Improvement of Mechanical Properties and Microstructure of Cementitious Materials in a Hot-Dry Environment

Abstract

The hot-dry environment in high geothermal tunnels negatively affects the development of properties of cementitious materials. In order to improve the mechanical properties of cementitious materials in such environments, a proper curing system was developed, the material compositions were taken into account, and the related mechanism was analyzed by microscopic tests. The results showed that the mechanical properties and microstructure of cementitious materials can be improved effectively by adopting film curing and an adequate dosage of fly ash in a hot-dry environment. Compared with standard curing, a hot-dry environment accelerates the evaporation of water in fresh cementitious materials and the early hydration rate of cementitious materials, resulting in lots of unhydrated cement particles and an uneven distribution of hydration products, seriously decreasing the properties of the cementitious materials. Film curing keeps the water from evaporating and ensures enough water is available for hydration at an early age. The addition of FA reduces the amount of cement clinker and decreases hydration reaction rate in high temperature environments at an early stage, avoiding the unevenness of hydration products. High temperatures stimulate the pozzolanic activity of fly ash and promote the degree of secondary hydration reaction, producing more hydration products, as well as contributing to the improvement of the mechanical properties and microstructure of cementitious materials. In this study, in a hot-dry environment, the mechanical properties of mortar with 25% fly ash and 2–3 d film curing are better than those of other experimental groups, including those of standard curing.

1. Introduction

In recent years, more and more long, large, and deep tunnels have been built in the world, and the problems of high geothermal tunnels have gradually emerged, attracting people’s attention [1,2,3,4,5]. Generally, a tunnel with a temperature above 28 °C should be regarded as a high geothermal tunnel [6]. Some of the high geothermal tunnels that have been built in the world are shown in

Table 1. High geothermal tunnel cases [7,8].

Country	Tunnel Name	Length/km	Maximum Depth of Burial/m	Maximum Rock Temperature/°C
China	Qirehataer hydropower station diversion tunnel	15.639	1720	98
China	Sangzhuling tunnel	16.449	1347	89.9
China	Padang Mountain Tunnel	2.865	33	76.4
Japan	Abo–Toge road tunnel	4.35	700	75
China	Dagashan tunnel	7.21	700	64
Italy	Apennine railway tunnel	18.507	2000	63.8
Switzerland	Simplon tunnel	19.8	2140	55.4
China	Jiuzhai tunnel	4.46	150	52
America	Tecolote tunnel	6.4	2287	47
Switzerland	New Gotthard Tunnel	57.09	2300	45
China	Fudang Tunnel	7.517	90	44
China	Qinling tunnel	18.448	1600	40
France, Italy	Lyon–Turin tunnel	57.5	2000	40

[7,8]. There are two kinds of high geothermal forms, one is a hot-humid environment with a high temperature and high humidity, and the other is a hot-dry environment with a high temperature and low humidity [9,10]. The hot-dry environment causes the temperature of concrete to rise after pouring and the free water in concrete to evaporate rapidly, leading to insufficient hydration of cement-based materials. Insufficient hydration results in the pore structure being coarse and a deterioration in concrete properties [11]. Therefore, the construction of tunnels in a hot-dry environment has also become a great challenge for tunnel engineers.

In China, there are more than 10 high geothermal tunnels from 28.7 °C to 86.0 °C under the construction of the Sichuan–Tibet Railway, which is an important and challenging project [12,13,14]. Therefore, improving the performance of tunnel concrete under high geothermal conditions is of great significance to keep the stability and service life of tunnel structures.

