- Email: [email protected]
Influence of various bentonites on the mechanical properties and impermeability of cement mortars
Construction and Building Materials 241 (2020) 118015
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of various bentonites on the mechanical properties and impermeability of cement mortars Liu Mengliang a, Hu Yang b, Lai Zhenyu a,⇑, Yan Tao a, He Xin a, Wu Jie a, Lu Zhongyuan a, Lv Shuzhen a a Southwest University of Science and Technology, School of Materials Science and Engineering, State Key Laboratory of Environmental-Friendly Energy Materials, Mianyang, Sichuan 621010, China b Sichuan Huashi Green Homeland Building Materials Co. Ltd., Chengdu, Sichuan 610081, China
h i g h l i g h t s Various bentonite on the properties of fresh cement mortar was studied. Magnesium bentonite has the best waterproof and impermeability of cement mortar. The mechanism of bentonite to the impermeability of cement mortar was discussed.
a r t i c l e
i n f o
Article history: Received 30 August 2019 Received in revised form 18 December 2019 Accepted 2 January 2020
Keywords: Bentonite Cement mortar Strength Impermeability
a b s t r a c t The durability of cement mortar depends mainly on its impermeability properties. In this study, three different types of bentonite—sodium bentonite (Na-bent), calcium bentonite (Ca-bent), and magnesium bentonite (Mg-bent)—were added to a cement mortar at different proportions to investigate their effect on the mortar strength and impermeability. The results show that the three types of bentonite provide a significant improvement in the performance of cement mortar, and with increasing bentonite content, this effect is increasingly obvious. For samples with a bentonite content of 10 wt%, the improvement in compressive strength, flexural strength, and permeability with Na-bent can reach 77.5%, 54.5%, and 115.7%, respectively; the corresponding improvements with Ca-bent can reach 62.2%, 47.9%, and 101.9%, while those with Mg-bent can reach 71.6%, 52.2%, and 137.3%, respectively. The waterproofing performance of cement mortar containing Mg-bent is the best, with a maximum impermeability pressure 2.37 times that of the reference group, followed by Na-bent, and finally Ca-bent; however, the lowest impermeability pressure is also 2.02 times that of the reference group. In general, the addition of bentonite improved the strength and impermeability of the cement mortar and has potential application value for improving the durability of cement mortars. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Cement mortar is made by mixing fine aggregate and water in a particular proportion, resulting in a heterogeneous and porous composite material. However, the pore structure of cement mortar can cause leakage in buildings, and the migration of water in the pore structure can introduce harmful ions such as CO2, Cl, and SO2 that can cause chemical damage to the structure [1–3]. 4 Reducing the permeability of the cement mortar is the primary method to solve this problem. Bentonite is currently widely used to improve the performance of cement mortars. Bentonite is a type of hydrated clay mineral ⇑ Corresponding author. E-mail address: [email protected] (Z. Lai). https://doi.org/10.1016/j.conbuildmat.2020.118015 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
consisting primarily of montmorillonite. Its chemical composition mainly includes silica, alumina, and water [4,5]. Based on the type and ratio of the interlayer ions of montmorillonite, bentonites can be divided into calcium bentonite (Ca-bent), sodium bentonite (Na-bent), and magnesium bentonite (Mg-bent) [6–11]. Because bentonite has excellent water swelling properties and is considered non-toxic and harmless, it can be added to mortar to fill the tiny pores in the mortar and reduce the migration of water in the pore structure, thus providing excellent waterproofing and impermeability characteristics [12–15]. Some researchers have prepared a humidity-adjustable mortar for the automatic adjustment of indoor humidity [16]. In addition, researchers have added bentonite to mortar and concrete by replacing part of the cement. The compressive strength of the replaced sample was similar to that of the original sample, and its sulfate attack resistance
2
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
was continuously improved, indicating that bentonite can be used as a low-cost auxiliary material [17–21]. Bentonite can also be applied as an impervious waterproofing material. Bentonite absorbs free water and swells in the mortar, which densifies the paste, thus reducing the porosity of the mortar; the formation of more calcium-silicate-hydrate (C-S-H) through the pozzolanic reaction between bentonite and calcium hydroxide is also beneficial for improving the impermeability of the mortar [22,23]. For example, in reservoir dams, concrete with high plasticity and low permeability can be obtained after incorporating bentonite [24,25]. Studies have shown that the addition of bentonite to the cement matrix used to cure radioactive waste and heavy metals can effectively reduce the leaching rate of the radionuclides and heavy metals [26–28]. While most of the above studies used bentonite, the chemical composition of bentonite in different regions varies, making it difficult to compare their results. Therefore, based on an investigation of the influence of bentonite on mortar performance, a follow-up study on the effects of different types of bentonite on mortar performance was proposed to facilitate the use of bentonite to improve the waterproofing performance of mortars [29]. In this paper, Na-bent, Ca-bent, Mg-bent, and quartz are mixed into mortars, and the flexural strength, compressive strength, and impermeability of the resulting mortars are measured. The influence of the bentonite type on the mortar properties was obtained by comparing the performance of the mortars, and the change in the microstructure of the mortars was also considered to explore the mechanism by which bentonite affects the waterproofing and impermeability of the mortar. 2. Experiment 2.1. Materials and procedures This study used ordinary Portland cement produced by the Lafarge Shuangma Cement Plant (Sichuan, China) with an average particle size of about 20 lm. The chemical composition is listed in Table 1. Na-bent, Ca-bent, and Mg-bent were supplied by the Shandong Weifang Shengshi Montana Stone Technology Co., Ltd. The average particle size is approximately 10 lm. The phase composition and chemical composition are presented in Fig. 1 and Table 1. The purity of the standard sand used is 98%, and the fineness modulus is 2.93. The quartz used is ground from ordinary quartz sand, and its average particle size is about 10 lm. In these experiments, the effects of different bentonite blends on the basic properties of ordinary Portland cement mortar mixtures were studied, considering Na-bent, Ca-bent, and Mg-bent, with quartz as a reference group. Cement mortars were prepared according to different dosages of bentonite and quartz. The mixing ratio of the mortar is cement:water: sand = 1:0.79:3.97. The amounts of bentonite or quartz added are 0 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%, and 10 wt% of the cement content. When preparing the samples, the cement, bentonite, and sand were first placed into a cement mortar mixer and dry mixed until uniform; water was then added and the mixture was stirred evenly. Samples were molded at an ambient temperature of 20 ± 3 and compacted with a vibrating table for 15 s. At the same time, all samples were covered with plastic sheets to prevent evaporation. After 24 h of curing, the mold was released, and these mortar specimens were cured in water at 20 ± 2 until the specified test age. 2.2. Test methods Three prisms with dimensions of 160 mm 40 mm 40 mm and 144 circular truncated cones with dimensions of 80 mm 70 mm 30 mm were prepared for each of the mixtures.
Fig. 1. Phase composition of bentonites.
