Properties of Cement-Based Materials with Low Water–Binder Ratios and Evaluation Mechanism under Further Hydration Effect

Abstract

Unhydrated cementitious materials in high-performance concrete (HPC) and ultra-high-performance concrete (UHPC) undergo further hydration when they are further supplied with water. A further hydration experiment on cement pastes was conducted to study the effects of temperature and humidity on their macroscopic properties. A rapid evaluation mechanism for further hydration was eventually presented. The results obtained under the four analysed humidity conditions indicated that the compressive strength and flexural strength increased by 22.6% and 75.2%, respectively, after further hydration for 180 d at a relative humidity (RH) of 95%. Considering water soaking under three analysed temperature conditions, water soaking at 60 °C had the largest impact on macroscopic properties, such as compressive strength and flexural strength, which showed an increase of 31.4% and a decrease of 13.8%, respectively, after further hydration for 180 d. Moreover, the expansion strain at 60 °C was 1.1 times higher than the strain determined under water soaking at 40 °C. Considering the stability of the evaluation indices, the combined water content for further hydration, expansion strain, and compressive strength were used to evaluate further hydration effect. Considering the acceleration and damaging effects, water soaking at 60 °C was an effective method to accelerate further hydration.

1. Introduction

Large amounts of carbon dioxide emissions are the primary cause of the greenhouse effect and will certainly have a significant impact on global warming [1,2]. The building materials sector, which is globally the third-largest carbon dioxide-emitting industrial sector, accounts for 10% of the total anthropogenic carbon dioxide emissions, most of which are related to the manufacturing of concrete [3]. In addition, cement production contributes approximately 85% of such carbon dioxide emissions [4]. Therefore, improving the usage efficiency of cement, especially that of clinkers, and reducing cement consumption by replacing it with high-quality mineral admixtures have emerged as important strategies to reduce carbon dioxide emissions in the concrete industry [5,6].

The scheme of mid-to-long-term development programs in science and technology in China requires that important traffic facilities such as sea-crossing channels, offshore deepwater harbours, and large-scale sea-crossing bridges should be constructed vigorously. This puts forward higher requirements for concrete performance. High-performance concrete (HPC) and ultra-high-performance concrete (UHPC) can satisfy the requirements for mechanical properties and durability of concrete for large-scale projects. Meanwhile, partial replacement of cement with industrial by-products such as silica fume (SF) from zirconium production, fly ash (FA) from coal-fired power plants, and ground granulated blast furnace slag (GGBS) from steel production is one promising solution to reduce the environmental impact of HPC and UHPC, and HPC and UHPC have already been developed into environmentally friendly engineering materials [7,8,9]. HPC and UHPC are produced with low water–binder ratios to improve the compactness of internal structures. Water-reducing admixture technology is currently used in a variety of applications to ensure or improve the workability of concrete mixtures. It enables the reduction of the water–binder ratio in concrete up to a value of 0.20 or lower [10]. However, when the water–binder ratio is less than or equal to 0.38, the cement hydration process is not completed, resulting in unhydrated cement particles in the hardened cement paste [11]. Unhydrated cement particles continue to hydrate when they are further supplied with water. This phenomenon is referred to as further hydration. The volume ratio of cement hydration products to unhydrated cement particles is generally assumed to be 2.1. The further hydration process can cause a volume expansion in the dense concrete material [12], which may damage its structure and affect its long-term performance. Wang et al. [13] reported that the splitting tensile strength of pre-damaged UHPC shows a tendency of increasing first and then decreasing with the increase in further hydration time, and the higher the preloading level, the greater the decrease in its strength during the late further hydration period. Li et al. [14] showed that concretes with low water–binder ratios gradually failed to provide adequate space for accommodating the new hydration products, and the internal stress led to the formation of microcracks during the late further hydration period. Liu [15] demonstrated that several microcracks with different orientations formed on C-S-H gel blocks around the pore, and this led to a reduction in compressive strength; and the decrease in strength was up to 12.3%.