Liu et al. [8] simulated the hot and dry environments through different temperature levels and studied the mechanical properties of the concrete in corresponding ages. It was found that the mechanical properties of concrete decreased with the increase in temperature, and the early strength increased only within a certain temperature range. Cui et al. [15] examined the deterioration mechanism and improvement measures of cement-based materials in the hot-dry environment of high geothermal tunnels. The results showed that the total porosity and the porosity of harmful pores in the hot-dry environment increased rapidly and the addition of fiber materials could reduce the total porosity and the porosity of harmful pores. Liu et al. [16] studied the mechanical behavior of tunnel lining in high temperatures by the thermal numerical method, and the results showed that the mechanical properties of the lining concrete could be improved by setting an insulation layer. He et al. [17,18] studied the influence of mineral admixtures, such as slag and fly ash, on the strength of concrete under the condition of a high rock temperature, and the results showed that the addition of slag and fly ash could improve the mechanical strength and the durability of concrete under a high rock temperature. Y. Zhang et al. [19] considered the modification effect of fiber on cement-based materials, analyzed the influence of fiber on the flexural strength of mortar, and found that the addition of fiber can improve long-term strength. Wang et al. [20] designed and developed a tunnel lining concrete with high temperature resistance by adding fly ash, fiber, and vitrified beads from the perspectives of heat insulation, enhancement, and heat resistance. At present, researchers focus on heat insulation methods and construction technologies to mitigate the adverse effects of high geothermal temperature on concrete properties, which will have a positive role in further research.

However, at present, a mature theoretical system has not yet formed to guide construction in hot-dry environments, and only relying on accumulated construction experience is not enough to solve the problem. Improving the performance of cementitious materials in hot-dry environments is of great significance for improving the stability and service life of tunnels. Therefore, studying methods to improve the performance of cementitious materials in hot-dry environments has become more important. In this paper, the method of improving the performance of cementitious materials under a hot-dry environment is studied by combining mechanical tests and microscopic experiments, hoping to provide theoretical guidance for future engineering practice.

2. Experimental Schemes

2.1. Materials

The type of cement (C) used in this experiment was P I 42.5, with an apparent density of 3120 kg/m³ and a specific surface area of 354 m²/kg. The mineral composition and chemical composition of cement are shown in

Table 2. Mineral composition of reference cement clinker (wt.%).

C3S	C2S	C3A	C4AF
57.55	17.82	7.54	11.19

and

Table 3. Oxides composition of reference cement and FA (wt.%).

Materials	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2Oeq	Other Oxides
C	20.58	5.03	3.38	63.32	2.01	2.45	0.55	2.68
FA	51.00	35.64	4.75	2.95	0.5	1.17	0.58	3.41

. The fly ash (FA) used was class F low-calcium FA produced by Hunan Xiangtan Power Plant. The apparent density and specific surface area of FA were 2450 kg/m³ and 480 m²/kg, respectively. The fineness modulus of sand (S) was 2.85. The apparent density and bulk density of S were 2660 kg/m³ and 1516 kg/m³, respectively. The water–cement ratio was 0.32, and the cement–sand ratio was 1:3. The workability was adjusted by a water reducer. The water reducer used here was polycarboxylate super-plasticizer (SP) with a water reduction rate of 32% and solid content of 33.1%.

2.2. Mix Proportion

The mass fractions of FA in the cement-fly ash mortar were 0%, 25%, and 45% (denoted as FA0-M, FA25-M, FA45-M, respectively). The specific mix proportions of the mortar are shown in

Table 4. Mix proportions of mortar (kg/m³).

MIX ID	C	S	Water	FA	SP
FA0-M	450	1350	135	0	2.25
FA25-M	337.5	1350	135	112.5	2.25
FA45-M	247.5	1350	135	202.5	2.25

. According to the same mix proportions as FA0-M and FA25-M in Table 4, paste specimens with sizes of 40 mm × 40 mm × 40 mm were formed and cured to 28 days (denoted as FA0-P, FA25-P, respectively). The specific mix proportions of paste are shown in

Table 5. Mix proportions of paste (kg/m³).

MIX ID	C	Water	FA	SP
FA0-P	450	135	0	1.35
FA25-P	337.5	135	112.5	1.35

.