The bentonite phases were determined by X-ray diffraction (PANalytical, X’Pert Pro, Netherlands; CuKa wavelength = 1.5406 nm, scanning step = 0.02 at 8°/min from 3° to 80°). According to the consistency testing method indicated in Chinese industry standard JGJ/T70-2009, the tip of the test cone of the mortar consistency meter and the surface of the mortar were recorded, the brake screw was loosened, and the screw was immediately tightened to record another reading in 10 s; the difference between the two readings is the consistency of the mortar. An NLD-3 cement mortar fluidity tester (Wuxi Jianyi Instrument & Machinery Co., Ltd.) was used to determine the fluidity of the mortar. The mixed mortar was quickly placed into the flow test mold in two layers. After flattening the surface, the test mold was opened and vibrated 30 times within 30 ± 1 s. The maximum diffusion diameter of the mortar was measured, and the diameter perpendicular to that was calculated; the average value was reported as the fluidity of the mortar. The flexural strength and compressive strength of the mortars were tested according to the Chinese building materials industry standard JC 474-2008. An SJS-1.5S digital mortar permeability meter (Wuxi Jianyi Machinery Co., Ltd.) was used to determine the permeability of the mortar. The cured samples were removed after the surface had dried, and the samples were sealed with a mixture of paraffin and grease and placed in a permeation apparatus for the water permeability test. The water pressure starts at a constant pressure of 0.2 MPa for 2 h, after which the pressure is increased by 0.1 MPa per hour. When three of the six test pieces exhibited water seepage, the test was stopped, and the corresponding water pressure value was recorded. The morphology of the bentonites and their influence on the morphology of the mortar hydration products were observed by scanning electron microscopy (SEM; MAIA3, TESCAN, Czech Republic) and energy-dispersive spectroscopy (EDS; OCTANE SUPER, AMETEK, USA). Measurement of the pore structure of the samples is based on mercury intrusion porosimetry (MIP) using a Micromeritics Autopore IV9500 MIP instrument (Micromeritics Instrument Co. Ltd., USA).
3. Results and discussion 3.1. Workability of cement mortars The effect of different types and amounts of bentonite on the workability of fresh mortar is shown in Fig. 2. The results show that the consistency and fluidity of the mortar decreased with an increasing amount of bentonite, which is consistent with the results reported in previous studies [30–32]. Comparing the results
Table 1 Chemical composition of bentonites and cement. Chemical Composition (%)
Ca-bent Na-bent Mg-bent Cement
SiO2
Al2O3
CaO
Fe2O3
MgO
K2O
TiO2
Na2O
SO3
other
64.32 62.62 66.29 19.26
18.24 16.46 16.70 4.33
7.13 5.92 5.38 65.46
3.8 3.26 3.15 3.06
3.68 3.50 6.34 1.60
1.74 1.26 1.14 0.77
0.4 0.34 0.35 –
0.31 6.31 0.33 0.13
0.03 0.05 – 4.45
0.34 0.27 0.33 0.94
3
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
absorption effect of quartz on the water is only due to its wetting effect on the surface, and thus it does not have a significant impact on the performance of the mortar. The water swelling effects of Cabent, Na-bent, and Mg-bent are shown in Fig. 3. It can be seen that Na-bent has the strongest water absorption, while the water absorption of Mg-bent and Ca-bent are similar and weaker than that of Na-bent. This is because the Na+ charge is low, and the force of the crystal layer is weak. Under hydration, the interlayer of Nabent can easily spread or even be peeled off, resulting in a strong swellability. In contrast, Mg2+ and Ca2+ have larger charges and the same size, and thus attraction effect between the layers is significant. As a result, the distance between the layers of Mg-bent and Ca-bent is small, and thus their ability to absorb water is limited. The difference in water swellability of the three bentonites resulted in a large loss of workability in the mortar containing Na-bent, while the effects of Mg-bent and Ca-bent were comparable and relatively small. It is worth noting that the consistency of the mortar with 10% Na-bent was reduced to 72 mm. If the dosage is increased further, it will cause difficulty in mortar preparation. 3.2. Strength of cement mortars
Fig. 2. Effect of bentonites on mortar performance: (a) consistency; (b) fluidity.
Na-bent
Ca-bent
Mg-bent
for the three types of bentonite, Na-bent had the most significant influence on the workability of the mortar. When 2, 4, 6, 8, and 10 wt% Na-bent was added to the mortar, its consistency decreased from 103 mm to 93, 95, 87, 79, and 72 mm, respectively; in other words, the maximum consistency decreased by 29.7% with the addition of 10 wt% Na-bent. The fluidity of the mortar with added 2, 4, 6, 8, and 10 wt% Na-bent decreased from 247 mm to 234, 240, 230, 221, and 207 mm, respectively, representing a decrease from the maximum value of 16.3%. The effect of adding Mg-bent on the mortar consistency was similar to that of Ca-bent. When the dosage was 10%, the mortar consistency was 80 mm and 83 mm, representing decreases of 22.3% and 19.4%, respectively. When the dosage was 6%, the effects of Mg-bent and Ca-bent on the fluidity of the mortar differed, but their effects were relatively similar under the other conditions. When the dosage was 10%, the fluidity of the mortar decreased to 221 mm and 225 mm with the addition of Mg-bent and Ca-bent, respectively, corresponding to decreases of 10.6% and 9.0%. Furthermore, quartz with the same particle size as the bentonite had little effect on the working performance of the fresh mortar; however, as the dosage increased, the consistency and fluidity also exhibited a decreasing trend. When the quartz dose was 10%, the consistency and fluidity decreased by 8.3% and 4.9%, respectively. Owing to the strong water absorption of bentonite, the bentonite will absorb water during mixing of the cement mortar, causing the amount of water in the fresh mortar to decrease, which results in decreased mortar consistency and fluidity. But the
The effect of different amounts of bentonite and quartz on the flexural strength of cement mortars is shown in Fig. 4. The results show that bentonite has a positive effect on the flexural strength of the mortar, and the overall trend shows that the flexural strength of the mortar gradually increases with increasing amount of bentonite in the mortar. Compared with the addition of quartz with the same particle size distribution, it can be found that the variation in the flexural strength of the quartz samples is small within the range of added contents in this study. Fig. 5 shows the effect of addition of 10 wt% Ca-bent, Na-bent, Mg-bent, and quartz on the flexural strength of the mortar. The results indicate that the 7-d flexural strengths of samples with 10 wt% addition of Ca-bent, Na-bent, Mg-bent, and quartz increased by 42.1%, 54.5%, 41.2%, and 21.8%, respectively, while the 28-d flexural strength increased by 47.9%, 54.5%, 52.2%, and 13.6%, respectively, compared to those of the blank sample. Thus, Na-bent had the most obvious effect on the flexural strength of the mortar. Moreover, the flexural strengths of the Mg-bent and
46mm 8mm
6mm
Fig. 3. Water swelling of bentonites.
4
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
Fig. 5. Effect of different bentonites on the flexural strength (10 wt% addition).
that of the Ca-bent mortar. This indicates that Mg-bent can more effectively promote the development of the late flexural strength of the mortar. The effect of different bentonite and quartz contents on the compressive strength of the mortar is shown in Fig. 6. The results show that the influence of bentonite addition on the compressive strength of mortars is similar to that of the flexural strength. With increasing bentonite content, the compressive strength of mortar also exhibits an increasing trend, while the amount of quartz added has a lesser effect on the compressive strength. The effect of different bentonites on the compressive strength of the mortar is shown in Fig. 7. When 10 wt% Ca-bent, Na-bent, Mgbent, and quartz are added to the mortar, the 7-d compressive strengths can be increased by 46.1%, 63.8%, 49.2%, and 20.3%, respectively, while the 28-d compressive strength can be increased by 62.2%, 77.5%, 71.6%, and 9.3%, respectively compared to the blank sample. This indicates that the bentonite has a positive effect on the development of the later strength of the mortar, and the effect of Mg-bent is most obvious. At the early age, the increase in the compressive strength with Mg-bent is close to that with Ca-bent; however, at 28 days, the strength of the Mg-bent mortar is significantly higher than that of the Ca-bent mortar. Of the three types of bentonites, Na-bent also has the most significant effect on improving the mortar compressive strength.