Currently, researchers consider that different temperature and humidity conditions can affect the degree of further hydration in cement-based materials. Wang et al. [16] demonstrated that the ratio of further hydration degree of cement pastes under water-soaking conditions for 90 d at temperatures of 60 °C and 20 °C was as high as 2.9. Yin et al. [17] reported that for cement mortar cured at 80 °C, the higher cement hydration rate corresponded to a significant decrease in humidity. Lu [18] showed that the splitting tensile strength of C60 concrete increased by 60.3% under water-soaking conditions at 30 °C. Under the condition, the further hydration process was more effective than those conducted at 50% and 70% humidity, as well as the dry–wet cycle. Wang [19] verified that the further hydration of high-strength concrete had a more significant effect at 70 ± 1 °C and relative humidity (RH) ≥ 95%.

Water migration inside cement-based materials is slow in the natural environment, as is the cement hydration rate. Therefore, the development of a rapid evaluation mechanism for further hydration is necessary to rapidly evaluate the long-term effects of further hydration. However, no universal method has been reported thus far for the rapid evaluation of further hydration. Hillemeier et al. [20] reported that at high temperatures, the higher water migration rate accelerated the hydration rate of unhydrated cement particles, indicating that a high temperature accelerated the further hydration process. In another study, Wang [21] reported that ettringite retained its stable structure below 80 °C and decomposed into an amorphous structure above 80 °C, but crystallised again to its original structure when exposed to water. Moreover, the evaluation of the further hydration effect could be performed using different parameters. Pushpalal et al. [22] considered the mass change rate, elastic modulus, and linear expansion rate in evaluating the further hydration effect. Yang et al. [23,24] evaluated the further hydration effect using the relative ultrasonic time, loss rate of compressive strength, cement hydration degree, water permeability resistance, and frost resistance. Hillemeier et al. [20] and Zhang et al. [25] focused on the number and width of microcracks on the surface of concrete and the loss rate of the compressive strength to evaluate the further hydration effect. Furthermore, Guan et al. [26] and Feng et al. [27,28] used the cement hydration degree, relative ultrasonic time, and loss rate of the compressive strength to evaluate the further hydration effect.

The influence of further hydration on the macroscopic properties of cement-based materials in different temperature and humidity environments has been extensively studied; however, no unified method has been developed for the rapid evaluation of further hydration. Therefore, the establishment of a rapid evaluation method for further hydration is urgently required. First, a further hydration experiment was conducted to study the influence of environmental humidity on the strength of cement paste. Next, based on the obtained results, the influence of the environmental temperature on the strength and expansion strain of cement pastes was further analysed. Finally, a rapid evaluation mechanism for further hydration was proposed based on the effects of the environmental temperature and humidity on further hydration.

2. Materials and Methods

2.1. Raw Materials

The density and specific surface area of the reference cement (P•I 42.5) were 3.16 g/cm³ and 342 m²/kg, respectively. The mineral and chemical compositions are listed in

Table 1. Mineral composition of cement.

Item	C3S	C2S	C3A	C4AF
mass fraction/%	56.23	22.08	6.40	10.28

and

Table 2. Chemical composition of cement.

Item	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2Oeq	f-CaO
mass fraction/%	22.19	4.48	3.23	63.10	2.41	2.57	0.53	0.88

, respectively. The water-reducing rate and solid content of polycarboxylate superplasticizer were 30% and 40%, respectively. In order to analyse the influences of temperature and humidity on the performance of cement pastes, considering that the water–binder ratio of high-performance and ultra-high-performance concrete is generally between 0.20 and 0.30, the mixing ratio was chosen and is listed in

Table 3. Mixing ratio.

Cement	Superplasticizer	Water	Water–Binder Ratio	Compressive Strength at the Age of 28 d/MPa	Flexural Strength at the Age of 28 d/MPa
1.00	0.01	0.194	0.20	91.9	8.1

.

