A comparison was made between the impact of raising the thermostatic temperature and the impact of prolonging the thermostatic time on the performance of steam-cured concrete containing a large portion of fly ash (FA) or ground granulated blast furnace slag (GGBS) by analysing the form removal strength, chemically combined water content, reaction degree, strength development, chloride permeability, and volume stability. For the materials and test conditions reported in this study, raising the thermostatic temperature is more favourable for concrete containing FA, as indicated by the significantly higher form removal strength and the higher growth of reaction degree of FA compared with prolonging the thermostatic time. With an increase in the thermostatic temperature, the hydration degree of a binder containing FA or GGBS initially increases and subsequently decreases. Although concrete containing FA can obtain satisfactory form removal strength with steam curing at 80°C, the late strength development of concrete containing FA is slow for the same curing conditions. The effect of the late performance of resistance to chloride ion permeability improved by FA is better than the effect improved by GGBS. The risk of destroying the structure of concrete containing a large portion of FA or GGBS due to delayed ettringite formation (DEF) is minimal when specimens were steam-cured at 80°C.
Concrete is one of the most common construction materials. Cast-in-situ concrete and precast concrete are two techniques that housing developers and construction workers often adopt. However, precast concrete members have been increasingly utilized in civil engineering construction in recent years due to their advantages: reliable quality assurance, simple production process, faster construction speed, and environmentally friendly building operations [
Currently available information indicates that the technique of steam curing is the most frequently employed technique among various production processes of prefabricated members [
Mineral admixtures are extensively applied in blended cement and concrete; this process is a substantial contribution to the field of civil engineering. The technology of steam curing has been primarily employed for pure cement concrete rather than concrete with a large portion of mineral admixtures as many researchers and housing developers have expressed their concern that the early compressive strength of concrete with a large portion of mineral admixtures is low [
Although the early strength of concrete with a large portion of mineral admixtures is low at room temperature, high temperatures can promote the early hydration of a binder. By adjusting the thermostatic time and the thermostatic temperature under steam curing conditions, concrete with a large portion of mineral admixtures may achieve the required form removal strength. To address the problem of form removal strength and promote the high performance of steam-cured concrete that incorporates mineral admixtures, this paper addresses the influence of prolonging the thermostatic time and improving the thermostatic temperature on the form removal strength of concrete that incorporates a large portion of mineral admixtures. Our study also focused on a comparison between the impact of prolonging the thermostatic time and the impact of raising the thermostatic temperature on the hydration degree of a binder, the strength development and the resistance to the chloride ion permeability of concrete, and the volume stability of steam-cured concrete with a large portion of mineral admixtures.
P.O 42.5 ordinary Portland cement (OPC), ground granulated blast furnace slag (GGBS), and fly ash (FA) were employed in this study. The chemical compositions and specific surface areas of these powder materials are shown in Table
Chemical compositions and specific surface areas of OPC, GGBS, and FA.
OPC | GGBS | FA | |
---|---|---|---|
SiO2 (%) | 21.10 | 31.76 | 53.33 |
Al2O3 (%) | 6.33 | 14.84 | 27.65 |
Fe2O3 (%) | 4.22 | 0.60 | 6.04 |
CaO (%) | 54.86 | 36.44 | 2.86 |
MgO (%) | 2.60 | 9.08 | 1.35 |
SO3 (%) | 2.66 | 1.94 | 0.45 |
|
0.53 | 0.56 | 0.64 |
Loss on ignition (%) | 2.42 | 0.86 | 4.71 |
Specific surface area (m2/kg) | 376 | 430 | 358 |
Table
Compositions of the pastes (%).
Sample | Binder | Water/binder ratio | ||
---|---|---|---|---|
OPC | GGBS | FA | ||
CC | 100 | 0 | 0 | 0.4 |
FF | 60 | 0 | 40 | 0.4 |
BB | 60 | 40 | 0 | 0.4 |
Mix proportions of the concrete (kg/m3).