3.3. Impermeability of the cement mortar
Fig. 4. Effect of different cement replacement amounts on the flexural strength of mortar: (a) Ca-bent; (b) Na-bent; (c) Mg-bent; (d) quartz.
Ca-bent mortars are similar at 7 days; however, at 28 days, the flexural strength of the Mg-bent mortar is significantly higher than
The cement mortars were measured with a step-by-step pressure method to determine the waterproofing ability and impermeability of the mortars. According to the requirements of Chinese building materials industry standard JC 474-2008, mortar test blocks with dimensions of 70 mm 80 mm 30 mm were prepared for the mortar impermeability tests, and the impermeability pressure was tested after 28 d of standard curing. The impermeability pressure values for the mortar samples with the addition of bentonites and quartz are listed in Table 2. The results indicate that the impermeability pressure of the mortar increases by 9.8%, 19.6%, 54.9%, 68.6%, and 101.9% with the addition of 2, 4, 6, 8, and 10 wt% Ca-bent, respectively. Correspondingly, with the addition of Na-bent the impermeability pressure increases by 17.6%, 35.3%, 56.9%, 76.5%, and 115.7%, respectively, and with the addition of Mg-bent, the impermeability pressure increases by 35.3%, 43.1%, 80.4%, 115.7%, and 137.3%, respectively. In general, the effects of the three types of bentonite on the impermeability of the mortar after 28 d of curing are very apparent. The greater the amount of
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
5
Fig. 7. Effect of different bentonites on the mortar compressive strength (10 wt% addition).
minimum impermeability pressure of the mortar can be increased to more than double that of the reference group. Of the bentonites, Mg-bent has the best water resistance, followed by Na-bent, and finally Ca-bent. A linear regression analysis between the bentonite content and the impermeability pressure reveals that there is a good linear relationship between these parameters. As shown in Fig. 8, the linear relationship for Mg-bent is the strongest, with R2 = 0.9780; followed by Na-bent, R2 = 0.9699; and finally, calcium-based bentonite, R2 = 0.9437. These linear correlations provide good quantitative relationships between the amount of bentonite added and the impermeability of the mortar. The slope of the fitted line between the Mg-bent content and impermeability pressure is k1 = 0.0702; the slope of the fitted line for Na-bent is k2 = 0.0565; the slope of the fitted line for Ca-bent is k3 = 0.0525. This indicates that for the same proportion of bentonite added to the mortar, the impermeability of the mortar is increased to a certain extent, and the required amount of bentonite to achieve the same increase in impermeability follows the order Ca-bent > Nabent > Mg-bent. Therefore, the use of a smaller amount of Mgbent allows the mortar to achieve superior water resistance. For the reference group, the addition of quartz powder provides a certain enhancement of the impermeability of the mortar, but its effect is weaker than that of the bentonites. This indicates that the improvement in the impermeability of the bentonite mortars is not only due to the filling of pore spaces by the particles, but also due to the properties of the bentonite itself.
3.4. Microstructure of the cement mortar
Fig. 6. Effect of the bentonite or quartz content on the compressive strength of mortars: (a) Ca-bent; (b) Na-bent; (c) Mg-bent; (d) quartz.
bentonite added, the greater the improvement in the impermeability will be. When 10 wt% bentonite is added to the mortar, the
Scanning electron microscopy and EDS analyses were used to observe the morphology of bentonite in the hydration products and its influence on the morphology of the hydration products. It can be seen from Fig. 9 that there are many needle-like phases on the surface of Na-bent. EDS analysis reveals that these needlelike products may be a C-S-H gel, and the amorphous product on the surface of the bentonite is C-A-S-H gel, which has a higher content of Al and Si. The surface of bentonite has many unsaturated chemical bonds (Al-O or Si-O), and these chemical bonds can serve as origin sites for the growth of hydration products during the hydration process of cement, thus promoting the growth of hydration products there. The hydration products have an obvious refinement effect on the pores, and the needle-like hydration products on the surface can be oriented and grow along the direction of
6
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
Table 2 Impermeability pressure of mortars (MPa).
0% 2% 4% 6% 8% 10%
a
Ca-bent
Na-bent
Mg-bent
Quartz
0.51 0.56 0.61 0.79 0.86 1.03
0.51 0.60 0.69 0.80 0.90 1.10
0.51 0.69 0.73 0.92 1.10 1.21
0.51 0.53 0.55 0.56 0.61 0.63
Area2
Spot1 Area3 Area1
Fig. 8. Linear relationship between the bentonite content and impermeability pressure.
the pores. In addition, there is a phenomenon of overlapping with the surrounding hydration products, which has a specific effect on the strength of the mortar. It can be seen from Figs. 10 and 11 that there is no formation of needle-like hydration products on the surface of the Ca-bent or Mg-bent mortars; only amorphous hydration products can be observed on their surfaces. Owing to the larger ionic valences of Ca2+ and Mg2+, the attraction effect on the crystal layer is also significant, making it difficult for the crystal layers to be stretched, thus limiting the growth of hydration products there. In addition, it can be seen that the bentonite particles are located at the large pores in the hydration product, allowing the particles to effectively fill the pores, thereby hindering the transport of water in the pores and enhancing the impermeability and strength of the mortar. Fig. 12 shows that the quartz particles can also act to fill the pores in the mortar matrix. As the quartz is inert, it will not participate in the hydration process of the cement, and thus there is no formation or enrichment of the hydration product, and the improvement in the mortar strength and impermeability is also weak. For the needle-like products around quartz particles, the results of a large number of literature investigation and preliminary experiments show that it is mainly ettringite formed by cement hydration [33–36]. 3.5. Pore structure of the cement mortars The impermeability and strength properties of cement-based materials are mainly related to the pore volume and pore structure characteristics of the material. MIP was used to determine changes in the pores inside the mortar. The size of the pores and the pressure at which the mercury is injected can be calculated with Eq. (1); the volume of mercury injected can represent the volume of the pores.