2.2. Sample Preparation and Test Methods

A cement paste mixer was used. First, cement was put into the mixing pot, and then superplasticizer was mixed into water and stirred uniformly. Finally, the water and superplasticizer were poured into the mixing pot. When cement pastes were stirred, they were stirred slowly for 2 min, and then fast for 2 min. The expansion degree, determined using a mortar expansion degree determinator, was 300 mm. The specimens were cast in moulds and compacted on a vibration table. To control the leakage of cement pastes during the mixing, moulding, and compacting processes, the sealant was brushed at the mould bottom. Three specimens of dimensions 40 mm × 40 mm × 160 mm were prepared for each group. The moulded specimens were placed at normal temperature for 1 d, after which the moulds were removed. The specimens were then placed in a standard curing room (20 ± 2 °C and RH ≥ 95%) for up to 28 d, and the completion of curing was selected as the starting point of the further hydration test. The specimens were placed under different temperature and humidity conditions to perform a further hydration experiment. The compressive strength, flexural strength, and expansion strain of the specimens were measured until the specified testing time.

Different humidity environments were realised in the following ways: (1) Sealing state: the surface moisture of the specimen was dried, and then the specimen was sealed using a plastic film and finally placed in an environment of 20 ± 2 °C; (2) RH50% ± 5%: the surface moisture of the specimen was dried, and the specimen was placed in an environment of 20 ± 2 °C and RH of 50 ± 5%; (3) RH ≥ 95%: the specimen was placed in a standard curing room. Different temperature environments were realised through water soaking at 20 °C, 40 °C, and 60 °C.

The ignition loss method was used to determine the chemically combined water content in three samples from each group. The crushed samples were ground into powder through a sieve with a diameter of 80 μm and the powder samples were placed in a dryer. During the test, each sample weighed about 10 g. The powder samples were put into a high-temperature furnace and heated to 105 °C for more than 3 h, then they were weighed after the samples were cooled. After weighing, they were placed in a high-temperature furnace again and heated to 950 °C for more than 3 h, then the samples were weighed again. The chemically combined water content of the sample (w_ne) was calculated according to Equations (1) and (2),

$$w_{ne}=\frac{w_1-w_2}{w_2}-\frac{r_{fc}}{1-r_{fc}},$$

(1)

where w₁ and w₂ are the masses of the sample after burning at 105 °C and 950 °C, respectively; r_fc is the total loss-on-ignition of cement and superplasticizer:

$$r_{fc}=p_c{r}_c+p_s{r}_s,$$

(2)

where p_c and p_s are the mass fractions of cement and superplasticizer (solid part) in cement pastes, respectively; and r_c and r_s are the loss-on-ignition of cement and superplasticizer, respectively.

The compressive and flexural strengths were measured according to cement—test methods—determination of strength (ISO 679: 2009) after further hydration for 0, 1, 3, 7, 14, 21, 28, 56, 90, and 180 d. Three samples of dimensions 40 mm × 40 mm × 160 mm were tested for flexural strength. Six samples, whose pressure faces were 40 mm × 40 mm, were tested for compressive strength. The heating and cooling rates of water were both controlled at 10 °C/h to avoid thermal deformation cracks in the specimens induced by sudden temperature fluctuations when the specimens were taken out. The expansion strain of the cement paste specimens caused by further hydration of the cementitious materials was considered equal to the free expansion strain during the further hydration test [15]. The free expansion strain was measured in accordance with the JC/T 603–2004 standard test method for the drying shrinkage of mortar through the comparator after further hydration for 0, 1, 2, 3, 7, 9, 14, 21, 28, 56, 72, 90, 120, 150, and 180 d. Subsequently, each specimen was removed from the water, its surface was quickly wiped dry, and its length was immediately determined.

3. Results

3.1. Effects of the Environmental Humidity on Strength

3.1.1. Compressive Strength

The compressive strength of the cement paste was evaluated under different humidity conditions. The results are shown in Figure 1. With an increase in the further hydration time, the compressive strength increased under all humidity conditions.