2.3. Specimen Molding

‘Chinese Railway code’ (Code for Durability Design on Concrete Structure of Railway (TB10005)) requires that the fly ash content of railway engineering concrete with the durability requirement should not be more than 50%. Therefore, this experiment studies the appropriate contents of fly ash, which are 0%, 25%, and 45%, respectively. In this experiment, 45 groups of 40 mm × 40 mm × 160 mm prism specimens were formed by steel mold, and different curing systems were adopted. Each group contained three mortar specimens. Figure 1 shows the photographs of the specimens formed by steel molds.

2.4. Curing Systems

In the experiment, the oven was used to simulate a hot-dry environment. The temperature was kept at 60 °C and the relative humidity was less than 20%. The molded specimens were divided into five groups, which were cured under five different curing systems. The parameters of different curing systems are shown in

Table 6. Parameters of different curing systems.

Curing System	Temperature/°C	Relative Humidity	Film Curing Days/d	Curing Age/d
SC	20	>95%	0	3, 7, 28
FC1	60	<20%	1	3, 7, 28
FC2	60	<20%	2	3, 7, 28
FC3	60	<20%	3	3, 7, 28
HD	60	<20%	0	3, 7, 28

.

The first curing system was standard curing (SC). After the mortar was formed, it was kept indoors for one day. One day later, the mold was removed. Then it was placed in a standard curing room where the relative humidity was not less than 95% and the temperature was (20 ± 2) °C.

The second to fourth curing systems were film curing (FC). In order to simulate the hot-dry environment of the construction site, once the specimens were formed, they were immediately covered with heat-resistant preservative films and directly put them into the oven. After half a day of curing, the mold was removed and covered with film again. The specimens would continue to be placed in the oven for curing. The preservative films were removed when the specimens were cured at a specific time. After that, they would continue to be cured in the oven to the corresponding age. In this paper, the film curing time is used to express the days of specimen covered with film. In engineering practice, if the film curing time is too long, the construction progress will be affected. Under the condition of meeting the strength requirements in advance, the time of film curing shall be controlled. Therefore, the film curing times of this experiment were 1 day, 2 days, and 3 days, respectively (denoted as FC1, FC2 and FC3, respectively).

The fifth curing system was hot-dry curing (HD). After being molded, the specimens were directly put into the oven for curing under the condition of a high temperature and low humidity.

2.5. Test Methods

(1)

The mortar was mixed following ASTM C305. According to the mix proportions in Table 4, each mix proportion was formed into fifteen groups, and there were three specimens in each group. The size of the specimens were 40 mm × 40 mm × 160 mm. The specimens were cured under five different curing systems after molding. There were three curing periods in this experiment, which were 3, 7, and 28 days, respectively. After being cured to the specified time, they were taken out and the mechanical strength was tested. The test equipment used in the experiment was TYA300B, and the continuous loading method was adopted. The flexural test was performed according to ASTM C348-19. After the flexural test, the specimen was divided into two parts and used them to test the compressive strength. The compressive test was performed according to ASTM C349-18.

(2)

According to the mix proportions of Table 5, five groups of specimens were formed with each mix proportion and cured under different curing systems. When curing to 28 days, they were broken, and immersed in isopropanol to terminate their hydration. After vacuum drying, microscopic performance tests were performed. The microstructure of cement-based materials under different curing systems was observed by scanning electron microscope (SEM). The hydration products were analyzed by X-ray diffraction (XRD) and thermogravimetric analysis (TG).

3. Results and Discussion

3.1. Effect of Curing Systems on Mortar Strength

3.1.1. Blank Cement Mortar

Figure 2 shows the change in the mechanical strength of blank cement mortar (FA0-M) under different curing systems. It can be seen from Figure 2 that under different curing systems, the strength of FA0-M increases with the growth of the age, but the growth rate is different. Compared with the SC condition, the strength of mortar specimens at different ages under the HD condition had been greatly reduced. At 28 d, the compressive strength and flexural strength of mortar under the HD condition are 38.5% and 29.6% lower than that under the SC condition, respectively. This indicates that the hot-dry environment has an extremely adverse effect on the growth of strength of mortar specimens.