D¼
4ccosh P
ð1Þ
Fig. 9. Micromorphology of Na-bent in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
where D is the equivalent pore diameter, c is the surface tension of mercury, h is the contact angle between the mercury and solid, and P is the pressure required for mercury injection. The MIP tests were carried out after the samples had cured for 28 d. The pore size distribution is shown in Fig. 13, and the porosity of the mortars is listed in Table 3. Fig. 13(a) shows that Ca-bent decreases the integral curve of the mortar pore volume with the pore diameter, and the greater the amount of Ca-bent added, the more obvious this downward trend will be, which indicates a reduction in the pores in the mortar and a smaller pore size. When the content of Ca-bent was 4 wt% and 8 wt%, the porosity of the mortar decreased from 27.16% to 25.63% and 24.59%, respectively. The main peak of the differential curve is shifted slightly to the left, which also indicates that the pore structure of the mortar has been refined to some extent. From the above analysis of the microstructure of bentonite in the mortar, it can be found that the surface of Ca-bent can generate more amorphous hydration products, which reduces the pore space in the mortar, and thus the strength of the mortar containing Ca-bent is higher than that of the reference group. Fig. 13(b) shows that when Na-bent is added to the mortar, the integral curve of the pore volume with pore size also exhibits a downward trend, and the extent of the decrease is larger than that with Ca-bent, indicating that Na-bent can more effectively reduce the porosity of the mortar and refine the pore size. The porosity of
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
a
Fig. 10. Micromorphology of Ca-bent in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
the mortars mixed with 4 wt% and 8 wt% Na-bent decreased from 27.16% to 24.87% and 22.58%, respectively. At the same time, the leftward shift of the main peak of the differential curve is much larger with Na-bent than Ca-bent. This also indicates that Na-bent has a stronger effect on the pore refinement of the mortar. From the analysis of the micromorphology, Na-bent in the mortar matrix can not only form amorphous hydration products on the mortar surface, but can also form acicular hydration products oriented toward the pores. This causes the porosity of the Na-bent mortar to decreases more significantly than that of the Ca-bent mortar, and thus the effect of Na-bent on the mortar strength is also greater than that of Ca-bent. Fig. 13(c) that the variation in the integral curve of the pore volume for Mg-bent mortar is similar to that of the Na-bent mortar, indicating that Mg-bent can also effectively reduce the porosity of the mortar and refine the pore structure. When 4 wt% and 8 wt% Mg-bent is added to the mortar, the porosity of the mortar can be reduced from 27.16% to 25.11% and 22.81%, respectively, which is equivalent to the results obtained with addition of Nabent. Analysis of the micromorphology of Mg-bent in the mortar indicates that the microstructure of Mg-bent is similar to that of Ca-bent; hydrated products are only formed on the surface, and there is no formation of acicular hydration products along the pores to reduce the porosity of the mortar further. However, Mg2+ between the Mg-bent layers forms Mg(OH)2 crystals around
7
a
Fig. 11. Micromorphology of Mg-bent in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
the bentonite through ion exchange, and ability of these crystals to fill the pores causes the Mg-bent mortar to also have a low porosity[37,38]. This is a reasonable explanation for the above trends in the effects of different types of bentonite on the mortar strength and impermeability. The effects of Mg-bent and Na-bent on the porosity of the mortar are similar, and thus their strength properties at the 28-d age are also similar. However, at 7 d of aging, the compressive strength and flexural strength of Mg-bent are similar to that of Ca-bent and significantly weaker than that of Nabent, which may be due to the slow exchange between Mg2+ in Mg-bent and Ca2+ in the hydration product pore solution. There is not a large amount of Mg(OH)2 crystal formation at 7 d of aging. In addition, because the surface of Mg-bent is similar to that of Cabent, the formation of hydration products is less than that of Nabent, and the porosity is larger than Na-bent; thus, the strength of the Mg-bent mortar is lower at 7 d. In the later process, with the progress of ion exchange and the formation of Mg(OH)2 crystals, the porosity of the mortar containing Mg-bent decreases rapidly, while the strength performance in the later stage increases rapidly. Although the porosity of the mortars containing Mg-bent and Na-bent is similar, the impermeability pressure of the Mgbent mortar is higher than that of the Na-bent mortar, mainly due to the formation of flake-like Mg(OH)2 crystals, which can increase the degree of tortuosity of the water permeation path in the pores of the mortar.
8
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
a
Fig. 12. Micromorphology of quartz in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
Fig. 13(d) shows that after incorporating quartz with a similar particle size to bentonite, the pore volume of the mortar changes little with the change in the pore diameter, indicating that the influence of quartz particles on the porosity of the mortar is small. After the incorporation of 4 wt% and 8 wt% quartz, the porosity of the mortar is 27.54% and 26.72%, respectively, which is very close to the 27.16% of the blank group. At a quartz content of 4 wt%, the porosity even increased slightly, although this increase of only 0.38% may be the result of instrumental errors. It can be seen from the differential curve that quartz has a certain refinement effect on the pore structure of the mortar. The permeability resistance of cement-based materials is determined not only by the overall porosity, but also by the characteristics of the pore structure. The addition of bentonite leads to the refinement of the pore structure, but is represented by a differential curve of the pore volume as a function of pore size. There is no specific data to describe this phenomenon. To understand the refinement effect of bentonite on the pore structure of the mortar more clearly, the full pore size range in the mortar (the pore size range of the MIP instrument used in the analysis is 6 nm to 360 lm) is divided into four different pore sizes categories: 6– 50 nm are classified as harmless or less harmful; 50–200 nm are harmful pores; 0.2–1 lm and 1–360 lm are also harmful pores. The volume fraction of pores (c) is also used to express the proportion of pores with different pore sizes in the whole pore size range.
Fig. 13. Pore size distribution of mortars: (a) Na-bent; (b) Ca-bent; (c) Mg-bent; (d) quartz.
The variation in c is used to understand the effect of bentonite on the refinement of mortar pores. The effects of different types of
9
M. Liu et al. / Construction and Building Materials 241 (2020) 118015 Table 3 Mortar porosity (%). Adding content (wt%)
Ca-bent
Na-bent
Mg-bent
Quartz
0 4 8
27.16 25.63 24.59
27.16 24.87 22.58
27.16 25.11 22.81
27.16 27.54 26.72
bentonite and similarly sized quartz particles on the volume fraction (c) of pores with different pore sizes in the mortar is shown in Fig. 14. Fig. 14 indicates that the effect of bentonite on the refinement of the pores of the mortar is mainly reflected in a decrease in macropores and increase in small pores. When 8 wt% Ca-bent, Na-bent, Mg-bent, and quartz were added to the mortar, the pore volume fraction, c6–50 nm, of the less harmful or harmless pores in the mortar increased by 12.3%, 17.5%, 11.5%, and 10.8%, respectively, at 28 d of curing; the pore volume fraction of harmful pores, c50–200 nm, changed little; the pore volume fraction of the multipore pores with sizes of 0.2–1 lm, c0.2–1 lm, increased by 6.4%, 13.3%, 18.1%, and 0.1%, respectively; and the volume fraction of the multi-pore pores with sizes of 1–360 lm, c1–360 lm, decreased by 17.5%, 29.6%, 28.2%, and 9.1%, respectively. The pores in the cement hydration products are caused by the evaporation of free water during the hydration process. Because bentonite has a strong water absorption capacity, its water absorption during the mortar mixing process reduces the amount of free water in the fresh mortar, thereby reducing the gap between the cement particles during the hydration process. As a result, the pores in the hydration product are smaller, and the data shows an increase in the c6–50 nm fraction and a decrease in the c1–360 lm fraction. In addition, through the above description of the micromorphology of bentonite in the mortar, it can be seen that the bentonite will release absorbed water during hydration of the mortar to form pores of approximately 1 lm at the junction of the bentonite and the matrix. Thus, the analysis of the change in the pore volume fractions of different pore sizes has also been effectively verified. With the incorporation of bentonite, the c0.2–1 lm fraction exhibits a certain increase. Na-bent has the best water absorption capacity, and its effect on the refinement of the pores of the mortar is also the most significant. The pore volume fraction of the less harmful or harmless pores, c6–50 nm, is the largest in the Na-bent mortar, and thus
Fig. 14. Volume fraction of pores with different pore sizes in the mortars (8 wt% addition).