Under the conditions of the sealing state, RH50% ± 5%, RH ≥ 95%, and water soaking at 20 °C, the compressive strength of the cement pastes after 180 d of further hydration increased by 10.1%, 15.3%, 22.6%, and 17.1%, respectively, compared to that before further hydration. After further hydration for 14 d, considering the sealing state, RH50% ± 5%, RH ≥ 95%, and water soaking at 20 °C, the increase in strength under the aforementioned conditions was equal to 3.0, 3.2, 4.2, and 5.3 MPa, respectively, compared to that before further hydration. After further hydration for 90 d, the increase in strength was 7.2, 9.2, 12.0, and 6.2 MPa, compared to that evaluated after 14 d of further hydration. After further hydration for 180 d under the conditions of the sealing state, RH50% ± 5%, RH ≥ 95%, and water soaking at 20 °C, the strength increased by 1.0, 1.6, 4.5, and 4.2 MPa, respectively, compared to that measured after 90 d of further hydration. Evidently, among the four humidity environments, RH ≥ 95% exerted the most significant influence on the compressive strength of the cement pastes. Under the water-soaking conditions at 20 °C, the highest and lowest strength increments were observed during the early (between 0 and 14 d) and middle stages (between 14 and 90 d) of further hydration, respectively, and a relatively large increment in strength was observed during the late stages of further hydration (between 90 and 180 d).

In the sealing state, large autogenous shrinkage deformations of cement pastes occurred, and the tensile stress on the surface of the cement pastes led to cracking. Meanwhile, the cement hydration rate was the lowest. Therefore, the rate of increase in strength in the sealing state was the lowest. Under RH50% ± 5% conditions, external water entered the specimen through diffusion, which reduced the degree of autogenous shrinkage and drying shrinkage. Meanwhile, under RH50% ± 5% conditions, the cement hydration was accelerated to a certain extent, yielding a higher rate of strength increase compared to that obtained in the sealing state. Under the conditions of RH ≥ 95% and water soaking at 20 °C, a satisfactory supply of water vapour and water from the external environment accelerated the further hydration process, and the further hydration products continued to fill the internal pores of the cement pastes. This increased the rate of increase in the compressive strength. Under the conditions of water soaking at 20 °C, the cement paste was exposed to an adequate supply of water during the early stage of further hydration, which enhanced the further hydration reaction, yielding the maximum increment in strength. The further hydration reaction during the early stage resulted in a high degree of pore filling, limiting the diffusion of external water in the cement paste and resulting in the minimum increment in strength. During the late stage of further hydration, the available space in the cement paste gradually decreased. When the remaining space could not accommodate further hydration products and the expansion stress of the products exceeded the tensile strength of the cement paste, the microcracks generated in the cement paste provided a new channel for water diffusion. Therefore, a large increment in strength was maintained.

3.1.2. Flexural Strength

The flexural strength of the cement paste was evaluated under different humidity conditions, and the results are shown in Figure 2. Under sealing and RH50% ± 5% conditions, the flexural strength of the cement paste decreased with an increase in the further hydration time. In contrast, under the conditions of RH ≥ 95% and water soaking at 20 °C, the flexural strength increased with further hydration time. This is consistent with the experimental results reported by Luo [29].

Under sealing and RH50% ± 5% conditions, the flexural strength of cement paste after further hydration for 180 d decreased by 50.1% and 40.1%, respectively, compared to that before further hydration. Under the conditions of RH ≥ 95% and water soaking at 20 °C, the flexural strength increased by 75.2% and 76.9%, respectively. Therefore, the evolution laws of flexural strength of cement paste under four different humidity conditions can be divided into two categories. The self-shrinkage of the cement paste under the sealing conditions and the self-shrinkage and dry shrinkage of the cement paste under the conditions of RH50% ± 5% caused damages or cracks. Compared to the compressive strength, the flexural strength was more sensitive to damages or cracks in the cement paste, particularly surface cracking. Thus, the flexural strength decreased under the sealing and RH50% ± 5% conditions. Notably, under the conditions of RH ≥ 95% and water soaking at 20 °C, the supply of water vapour and water in the external environment accelerated the further hydration reaction of the cement paste, and the further hydration products continued to fill the internal pores of the cement paste, resulting in an increase in its flexural strength.

