.12 |
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SSP-15 | 0.14 | 0.25 | 0.04 | 0.14 |
SSP-30 | 0.10 | 0.14 | −0.08 | 0.16 |
SSP-45 | −0.10 | −0.01 | −0.19 | 0.07 |
The contribution rate of GP increases along with the curing time. The value of contribution rate developing from early negative to the late positive reveals that the pozzolanic reaction degree of GP at the later stage is significantly higher than that at the early stage. LP’s early contribution rate is high and decreases along with the curing time in general, which indicates that the early filling effect and accelerating effect obviously improve the strength of the paste, but the later hydration activity is low. SSP has better early filling effect and the later hydration activity works a long time; thus the SSP group has higher early and later contribution rate than GP group.
Figures
XRD patterns of PC paste at different curing ages.
XRD patterns of GP-30 cement paste at different curing ages.
XRD patterns of LP-30 cement paste at different curing ages.
XRD pattern of SSP.
Compared with GP group, Figure
Figure
XRD patterns of SSP-30 cement paste at different curing ages.
The TG-DTA curves of the pastes are shown in Figures
TG-DTA curves of PC paste at different curing ages.
TG-DTA curves of GP-30 cement paste at different curing ages.
TG-DTA curves of LP-30 cement paste at different curing ages.
TG-DTA curves of SSP-30 cement paste at different curing ages.
Based on the endothermic peak and the weight loss rate of TG-DTA curves, the CH content can be calculated quantitatively. The dehydrated decomposition of CH will occur at 400 to 500°C which will result in a loss in quality and the content of CH can be calculated as formula (
CH content of the pastes at different curing ages.
Figure
SEM images of the pastes at 28 and 90 days are shown in Figures
SEM images of PC samples at curing age of (a) 28 days and (b) 90 days.
SEM images of GP-30 cement samples at curing age of (a) 28 days and (b) 90 days.
SEM images of LP-30 cement samples at curing age of (a) 28 days and (b) 90 days.
SEM images of SSP-30 cement samples at curing age of (a) 28 days and (b) 90 days.
Similar to the PC sample, a large number of fibrous C-S-H gel can also be observed in Figure
Figure
Figure
(1) GP has negative effect on the early strength but a high strength development due to the pozzolanic reaction at later stage. LP can obviously improve the early strength, especially at 3 and 7 days; it mainly plays a role of filling effect and nucleating effect at the early stage and reactive effect at later stage. SSP has a high contribution to the early and later strength of the paste when its content is under 15%. The early filling effect and the later hydration of SSP improve the strength contribution rate over a long time.
(2) GP has little effect on the kind of hydration products, but relatively large effect on the quantity. CH content of PC sample increases gradually along with the curing age, while the GP group is contrary. The CH content produced by cement hydration gradually increases, which stimulates the pozzolanic activity of GP; GP consumes CH more than that cement hydration generates. And CH content of LP and SSP samples are similar to PC group which increases along with curing age. The LP hydration product calcium carboaluminate hydrates can be detected by XRD and also be confirmed by SEM. The active substances C2S of SSP generate CH at the later stage.
(3) GP, LP, and SSP mainly play the role of filling effects at the early stage, and the nucleating effect of LP promotes the early hydration of cement. The surface of GP, LP, and SSP has been eroded at 90 days indicating that they take part in hydration reaction at later stage, which is also proven by the strength tests.
The authors declare that they have no conflicts of interest.
This project is funded by the National Key R&D Program of China (2016YFC0401907) and Opening Funds of Guangxi Key Laboratory of New Energy and Building Energy Saving (15-J-22-4-001).