the permeability resistance of the mortar can improve significantly. Ca-bent has a weak water absorption capacity, and thus its ability to refine the pores of the mortar is also weak. The increase in the c6–50 nm fraction is only 12.3% with Ca-bent, and the increase in the corresponding c0.2–1 lm fraction is also low, only 6.4%. The water absorption capacity of Mg-bent is comparable to that of Ca-bent, and thus the increase in the c6– 50 nm fraction of the Mg-bent mortar is also only 11.5%. However, the c0.2–1 lm fraction of the mortar doped with Mg-bent is larger than that with the other types of bentonite, indicating that a 2– 3 lm sheet of Mg(OH)2 crystals is formed in the macropores around the bentonite, and thus the macropores are refined into pores with sizes between 0.2 lm and 1 lm. Quartz is inert during the hydration process of the mortar, but because its particles can fill the pores in the mortar, it can convert a small number of harmful pores with sizes of 1–360 lm into less harmful or harmless pores with sizes of 6–50 nm, thereby increasing the impermeability pressure of the mortar. It can be seen that the bentonite particles of the same size also have the function of filling the pores in the mortar to achieve a degree of refinement. In addition, it is worth noting that the pore volume fraction of the harmless or less harmful pores of the mortar, c6–50 nm, after adding quartz, Ca-bent, and Mg-bent with equal grain sizes is 25.4%, 26.8%, and 26.0%, respectively; the three are almost equal. However, the impermeability pressure pressures of Ca-bent and Mg-bent are much higher than that of quartz with an equal particle size. In particular, the highest impermeability pressure of Mg-bent is 1.92 times that of the quartz mortar sample with the same dosage.
3.6. Mechanism for the effects of bentonite on the cement mortar Through the discussion of a series of macroscopic properties of bentonite mortars combined with the influence of bentonite on the microstructure of mortar hydration products, the following mechanism by which bentonite improves the waterproofing effect of cementitious materials has been proposed. This section will explain the waterproofing mechanism of bentonite from two perspectives: the influence of bentonite on the hydration process of cement-based materials, and the effect of bentonite when water passes through the pores. Fig. 15 shows the effect of bentonite on the hydration of the mortar and the infiltration of water into the pores. As Fig. 15 shows, the bentonite in the fresh mortar can absorb some of the free water, thus expanding the volume of the bentonite. At the same time, the gap between the unhydrated cement particles and the amount of free water are also reduced relative to the unblended bentonite, which leads to smaller pore sizes in the formed hydration products, expressed as the harmless or less harmful pores(Fig. 14). In addition, the surface of the bentonite in the cement hydration products can generate a large number of amorphous or needle-like hydration products, which can reduce the pore space in the matrix, thereby enhancing the impermeability to water of the cement-based material. The water-expanded bentonite gradually releases the absorbed water as the humidity in the mortar decreases, causing the volume of the bentonite to shrink, and forming micron-sized harmful pores between the bentonite and the matrix. However, when external water penetrates
10
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
(a)
(b)
Pore
Free water between particles
Cement granule (d)
(c)
Outside water
Hydrated slurry (e)
(f)
Quartz particles
(g)
(h)
Bentonite surface hydration product
(i)
Swelling bentonite Bentonite in hydration products
Bentonite expands again with water
Fig. 15. Waterproofing mechanism of bentonite: (a) (b) (c) reference mortar; (d) (e) (f) mortar mixed with quartz; (g) (h) (i) mortar mixed with bentonite.
(a)
(b)
(c)
Mg(OH)2 Fig. 16. Waterproofing mechanism of modified bentonite: (a) fresh mortar; (b) mortar mixed with Mg-bentonite; (c) water penetration process in pores.
the matrix, the bentonite can re-expand to fill the surrounding pores, thereby effectively blocking the passage of water through the pores. As mentioned above, Ca-bent can significantly improve the waterproofing performance of the mortar compared to the quartz particles with an equal diameter. However, by comparing the volume fractions of pores with different pore sizes in the quartz and Ca-bent mortars, it can be found that the pore volume fraction of the harmless or less harmful pores is similar. The difference is that the 0.2–1 lm pores in the Ca-bent mortar increase, and because these pores are classified as harmful pores, the improvement in the penetration resistance of the cement-based material does not have a positive effect or even a negative effect. Therefore, the waterproofing mechanism described above for the reexpansion of bentonite in the mortar provides a more appropriate interpretation of the observed phenomena. From this, it can be concluded that the improvement in the waterproofing and impermeability of cement-based materials by bentonite is mainly caused by the swelling of the bentonite itself. For Na-bent, because of its stronger water absorption (Fig. 3), it has a better refining effect on the pores after hydration and can convert more harmful macropores into harmless or less harmful pores. In addition, owing to the looser surface of Na-bent particles, more nucleation sites can be provided for the growth of surface
hydration products; the more hydrated products are formed, the lower the porosity of the matrix will be(Fig. 9). These two factors cause Na-bent to have a better influence on the waterproofing and impermeability of mortar than Ca-bent. In addition, because quartz is inert and cannot expand, it can only enhance the impermeability of the mortar by the act of particles filling the pores (Fig. 12). For Mg-bent, its swellability is similar to that of Ca-bent, but its improvement in the anti-penetration performance of the mortar is the best among the three bentonites (Table 2, Fig. 8). The effect of Mg-bent on the hydration of cement-based materials and the penetration of water into the pores is shown in Fig. 16. Owing to the ion exchange of Mg2+ with Ca2+ in the pore solution of the hydration products, Mg2+ is exchanged between the layers of Mg-bent. Thus, Mg(OH)2 crystals are formed under highly alkaline conditions, which generally occur in the form of flakes near the bentonite to create the micromorphology shown in Figs. 16(b), 11. When external water passes through the pores, the Mg-bent not only has the water-swelling and waterproofing properties described above, but the formed Mg(OH)2 crystals can also reduce the pores and increase the tortuosity of the water permeation path. Therefore, mortar mixed with Mg-bent can achieve excellent water resistance under the dual action of Mg-bent and the formed Mg
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
(OH)2 crystals. Recently, studies have also shown that there is a pozzolanic reaction between bentonite and calcium hydroxide [39–41]. The effects of bentonite on the cement mortar were not only the waterproof mechanism mentioned but also the pozzolanic reaction consumes part of calcium hydroxide, which generated more C-S-H and made the matrix more compact. 4. Conclusion By comparing the effects of different types of bentonite and quartz on the workability, strength properties, and impermeability of cement mortars, combined with consideration of the changes in the microstructure and pore structure of the mortars, the following conclusions could be drawn: (1) Bentonite has a significant influence on the workability of the mortar. The main result is that the consistency and fluidity of the fresh mortar decrease with increasing bentonite content; the loss of mortar fluidity is mainly caused by the absorption of water by bentonite. (2) The addition of bentonite or quartz with an equal grain size can increase the flexural strength and compressive strength of the mortar; the larger the added amount of bentonite or quartz, the more obvious the increase will be. (3) The mortar made with Mg-bent has the best impermeability, and the maximum impermeability pressure can reach 2.37 times that of the blank group. The impermeability pressure of the mortar generally increases with increasing bentonite content and exhibits a strong linear relationship. (4) Bentonite can refine the pore structure of the mortar; the increase in micron-sized pores between 0.2 lm and 1 lm is a characteristic unique to the incorporation of bentonite. It is worth noting that although Na-bent can provide the mortar with better impermeability, it has a significant influence on the workability of the fresh mortar, and Mg-bent can achieve an excellent waterproofing effect with small fluid loss of the mortar. Author contributions Lai Zhenyu developed the idea for the study, Liu Mengliang and Hu Yang analysed most of the data, and wrote the initial draft of the paper. The other authors contributed to refining the ideas, carrying out additional analyses and finalizing this paper. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the projects supported by the National Key Research and Development Plan (2016YFC0701004), Sichuan Science and Technology Program (No.2019ZDZX0024), Open Fund of Southwest University of Science and Technology (17FKSY0108, 19FKSY05). References [1] J.Y. Yoo, H.S. Lee, Y.J. Kim, Experimental study on the water penetration into mortar under water pressure condition, Key Eng. Mater. 385–387 (2008) 681–684. [2] Y. Farnam, C. Villani, T. Washington, M. Spence, J. Jain, W. Jason Weiss, Performance of carbonated calcium silicate based cement pastes and mortars exposed to NaCl and MgCl2 deicing salt, Constr. Build. Mater. 111 (2016) 63– 71.