3.2. Effects of the Environment Temperature on Further Hydration

3.2.1. Compressive Strength

At RH ≥ 95%, further hydration significantly affected the strength of the cement paste. Therefore, to evaluate the effect of temperature on further hydration, the compressive strength of the cement paste under water-soaking conditions at 40 and 60 °C was measured. The results are reported in Figure 3. As the further hydration time increased, the compressive strength of the cement paste increased under all temperature conditions. This is consistent with the experimental results reported by Ge [30].

Under water-soaking conditions at 40 and 60 °C, the compressive strength of the cement paste tended to plateau after further hydration for 56 and 28 d, respectively, and the higher the water temperature, the earlier the compressive strength plateaued. The high temperature promoted cement hydration, forming further hydration products at an excessive generation rate. The products were unevenly diffused in the solid phase. When the further hydration reaction occurred, the compactness of the cement paste intensified. This hindered the uniform diffusion of the products, which caused most products to accumulate around the cement particles. Concurrently, the grain size of the cement hydration products under high-temperature water-soaking conditions was coarse [31,32], yielding the compressive strength plateau. Furthermore, the higher the water temperature, the earlier the plateau appeared.

Under RH ≥ 95% and water-soaking conditions at 40 and 60 °C, the compressive strength of the cement paste after further hydration for 180 d increased by 22.6%, 26.2%, and 31.4%, respectively. The higher the temperature, the higher the rate of increase in the compressive strength. This was because higher temperatures accelerated the further hydration of cement during the further hydration process, and the new products filled the pores of the cement paste to a higher degree, which increased the rate of increase in the compressive strength.

3.2.2. Flexural Strength

The flexural strengths of the cement pastes determined at different temperatures are shown in Figure 4. Under the conditions of RH ≥ 95% and water soaking at 40 °C, the flexural strength of the cement paste increased with the increase in the further hydration time. However, under water-soaking conditions at 60 °C, it exhibited an initial steep increase followed by a rapid decrease. This is consistent with the experimental results reported by Ge [30].

Under the conditions of RH ≥ 95% and water soaking at 40 °C, the flexural strength of the cement paste after further hydration for 180 d increased by 75.2% and 104.1%, respectively. Under water-soaking conditions at 60 °C, the flexural strength increased by 98.8% after further hydration for 7 d and decreased by 13.8% after further hydration for 180 d. The variations in the flexural strength under the three temperature conditions can be divided into two categories. During the early stage of further hydration, the new products continuously filled the pores of the cement paste, resulting in an increase in the flexural strength. The high temperature under water soaking at 60 °C accelerated the hydration of the cement paste, gradually decreasing the available space within the pores. When the remaining space could not accommodate the further hydration products and the expansion stress of the products exceeded the tensile strength of the cement paste, microcracks were observed in the cement paste. Compared to the compressive strength, the flexural strength was more sensitive to damages or cracks in the cement paste, particularly the surface cracks, which sharply reduced the flexural strength.

3.2.3. Expansion Strain

The results for compressive and flexural strength reveal that the improvement in strength under water soaking at 40 °C was more than that observed at RH ≥ 95%. Therefore, to further analyse the influence of temperature on the expansion strain of cement pastes, the expansion strain of the cement paste under water-soaking conditions at 40 and 60 °C was measured, and the results are shown in Figure 5. With an increase in further hydration time, the expansion strain of the cement paste increased at both temperatures. This is consistent with the experimental results reported by Igarashi et al. [33].

Under water-soaking conditions at 40 and 60 °C, expansion strains after further hydration for 180 d were 1152 and 1221 με, respectively. The strain value under water soaking at 60 °C was 1.1 times higher than that at 40 °C. Evidently, during the further hydration process, the higher the temperature, the larger the expansion strain of the cement paste. This was because the high-temperature water soaking accelerated the hydration of the cement.

4. Rapid Evaluation Mechanism for Further Hydration

Currently, no uniform standard protocol has been developed for the rapid evaluation of further hydration. Therefore, to clarify the evaluation mechanism in this study, a comparison of the macroscopic performance of the cement paste in different hydration environments after further hydration for 180 d is shown in Figure 6. Results indicate that high humidity or temperature increased the combined water content for further hydration, expansion strain, and the rate of increase in the compressive strength of the cement paste. In contrast, the rate of increase in flexural strength was not proportional to temperature.