It can be seen from the Figure 2 that the strength under the condition of FC is higher than that under the condition of HD. The strength under the condition of FC1 is not improved compared with the condition of SC, but the strength under the conditions of FC2 and FC3 is improved significantly. Compared with the condition of SC, the compressive strength of mortar under the conditions of FC2 and FC3 is increased by 4.7% and 6.3%, respectively. The flexural strength of mortar under the conditions of FC2 and FC3 is increased by 4.9% and 11.1%, respectively. These indicate that a certain time of film curing is helpful to keep the increase in mortar strength in a hot-dry environment.

3.1.2. Cement-Fly Ash Mortar (25%FA)

Figure 3 shows the changes in mechanical strength of cement-fly ash mortar (25% fly ash, FA25-M) under different curing systems. As can be seen from Figure 3, under different curing systems, the compressive strength of FA25-M develops with the increase in the age. Compared with the condition of SC, the strength of mortar under the condition of HD is lower at all ages, and the compressive and flexural strengths of mortar at 28 d are reduced by 37.6% and 25.6%, respectively. This indicates that the HD condition also has a great adverse effect on the strength of cement-fly ash mortar.

It can also be seen from Figure 3 that the strength under the condition of FC1 is not significantly different from that under the condition of SC. However, compared with the condition of SC, the strength of mortar under the conditions of FC2 and FC3 has a significant improvement. The compressive strength of mortar under the conditions of FC2 and FC3 is increased by 18.1% and 25.4%, respectively. The flexural strength of mortar under the conditions of FC2 and FC3 is increased by 8.9% and 12.2%, respectively. These indicate that when 25% fly ash is mixed, the early film curing plays a key role in promoting the hydration of the cement-fly ash mortar system, but different film curing times have different effects on the 28 d strength.

When the mass fraction of fly ash is 25%, the strength of mortar at each age under the FC condition is higher than that under the HD condition. Compared with other curing conditions, the strength of mortar under the condition of FC3 is the highest. That is, in a hot-dry environment, increasing the time of film curing is conducive to the growth of mechanical strength.

3.1.3. Cement-Fly Ash Mortar (45%FA)

Figure 4 presents the changes in mechanical strength of the cement-fly ash mortar (45% fly ash, FA45-M) under different curing systems. As can be seen from Figure 4, under different curing systems, the mechanical strength of the FA45-M group develops with the increase in age. At 28 d, compared with the SC condition, the compressive strength is reduced by 29%, and the flexural strength is reduced by 23.7%, indicating that the HD condition negatively affects the strength of FA45-M.

It can also be seen from Figure 4 that the mechanical strength of mortar at all ages under the condition of FC is higher than those under the condition of SC. Compared with the condition of SC, the compressive strength of mortar with 1–3 d film curing is increased by 1.1%, 13.3%, and 17.6%, respectively, and the flexural strength of mortar with 1–3 d film curing is increased by 10.5%, 14.5%, and 19.7%, respectively.

When the mass fraction of fly ash is 45%, the strength of mortar at each age under the FC condition is higher than that of the HD condition. Compared with the HD condition, film curing plays a role in reducing the evaporation of free water in the early stage, which is conducive to the hydration reaction. Compared with different curing systems, the mechanical strength of mortar under the conditions of FC2 and FC3 achieved better growth with the increase in age. That is, under the hot-dry condition, a certain time of film curing and proper fly ash content is beneficial for the growth of mechanical strength.