11
[3] J.M. Loche, A. Ammar, P. Dumargue, Influence of the migration of chloride ions on the electrochemical impedance spectroscopy of mortar paste, Cem. Concr. Res. 35 (9) (2005) 1797–1803. [4] B. Meng, Q. Guo, X. Men, S. Ren, W. Jin, B. Shen, Preparation of modified bentonite by polyhedral oligomeric silsesquioxane and sodium dodecyl sulfate in aqueous phase and its adsorption property, Mater. Lett. 253 (2019) 71–73. [5] Additives EPo, Products or Substances used in Animal F. Scientific Opinion on the efficacy of Bentonite (dioctahedral montmorillonite) for all species. EFSA J 2011;9(6):2276. [6] M. Segad, B. Jonsson, T. Akesson, B. Cabane, Ca/Na montmorillonite: structure, forces and swelling properties, Langmuir 26 (8) (2010) 5782–5790. [7] M.H. Köster, L.B. Williams, P. Kudejova, H.A. Gilg, The boron isotope geochemistry of smectites from sodium, magnesium and calcium bentonite deposits, Chem. Geol. 510 (2019) 166–187. [8] G. Sposito, K.M. Holtzclaw, L. Charlet, C. Jouany, A.L. Page, Sodium-calcium and sodium-magnesium exchange on wyoming bentonite in perchlorate and chloride background ionic media, Soil Sci. Soc. Am. J. 47 (1983) 51–56. [9] L. Liu, Prediction of swelling pressures of different types of bentonite in dilute solutions, Colloids Surf., A 434 (2013) 303–318. [10] M. Muzakky, C. Supriyanto, Modification of three types of bentonite with zirconium oxide chloride (ZOC) of local products using intercalation process, Indonesian J. Chem. 16 (2016) 14–19. [11] R. Fernandez, J. Cuevas, U.K. Mader, Modelling concrete interaction with a bentonite barrier, Eur. J. Mineral. 21 (1) (2009) 177–191. [12] S. Andrejkovicˇová, C. Alves, A. Velosa, F. Rocha, Bentonite as a natural additive for lime and lime–metakaolin mortars used for restoration of adobe buildings, Cem. Concr. Compos. 60 (2015) 99–110. [13] P. Leroy, A. Revil, D. Coelho, Diffusion of ionic species in bentonite, J. Colloid Interface Sci. 296 (1) (2006) 248–255. [14] H.H. Lee, C.W. Wang, experimental study on cement mortar with bentonite, Adv. Mater. Res. 671–674 (2013) 1741–1744. [15] A.K. Parande, B. Ramesh Babu, M. Aswin Karthik, K.K. Deepak Kumaar, N. Palaniswamy, Study on strength and corrosion performance for steel embedded in metakaolin blended concrete/mortar, Constr. Build. Mater. 22 (3) (2008) 127–134. [16] M. Li, Z. Wu, Preparation, characterization, and humidity-control performance of organobentonite/sodium polyacrylate mortar, J. Macromol. Sci. Part B 51 (8) (2012) 1647–1657. [17] J. Mirza, M. Riaz, A. Naseer, F. Rehman, A.N. Khan, Q. Ali, Pakistani bentonite in mortars and concrete as low cost construction material, Appl. Clay Sci. 45 (4) (2009) 220–226. [18] S. Ahmad, S.A. Barbhuiya, A. Elahi, J. Iqbal, Effect of Pakistani bentonite on properties of mortar and concrete, Clay Miner. 46 (1) (2018) 85–92. [19] J.P. Gonçalves, L.M. Tavares, R.D. Toledo Filho, E.M.R. Fairbairn, Performance evaluation of cement mortars modified with metakaolin or ground brick, Constr. Build. Mater. 23 (5) (2009) 1971–1979. [20] N. Mesboua, K. Benyounes, A. Benmounah, S.K. Shukla, Study of the impact of bentonite on the physico-mechanical and flow properties of cement grout, Cogent Eng. 5 (1) (2018). [21] I. Garcia-Lodeiro, N. Boudissa, A. Fernandez-Jimenez, A. Palomo, Use of clays in alkaline hybrid cement preparation. The role of bentonites, Mater. Lett. 233 (2018) 134–137. [22] F. Raúl, A.I. Ruiz, J. Cuevas, Formation of C-A-S-H phases from the interaction between concrete or cement and bentonite, Clay Miner. 51 (2) (2018) 223– 235. [23] J. Wei, B. Gencturk, Hydration of ternary Portland cement blends containing metakaolin and sodium bentonite, Cem. Concr. Res. 123 (2019) 105772. [24] M.A. Fadaie, M. Nekooei, P. Javadi, Effect of dry and saturated bentonite on plastic concrete, KSCE J. Civ. Eng. 23 (8) (2019) 3431–3442. [25] A.T. Amlashi, S.M. Abdollahi, S. Goodarzi, A.R. Ghanizadeh, Soft computing based formulations for slump, compressive strength, and elastic modulus of bentonite plastic concrete, J. Cleaner Prod. 230 (2019) 1197–1216. [26] I. Plecas, R. Pavlovic, S. Pavlovic, Leaching behavior of 60Co and 137Cs from spent ion exchange resins in cement–bentonite clay matrix, J. Nucl. Mater. 327 (2) (2004) 171–174. [27] I. Sato, K. Maeda, M. Suto, M. Osaka, T. Usuki, S. Koyama, Penetration behavior of water solution containing radioactive species into dried concrete/mortar and epoxy resin materials, J. Nucl. Sci. Technol. 52 (4) (2015) 580–587. [28] M. Katsioti, N. Katsiotis, G. Rouni, D. Bakirtzis, M. Loizidou, The effect of bentonite/cement mortar for the stabilization/solidification of sewage sludge containing heavy metals, Cem. Concr. Compos. 30 (10) (2008) 1013–1019. [29] H. Yang, D. Long, L. Zhenyu, H. Yuanjin, Y. Tao, H. Xin, et al., Effects of bentonite on pore structure and permeability of cement mortar, Constr. Build. Mater. 224 (2019) 276–283. [30] C.H. Yu, G.H. Li, J.L. Gao, B. Lan, Q. Wei, D.Z. Xu, Effect of bentonite on the performance of the limestone manufactured-sand mortar, Appl. Mech. Mater. 357–360 (2013) 1374–1378. [31] A. Kaci, M. Chaouche, P.A. Andréani, Influence of bentonite clay on the rheological behaviour of fresh mortars, Cem. Concr. Res. 41 (4) (2011) 373– 379. [32] B. Abu-Jdayil, Rheology of sodium and calcium bentonite–water dispersions: effect of electrolytes and aging time, Int. J. Miner. Process. 98 (3) (2011) 208– 213. [33] R. Komatsu, N. Mizukoshi, K. Makida, K. Tsukamoto, In-situ observation of ettringite crystals, J. Cryst. Growth 311 (3) (2009) 1005–1008.