Under water-soaking conditions at 60 °C, the combined water content for further hydration and rate of increase in compressive strength were 0.02751 and 31.4%, respectively, which were 3.6 and 1.4 times higher than those at RH ≥ 95%. At this temperature, the expansion strain was 1221 με, which was also 1.1 times higher than that under water-soaking conditions at 40 °C. In contrast, the flexural strength decreased by 13.8%. The combined water content for further hydration, expansion strain, and rate of increase in the compressive strength of the cement paste reached a maximum value under water-soaking conditions at 60 °C. Notably, although the acceleration effect of the further hydration under water-soaking conditions at 60 °C was more significant compared to those under other analysed conditions, it caused visible damage. Therefore, the combined water content for further hydration, expansion strain, and compressive strength can be used to evaluate the further hydration effect, considering the stability of the evaluation indices.

The combined water content for further hydration under water soaking at 20 °C was higher than RH ≥ 95%, but its rate of increase in compressive strength was lower. Under the conditions of water soaking at 20 °C, the cement paste was exposed to an adequate supply of water during the early stage of further hydration, which enhanced the further hydration reaction, yielding a larger value in the combined water content for further hydration compared with the condition of RH ≥ 95% after further hydration for 180 d. However, the further hydration reaction during the early stage resulted in a high degree of pore filling, limiting the diffusion of external water into the cement paste and resulting in a smaller rate of increase in the compressive strength after further hydration for 180 d.

At temperatures above 70 °C, the ettringite inside the cement-based materials would decompose [35]. Furthermore, considering the acceleration and damaging effects caused by the further hydration experiment and to avoid the influence of ettringite decomposition, water soaking at 60 °C is recommended to accelerate further hydration.

5. Conclusions

In this study, the influence of further hydration under different humidity and temperature conditions was evaluated, along with its impacts on the mechanical properties of cement pastes. First, the effect of further hydration was studied under the following humidity conditions: sealing state, RH50% ± 5% and RH ≥ 95%, while the water-soaking condition was maintained constant at 20 °C. Thereafter, its impacts were evaluated under the following temperature conditions: water soaking at 40 and 60 °C while maintaining RH ≥ 95% at 20 °C. Finally, a rapid evaluation mechanism for further hydration was presented. Based on the experimental results, the following conclusions can be drawn:

(1)

Among the four humidity environments: sealing state at 20 °C, RH50% ± 5% at 20 °C, RH ≥ 95% at 20 °C, and water soaking at 20 °C, RH ≥ 95% at 20 °C had the most significant improvement effect, leading to an increase in the compressive strength and flexural strengths of 22.6% and 75.2%, respectively, after further hydration for 180 d.

(2)

Among the three temperature environments: RH ≥ 95% at 20 °C and water soaking at 40 and 60 °C, water soaking at 60 °C showed the most significant effect on the strength and expansion strain of the cement paste. After further hydration for 180 d, the compressive strength increased by 31.4% compared to that before further hydration, whereas the flexural strength decreased by 13.8%, and the expansion strain was 1.1 times higher than that under water-soaking conditions at 40 °C.

(3)

Considering the stability of the evaluation indices, further hydration was evaluated using the following parameters: the combined water content for further hydration, expansion strain, and compressive strength. Moreover, considering the acceleration and damaging effects of further hydration experiments, water soaking at 60 °C optimally accelerated further hydration.

For HPC and UHPC structures that have been in long-term service in aqueous environments such as sea-crossing channels, offshore deepwater harbours, and large-scale sea-crossing bridges, the influence of further hydration on the properties of hardened cement pastes cannot be overlooked. This study can serve as a foundation for evaluating and designing the long-term performance of cement-based materials with low water–binder ratios. While the results of the study are valid for the range of parameters considered herein, further experiments should be conducted to support and expand the knowledge gathered in this work.