3.2. Effect of Film Curing Time on Mortar Strength

Figure 5 shows the change in mechanical strength at 28 days under different film curing times. It can be seen from Figure 5a that with the increase in film curing time, the 28 d compressive strength gradually increases, but the growth rate of mortar in each group is different. Compared with the no-film curing group, the compressive strength of FA0-M with 1–3 d film curing is increased by 58.9%, 70.2%, and 72.8%, respectively; the compressive strength of FA25-M with 1–3 days film curing is increased by 53.6%, 83.3%, and 91.8%, respectively; the compressive strength of FA45-M with 1–3 days film curing is increased by 42.5%, 59.6%, and 65.7%, respectively. These show that with the increase in film curing time, the compressive strength of mortar is improved.

It can be seen from Figure 5b, compared with the no-film curing group, the flexural strength of FA0-M with 1–3 days of film curing is increased by 42.1%, 49.1%, and 54.4%, respectively; the flexural strength of FA25-M with 1–3 d film curing is increased by 38.8%, 46.3%, and 50.7%, respectively; and the flexural strength of FA45-M with 1–3 d film curing is increased by 44.8%, 53.7%, and 58.6%, respectively. These show that with the increase in film curing time, the flexure strength of mortar is improved.

The increase in film curing time is beneficial to increase mechanical strength. It can also be seen from Figure 5 that the compressive strength of FA25-M is relatively low when it is cured without film. However, with the increase in film curing time, the strength of FA25-M increases greatly, which is higher than that of FA45-M. It can be observed that the highest value of 28 d strength is FA25-M with 3 days of film curing. Therefore, 25% fly ash content with 3 days film curing is the most effective method to improve the mechanical strength of mortar in this study.

3.3. Effect of Curing Systems on Microstructure

3.3.1. Hot-Dry Environment

In order to analyze the effect of a hot-dry environment on the microstructure of cementitious materials, the specimens of FA0-P were cured under the conditions of SC and HD to 28 d and then used to perform the SEM, XRD, and TG-DTG tests.

Figure 6 shows the microstructure of FA0-P under the SC and HD curing systems. As can be seen from Figure 6a, under the condition of SC, the internal structure of paste is compact. As can be seen from Figure 6b, under the condition of HD, the internal structure of cement paste is loose and porous, and the distributions of hydration products are uneven. Large hexagonal lamellar CH (calcium hydroxide) crystals exist in hydration products. The loose and porous internal structure is an important reason for the sharp decline in the mechanical strength of mortar under the condition of HD.

Figure 7 shows the XRD test results of FA0-P under the SC and HD curing systems. It can be seen from Figure 7 that hydration products under the SC condition are mainly CH crystals, Aft, and unhydrated C₂S. However, under the HD condition, the hydration products are mainly CH crystals, unhydrated C₂S (Dicalcium silicate), and C₃S (Tricalcium silicate); Aft (Ettringite) is not found. Moreover, the diffraction peak intensity of CH under the SC condition is higher than that under the HD condition. This shows that compared with the SC condition, lots of cement at 28 d is still not hydrated under the HD condition, leading to the deterioration of mortar performance.

Figure 8 shows the thermal analysis test results of FA0-P under the SC and HD curing systems. As shown in Figure 8, under different curing systems, the trend of the curves is basically the same, but there is a great difference in the peak intensity. This indicates that the hydration products of FA0-P under the SC and HD curing systems are basically the same, but the contents of the hydration products are different. Three peaks can be clearly seen in Figure 8. The endothermic peaks between 80 °C and 200 °C are generated by the dehydration of ettringite, AFm, and CSH gel. The endothermic peaks at about 460 °C are mainly caused by the decomposition of hydration product CH. The decomposition peaks of calcium carbonate are about 700 °C. The endothermic peaks of CSH gel and CH under the HD condition are significantly lower than those under the SC condition, indicating that the number of hydration products of cement under the HD condition is significantly less than that under the SC condition. This is consistent with the XRD test results; that is, the hydration degree of cement under the HD condition is low.