12
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
[34] S. Luo, M. Liu, L. Yang, J. Chang, Effects of drying techniques on the crystal structure and morphology of ettringite, Constr. Build. Mater. 195 (2019) 305–311. [35] P.K. Mehta, Scanning electron micrographic studies of ettringite formation, Cem. Concr. Res. 6 (2) (1976) 169–182. [36] K. Ogawa, D.M. Roy, C4A3S hydration, ettringite formation, and its expansion mechanism: II. Microstructural observation of expansion, Cem. Concr. Res. 12 (1) (1982) 101–109. [37] T.S. Qureshi, A. Kanellopoulos, A. Al-Tabbaa, Encapsulation of expansive powder minerals within a concentric glass capsule system for self-healing concrete, Constr. Build. Mater. 121 (2016) 629–643. [38] T. Qureshi, A. Kanellopoulos, A. Al-Tabbaa, Autogenous self-healing of cement with expansive minerals-I: impact in early age crack healing, Constr. Build. Mater. 192 (2018) 768–784.
ˇ e˙sniene˙, A. Baltušnikas, I. Lukošiu te˙, K. Brinkiene˙, R. Kalpokaite˙-Dicˇkuviene˙, [39] J. C Influence of organoclay structural characteristics on properties and hydration of cement pastes, Constr. Build. Mater. 166 (2018) 59–71. [40] J. Calabria-Holley, S. Papatzani, B. Naden, J. Mitchels, K. Paine, Tailored montmorillonite nanoparticles and their behaviour in the alkaline cement environment, Appl. Clay Sci. 143 (2017) 67–75. [41] J. Jiang, Z. Lu, J. Li, Y. Xie, K. Luo, Y. Niu, Preparation and properties of nanopore-rich lightweight cement paste based on swelled bentonite, Constr. Build. Mater. 199 (2019) 72–81.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of various bentonites on the mechanical properties and impermeability of cement mortars Liu Mengliang a, Hu Yang b, Lai Zhenyu a,⇑, Yan Tao a, He Xin a, Wu Jie a, Lu Zhongyuan a, Lv Shuzhen a a Southwest University of Science and Technology, School of Materials Science and Engineering, State Key Laboratory of Environmental-Friendly Energy Materials, Mianyang, Sichuan 621010, China b Sichuan Huashi Green Homeland Building Materials Co. Ltd., Chengdu, Sichuan 610081, China
h i g h l i g h t s Various bentonite on the properties of fresh cement mortar was studied. Magnesium bentonite has the best waterproof and impermeability of cement mortar. The mechanism of bentonite to the impermeability of cement mortar was discussed.
a r t i c l e
i n f o
Article history: Received 30 August 2019 Received in revised form 18 December 2019 Accepted 2 January 2020
Keywords: Bentonite Cement mortar Strength Impermeability
a b s t r a c t The durability of cement mortar depends mainly on its impermeability properties. In this study, three different types of bentonite—sodium bentonite (Na-bent), calcium bentonite (Ca-bent), and magnesium bentonite (Mg-bent)—were added to a cement mortar at different proportions to investigate their effect on the mortar strength and impermeability. The results show that the three types of bentonite provide a significant improvement in the performance of cement mortar, and with increasing bentonite content, this effect is increasingly obvious. For samples with a bentonite content of 10 wt%, the improvement in compressive strength, flexural strength, and permeability with Na-bent can reach 77.5%, 54.5%, and 115.7%, respectively; the corresponding improvements with Ca-bent can reach 62.2%, 47.9%, and 101.9%, while those with Mg-bent can reach 71.6%, 52.2%, and 137.3%, respectively. The waterproofing performance of cement mortar containing Mg-bent is the best, with a maximum impermeability pressure 2.37 times that of the reference group, followed by Na-bent, and finally Ca-bent; however, the lowest impermeability pressure is also 2.02 times that of the reference group. In general, the addition of bentonite improved the strength and impermeability of the cement mortar and has potential application value for improving the durability of cement mortars. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Cement mortar is made by mixing fine aggregate and water in a particular proportion, resulting in a heterogeneous and porous composite material. However, the pore structure of cement mortar can cause leakage in buildings, and the migration of water in the pore structure can introduce harmful ions such as CO2, Cl, and SO2 that can cause chemical damage to the structure [1–3]. 4 Reducing the permeability of the cement mortar is the primary method to solve this problem. Bentonite is currently widely used to improve the performance of cement mortars. Bentonite is a type of hydrated clay mineral ⇑ Corresponding author. E-mail address: [email protected] (Z. Lai). https://doi.org/10.1016/j.conbuildmat.2020.118015 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
consisting primarily of montmorillonite. Its chemical composition mainly includes silica, alumina, and water [4,5]. Based on the type and ratio of the interlayer ions of montmorillonite, bentonites can be divided into calcium bentonite (Ca-bent), sodium bentonite (Na-bent), and magnesium bentonite (Mg-bent) [6–11]. Because bentonite has excellent water swelling properties and is considered non-toxic and harmless, it can be added to mortar to fill the tiny pores in the mortar and reduce the migration of water in the pore structure, thus providing excellent waterproofing and impermeability characteristics [12–15]. Some researchers have prepared a humidity-adjustable mortar for the automatic adjustment of indoor humidity [16]. In addition, researchers have added bentonite to mortar and concrete by replacing part of the cement. The compressive strength of the replaced sample was similar to that of the original sample, and its sulfate attack resistance
2
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
was continuously improved, indicating that bentonite can be used as a low-cost auxiliary material [17–21]. Bentonite can also be applied as an impervious waterproofing material. Bentonite absorbs free water and swells in the mortar, which densifies the paste, thus reducing the porosity of the mortar; the formation of more calcium-silicate-hydrate (C-S-H) through the pozzolanic reaction between bentonite and calcium hydroxide is also beneficial for improving the impermeability of the mortar [22,23]. For example, in reservoir dams, concrete with high plasticity and low permeability can be obtained after incorporating bentonite [24,25]. Studies have shown that the addition of bentonite to the cement matrix used to cure radioactive waste and heavy metals can effectively reduce the leaching rate of the radionuclides and heavy metals [26–28]. While most of the above studies used bentonite, the chemical composition of bentonite in different regions varies, making it difficult to compare their results. Therefore, based on an investigation of the influence of bentonite on mortar performance, a follow-up study on the effects of different types of bentonite on mortar performance was proposed to facilitate the use of bentonite to improve the waterproofing performance of mortars [29]. In this paper, Na-bent, Ca-bent, Mg-bent, and quartz are mixed into mortars, and the flexural strength, compressive strength, and impermeability of the resulting mortars are measured. The influence of the bentonite type on the mortar properties was obtained by comparing the performance of the mortars, and the change in the microstructure of the mortars was also considered to explore the mechanism by which bentonite affects the waterproofing and impermeability of the mortar. 2. Experiment 2.1. Materials and procedures This study used ordinary Portland cement produced by the Lafarge Shuangma Cement Plant (Sichuan, China) with an average particle size of about 20 lm. The chemical composition is listed in Table 1. Na-bent, Ca-bent, and Mg-bent were supplied by the Shandong Weifang Shengshi Montana Stone Technology Co., Ltd. The average particle size is approximately 10 lm. The phase composition and chemical composition are presented in Fig. 1 and Table 1. The purity of the standard sand used is 98%, and the fineness modulus is 2.93. The quartz used is ground from ordinary quartz sand, and its average particle size is about 10 lm. In these experiments, the effects of different bentonite blends on the basic properties of ordinary Portland cement mortar mixtures were studied, considering Na-bent, Ca-bent, and Mg-bent, with quartz as a reference group. Cement mortars were prepared according to different dosages of bentonite and quartz. The mixing ratio of the mortar is cement:water: sand = 1:0.79:3.97. The amounts of bentonite or quartz added are 0 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%, and 10 wt% of the cement content. When preparing the samples, the cement, bentonite, and sand were first placed into a cement mortar mixer and dry mixed until uniform; water was then added and the mixture was stirred evenly. Samples were molded at an ambient temperature of 20 ± 3 and compacted with a vibrating table for 15 s. At the same time, all samples were covered with plastic sheets to prevent evaporation. After 24 h of curing, the mold was released, and these mortar specimens were cured in water at 20 ± 2 until the specified test age. 2.2. Test methods Three prisms with dimensions of 160 mm 40 mm 40 mm and 144 circular truncated cones with dimensions of 80 mm 70 mm 30 mm were prepared for each of the mixtures.