Sample | OPC | GGBS | FA | Fine aggregates | Coarse aggregates | Water |
---|---|---|---|---|---|---|
C | 350 | 0 | 0 | 812 | 1077 | 161 |
F30 | 245 | 0 | 105 | 812 | 1077 | 161 |
F40 | 210 | 0 | 140 | 812 | 1077 | 161 |
F50 | 175 | 0 | 175 | 812 | 1077 | 161 |
B30 | 245 | 105 | 0 | 812 | 1077 | 161 |
B40 | 210 | 140 | 0 | 812 | 1077 | 161 |
B50 | 175 | 175 | 0 | 812 | 1077 | 161 |
B60 | 140 | 210 | 0 | 812 | 1077 | 161 |
The precuring time for steam curing was three hours (20°C). The heating and cooling rate was
The chemically combined water (
This study involves an experiment on the volume stability of concrete. Concrete that was used to measure volume stability was cured in a saturated Ca(OH)2 solution at 20°C after steam curing. As water is a necessity to DEF, the specimens after steam curing were placed in a saturated Ca(OH)2 solution to keep concrete completely wet during curing. This type of practice can prevent Ca(OH)2 dissolution and drying shrinkage. The consequence of the volume stability analysis was confirmed by measuring the lengths of the concrete specimens using a comparator at scheduled ages. Test probes were installed in advance on both ends of the concrete specimens. Concrete that was used to measure volume stability and the process of measuring volume stability are shown in Figure
The measurement of volume stability of concrete.
The influence of thermostatic time and thermostatic temperature on the form removal strength of steam-cured concrete is presented in Table
Removal strengths under different curing conditions.
Samples | Thermostatic temperature/°C | Thermostatic time/h | Removal strength/MPa |
---|---|---|---|
C | 60 | 9 | 28.7 |
F30 | 60 | 11 | 23.7 |
80 | 9 | 35.7 | |
F40 | 60 | 11 | 17.7 |
80 | 9 | 29.7 | |
F50 | 60 | 13 | 10.4 |
80 | 11 | 26.8 | |
B30 | 60 | 11 | 27.8 |
80 | 9 | 31.0 | |
B40 | 60 | 11 | 32.7 |
80 | 9 | 27.2 | |
B50 | 60 | 11 | 20.0 |
80 | 9 | 22.6 | |
B60 | 60 | 11 | 19.4 |
80 | 10 | 27.8 |
By utilizing the form removal strength of pure cement concrete under steam curing at 60°C for 9 h as the reference, the form removal strength of concrete B30 and B40 cured at 80°C for 9 h is similar to the form removal strength of the control group. The form removal strength of concrete B50 that was cured at 80°C for 9 h is obviously lower than the form removal strength of the control group. However, by prolonging the thermostatic time to 10 h, the form removal strength of concrete B60 that was cured at 80°C is similar to the form removal strength of the control group. By prolonging the thermostatic time to 11 h and controlling the thermostatic temperature (60°C) as a constant, the form removal strengths of concrete B30 and B40 are similar to the form removal strength of the control group and the form removal strengths of concrete B50 and B60 are lower than the form removal strength of the control group. Both raising the thermostatic temperature and prolonging the thermostatic time enhance the form removal strength of concrete that incorporates a large portion of GGBS. When the content of GGBS in cementing materials exceeds 50%, methods of prolonging the thermostatic time and raising the thermostatic temperature need to be simultaneously employed to obtain the satisfactory form removal strength as the influence of prolonging the thermostatic time on enhancing the form removal strength is limited.
The influence of thermostatic time on
Sample CC
Sample BB
Sample FF
Figure
Conclusions can be drawn from Figures
The influence of the thermostatic temperature on
The influence of thermostatic temperature on
Sample CC
Sample BB
Sample FF
Figure
Conclusions can be drawn from Figures
This result indicates that increasing the thermostatic temperature to 80°C can effectively enhance the hydration degree of paste containing a large portion of FA or GGBS. Compared with Figure
The influence of the thermostatic time on the reaction degree of GGBS and FA is presented in Table
Reaction degree of fly ash and GGBS/%.
Thermostatic temperature/°C | Thermostatic time/h | |||
---|---|---|---|---|
8 | 12 | 16 | ||
Fly ash | 60 | 2.11 | 4.26 | 6.39 |
80 | 5.79 | 7.96 | 9.73 | |
GGBS | 60 | 7.86 | 10.15 | 13.18 |
80 | 9.84 | 13.15 | 15.25 |
Table
The following can also be concluded from Table
From the perspective of the reaction degree of mineral admixtures as well as the hydration degree of the whole binder, it is obvious that the promoting effect of increasing the thermostatic temperature is more significant than the promoting effect of prolonging the thermostatic time on the early hydration of the binder containing a large portion of FA. Increasing the thermostatic temperature and prolonging the thermostatic time have a significant role in promoting the hydration of GGBS. This case also applies to the influence of the thermostatic temperature and thermostatic time on the form removal strength of concrete.