From the above microscopic tests, it can be known that under the condition of SC, the hydration reaction can proceed normally and the degree of hydration is high at 28 d. Therefore, the internal structure of paste is compact. However, under the hot-dry environment, water on the surface of the specimen evaporates faster than standard curing. The internal structure can still carry out the hydration reaction, but the degree of hydration is low. The high temperature can accelerate the early hydration reaction rate of cement. The early hydration rate is too fast to allow the hydration products to diffuse in time, resulting in hydration products accumulating irregularly on the surface of cement particles. Therefore, the hydration reaction rate becomes slow and even stops when the uneven hydration products cover the surfaces of the cement particles. Furthermore, under the hot-dry condition, the rapid evaporation of water will seriously affect the further progress of the hydration reaction. The hot-dry condition could not change the kind of hydration products, but it can result in the uneven distribution of hydration products, which has a great influence on the degree of hydration. As a result, under hot-dry condition the inner structure of paste is loose and porous.

3.3.2. Film Curing Time

Fly ash is a kind of mineral admixture with pozzolanic activity. When cement is mixed with fly ash, fly ash particles will undergo a secondary hydration reaction [21,22]. A secondary hydration reaction is the reaction of calcium hydroxide (CH) produced by cement hydration with the active component of fly ash to produce new hydration product. A secondary hydration reaction will increase the content of hydration products and refine the pore structure of the cement paste [23,24,25,26,27,28]. The activity of fly ash is more easily activated at a high temperature, which is conducive to the secondary hydration reaction. However, the process of the secondary hydration reaction is also slow. From the previous mechanical experiments, it can be seen that the strength of FA25 -M under the film curing condition is the best (Figure 5). In order to study the influence of different curing systems (film curing time) on cement-fly ash mortar, the specimens of FA25-P group were cured under different curing systems to 28 d and used to perform SEM, XRD, and TG-DTG tests.

Figure 9 shows the SEM pictures of FA25-P under the SC condition. As can be seen from Figure 9a,b, under the condition of SC, a large number of hydration products are generated and distributed evenly. The fly ash particles inside the paste have taken part in the hydration reaction. The internal structure of paste is compact, and there are some hydration products attached on the surface of the fly ash.

Figure 10 shows the pictures of SEM of FA25-P under the HD condition. As can be seen from Figure 10a, under the HD condition, the hydration products in the paste are loose. The distribution of hydration products is uneven, and the porosity is obviously higher than that under the condition of SC. Figure 10b shows that, under the condition of HD, there are some hydration products on the surface of fly ash, but the distribution of hydration products around the fly ash particles is uneven, and there are many pores around the fly ash particle.

Figure 11, Figure 12 and Figure 13 respectively show the SEM pictures of FA25-P under FC1–FC3 conditions. By comparing Figure 11, Figure 12 and Figure 13, it can be seen that the microstructure of paste under the FC1 condition is compacter than HD, but there are still many pores, and the irregular accumulation of hydration products can be observed clearly. Under the FC2 condition, the compactness of paste is improved, but the distribution of hydration products is still uneven. Under the FC3 condition, the compactness of the structure is significantly improved, and the porosity is significantly reduced. Therefore, with the increase in film curing time, the compactness of internal structure is enhanced. It can be observed from Figure 10b, Figure 11b, Figure 12b and Figure 13b that hydration products are attached on the surface of fly ash under different curing systems. It shows that the hydration reaction of fly ash occurs under the hot-dry environment, and the activity of fly ash is stimulated by the high temperature. However, the degree of hydration reaction of fly ash is different under different film curing times. It can be seen from Figure 13b that the hydration reaction of fly ash particles is almost fully under the condition of FC3.

Figure 14 shows the XRD test results of FA25-P cured to 28 d under different curing systems. As can be seen from Figure 14, the types of hydration products under different curing systems are almost the same, but the peak intensity is different. Under the SC condition, the peak of AFt can be clearly observed, but under other curing systems, the peak of AFt is not obvious. The diffraction peak intensity of CH is the highest under the SC condition and the lowest under the HD condition. The content of CH under the conditions of FC1–FC3 is higher than that under the condition of HD, indicating that the early film curing is conducive to the further hydration reaction. The activity of FA is stimulated at a high temperature, which promotes the secondary hydration reaction between FA and CH and produces more hydration products. The ${\mathrm{SiO}}_2$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$ in fly ash react with the CH formed by cement hydration to produce hydrated calcium silicate, hydrated calcium aluminate, and other crystals [29]. CH is consumed and the content of CH is reduced. As can be seen from Figure 14, under the condition of FC, the CH content from low to high is FC3, FC2, and FC1. It shows that with the increase in film curing time, the content of CH gradually decreases; that is, the degree of secondary hydration was improved.

$${\mathrm{SiO}}_2+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+n{\mathrm{H}}_2\mathrm{O}\to \mathrm{CaO}·{\mathrm{SiO}}_2·\left(n+1\right){\mathrm{H}}_2\mathrm{O}$$

$${\mathrm{Al}}_2{\mathrm{O}}_3+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+n{\mathrm{H}}_2\mathrm{O}\to \mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·\left(n+1\right){\mathrm{H}}_2\mathrm{O}$$

Figure 15 shows the TG-DTG test results of FA25-P cured to 28 d under different curing systems. Just as the result from Figure 8, there are three obvious weight-loss plateaus in the TG curves for all conditions, corresponding to endothermic peaks in the DTA curves. The peaks between 80 °C and 200 °C are endothermic peaks generated by the dehydration of the AFm and C-S-H gel. The endothermic peaks at about 450 °C are mainly caused by the decomposition of CH. The endothermic peaks of CaCO₃ are between 600 °C and 720 °C. The mass loss from high to low is as follows: SC, FC3, FC2, FC1, and HD, and the difference between FC2 and FC3 is small. The DTG curves between 400 °C and 500 °C are too dense to distinguish clearly, so the enlarged figure is shown. It can be observed that the order of the size of the area enclosed by the DTG curve is also the same. This shows that with the increase in film curing time, the degree of hydration increases and the number of hydration products increases.

According to the above analysis, high temperatures can stimulate the pozzolanic activity of fly ash and promote the degree of secondary hydration reaction. The generated hydration products are filled in the pores and refine the pore structure. It is proved that film curing in the early stage in a hot-dry environment can effectively promote the degree of hydration reaction. Moreover, the microstructure becomes denser as the film curing time increases. Film curing and the addition of FA can improve the microstructure.

4. Conclusions

The strength of concrete in a hot-dry environment will be greatly reduced, in order to improve the performance of concrete under a hot-dry environment. Based on the experimental results and analysis presented in this paper, the following conclusions may be drawn:

(1)

Compared with the different curing systems, the strength of mortar under the condition of film curing is higher than that under the condition of hot-dry curing. Film curing at early age can effectively improve the mechanical strength of mortar under the hot-dry environment.

(2)

Compared with hot-dry curing, the degree of hydration is obviously improved under the condition of film curing. According to the microscopic texts, with the increase in film curing time, the hydration products gradually increase. With the increase in film curing time, the microstructure becomes denser and denser.

(3)

Film curing can ensure enough water is available for hydration at an early age, and improve the degree of secondary hydration. The addition of FA reduces the amount of cement clinker and reduces the unevenness of hydration products in high temperature environments in the early stage. The generated hydration products filled in the pores and refined the pore structure.

(4)

In this study, in the hot-dry environment, the mechanical properties of mortar with 25% fly ash and 2–3 d film curing time are better than those of other groups. This shows that the mechanical properties of cementitious materials under hot-dry environments can be effectively improved by adopting certain technical measures in material composition parameters and the curing process.