The comparison between the strength development of pure cement concrete after steam curing and the strength development of steam-cured concrete containing a large portion of GGBS and FA is shown in Figure
Comparison of strength development between pure cement concrete and concrete containing large portion FA or GGBS under steam curing.
Concrete containing FA at 80°C
Concrete containing FA at 60°C
Concrete containing GGBS at 80°C
Concrete containing GGBS at 60°C
Figure
Figures
The comparison between chloride ion permeability of concrete containing a large portion of FA or GGBS and the chloride ion permeability of pure cement concrete after steam curing is illustrated in Table
Chloride ion permeability of concrete.
Samples | Thermostatic temperature/°C | Thermostatic time/h | 28 d | 90 d | ||
---|---|---|---|---|---|---|
Charge passed/C | Permeability level | Charge passed/C | Permeability level | |||
C | 60 | 9 | 7111 | High | 4338 | High |
F30 | 60 | 11 | 1714 | Low | 700 | Very low |
80 | 9 | 644 | Very low | 503 | Very low | |
F40 | 60 | 11 | 1707 | Low | 510 | Very low |
80 | 9 | 682 | Very low | 278 | Very low | |
F50 | 60 | 13 | 1952 | Low | 607 | Very low |
80 | 11 | 518 | Very low | 200 | Very low | |
B30 | 60 | 11 | 2094 | Moderate | 1350 | Low |
80 | 9 | 2628 | Moderate | 1709 | Low | |
B40 | 60 | 11 | 2424 | Moderate | 1535 | Low |
80 | 9 | 2765 | Moderate | 1650 | Low | |
B50 | 60 | 11 | 1668 | Low | 1117 | Low |
80 | 9 | 2075 | Moderate | 1213 | Low | |
B60 | 60 | 11 | 1515 | Low | 813 | Very low |
80 | 10 | 1150 | Low | 775 | Very low |
At the age of 28 days, the permeability of steam-cured concrete containing GGBS falls in the “Moderate” or “Low” levels. At the age of 90 days, the permeability of steam-cured concrete containing GGBS falls in the “Low” or “Very Low” levels. The greater is the mixing amount of GGBS, the better is the resistance to the chloride ion permeability of concrete. Compared with pure cement concrete, the concrete containing GGBS can achieve better resistance to chloride ion permeability. In addition, the chloride ion permeability of steam-cured concrete containing GGBS of each group is not substantially different due to the difference in the curing systems, as both prolonging the thermostatic time and improving the thermostatic temperature can stimulate the reaction activity of GGBS and substantially improve the pore structure of concrete. The effect of the late performance of resistance to chloride ion permeability improved by FA is better than the effect by GGBS. Although the reaction degree of GGBS is higher than the reaction degree of FA after steam curing, the pozzolanic reaction of FA can consume a mass of Ca(OH)2. The amount of Ca(OH)2 consumed by GGBS is minimal. Therefore, the reaction of FA plays a significant role in improving the pore structure of concrete.
The comparison between the volume deformation of steam-cured concrete containing a large portion of FA or GGBS and the volume deformation of pure cement concrete after steam curing is presented in Figure
Volume deformation of steam-cured concrete.
Cured at 90°C
Concrete containing FA at 80°C
Concrete containing GGBS at 80°C
The volume deformation of steam-cured concrete containing a large portion of FA and GGBS with steam curing at 80°C is illustrated in Figures
Improving the thermostatic temperature is more favourable for concrete containing FA, as indicated by the significantly higher form removal strength and higher growth of reaction degree of FA compared with the method of prolonging the thermostatic time. Both improving the thermostatic temperature and prolonging the thermostatic time contribute to a distinct enhancement of the form removal strength of concrete that incorporates a large portion of GGBS and the reaction degree of GGBS. With an increase in the thermostatic temperature from 60°C to 90°C, the hydration degree of binder containing FA or GGBS initially increases and subsequently decreases. Concrete containing FA can obtain satisfactory form removal strength with steam curing at 80°C; however, the late strength growth rate of concrete containing FA is low for the same curing conditions. The effect of late performance of resistance to chloride ion permeability improved by FA is better than the same effect achieved by GGBS. The risk of destroying the structure of concrete containing FA or GGBS due to DEF when specimens were steam-cured at 80°C is minimal.
The authors declare that they have no conflicts of interest.
The authors would like to acknowledge National Natural Science Foundation of China (no. 51478248) and the Tsinghua University Initiative Scientific Research Program (20131089239).