Fig. 1. Phase composition of bentonites.
The bentonite phases were determined by X-ray diffraction (PANalytical, X’Pert Pro, Netherlands; CuKa wavelength = 1.5406 nm, scanning step = 0.02 at 8°/min from 3° to 80°). According to the consistency testing method indicated in Chinese industry standard JGJ/T70-2009, the tip of the test cone of the mortar consistency meter and the surface of the mortar were recorded, the brake screw was loosened, and the screw was immediately tightened to record another reading in 10 s; the difference between the two readings is the consistency of the mortar. An NLD-3 cement mortar fluidity tester (Wuxi Jianyi Instrument & Machinery Co., Ltd.) was used to determine the fluidity of the mortar. The mixed mortar was quickly placed into the flow test mold in two layers. After flattening the surface, the test mold was opened and vibrated 30 times within 30 ± 1 s. The maximum diffusion diameter of the mortar was measured, and the diameter perpendicular to that was calculated; the average value was reported as the fluidity of the mortar. The flexural strength and compressive strength of the mortars were tested according to the Chinese building materials industry standard JC 474-2008. An SJS-1.5S digital mortar permeability meter (Wuxi Jianyi Machinery Co., Ltd.) was used to determine the permeability of the mortar. The cured samples were removed after the surface had dried, and the samples were sealed with a mixture of paraffin and grease and placed in a permeation apparatus for the water permeability test. The water pressure starts at a constant pressure of 0.2 MPa for 2 h, after which the pressure is increased by 0.1 MPa per hour. When three of the six test pieces exhibited water seepage, the test was stopped, and the corresponding water pressure value was recorded. The morphology of the bentonites and their influence on the morphology of the mortar hydration products were observed by scanning electron microscopy (SEM; MAIA3, TESCAN, Czech Republic) and energy-dispersive spectroscopy (EDS; OCTANE SUPER, AMETEK, USA). Measurement of the pore structure of the samples is based on mercury intrusion porosimetry (MIP) using a Micromeritics Autopore IV9500 MIP instrument (Micromeritics Instrument Co. Ltd., USA).
3. Results and discussion 3.1. Workability of cement mortars The effect of different types and amounts of bentonite on the workability of fresh mortar is shown in Fig. 2. The results show that the consistency and fluidity of the mortar decreased with an increasing amount of bentonite, which is consistent with the results reported in previous studies [30–32]. Comparing the results
Table 1 Chemical composition of bentonites and cement. Chemical Composition (%)
Ca-bent Na-bent Mg-bent Cement
SiO2
Al2O3
CaO
Fe2O3
MgO
K2O
TiO2
Na2O
SO3
other
64.32 62.62 66.29 19.26
18.24 16.46 16.70 4.33
7.13 5.92 5.38 65.46
3.8 3.26 3.15 3.06
3.68 3.50 6.34 1.60
1.74 1.26 1.14 0.77
0.4 0.34 0.35 –
0.31 6.31 0.33 0.13
0.03 0.05 – 4.45
0.34 0.27 0.33 0.94
3
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
absorption effect of quartz on the water is only due to its wetting effect on the surface, and thus it does not have a significant impact on the performance of the mortar. The water swelling effects of Cabent, Na-bent, and Mg-bent are shown in Fig. 3. It can be seen that Na-bent has the strongest water absorption, while the water absorption of Mg-bent and Ca-bent are similar and weaker than that of Na-bent. This is because the Na+ charge is low, and the force of the crystal layer is weak. Under hydration, the interlayer of Nabent can easily spread or even be peeled off, resulting in a strong swellability. In contrast, Mg2+ and Ca2+ have larger charges and the same size, and thus attraction effect between the layers is significant. As a result, the distance between the layers of Mg-bent and Ca-bent is small, and thus their ability to absorb water is limited. The difference in water swellability of the three bentonites resulted in a large loss of workability in the mortar containing Na-bent, while the effects of Mg-bent and Ca-bent were comparable and relatively small. It is worth noting that the consistency of the mortar with 10% Na-bent was reduced to 72 mm. If the dosage is increased further, it will cause difficulty in mortar preparation. 3.2. Strength of cement mortars
Fig. 2. Effect of bentonites on mortar performance: (a) consistency; (b) fluidity.
Na-bent
Ca-bent
Mg-bent
for the three types of bentonite, Na-bent had the most significant influence on the workability of the mortar. When 2, 4, 6, 8, and 10 wt% Na-bent was added to the mortar, its consistency decreased from 103 mm to 93, 95, 87, 79, and 72 mm, respectively; in other words, the maximum consistency decreased by 29.7% with the addition of 10 wt% Na-bent. The fluidity of the mortar with added 2, 4, 6, 8, and 10 wt% Na-bent decreased from 247 mm to 234, 240, 230, 221, and 207 mm, respectively, representing a decrease from the maximum value of 16.3%. The effect of adding Mg-bent on the mortar consistency was similar to that of Ca-bent. When the dosage was 10%, the mortar consistency was 80 mm and 83 mm, representing decreases of 22.3% and 19.4%, respectively. When the dosage was 6%, the effects of Mg-bent and Ca-bent on the fluidity of the mortar differed, but their effects were relatively similar under the other conditions. When the dosage was 10%, the fluidity of the mortar decreased to 221 mm and 225 mm with the addition of Mg-bent and Ca-bent, respectively, corresponding to decreases of 10.6% and 9.0%. Furthermore, quartz with the same particle size as the bentonite had little effect on the working performance of the fresh mortar; however, as the dosage increased, the consistency and fluidity also exhibited a decreasing trend. When the quartz dose was 10%, the consistency and fluidity decreased by 8.3% and 4.9%, respectively. Owing to the strong water absorption of bentonite, the bentonite will absorb water during mixing of the cement mortar, causing the amount of water in the fresh mortar to decrease, which results in decreased mortar consistency and fluidity. But the
The effect of different amounts of bentonite and quartz on the flexural strength of cement mortars is shown in Fig. 4. The results show that bentonite has a positive effect on the flexural strength of the mortar, and the overall trend shows that the flexural strength of the mortar gradually increases with increasing amount of bentonite in the mortar. Compared with the addition of quartz with the same particle size distribution, it can be found that the variation in the flexural strength of the quartz samples is small within the range of added contents in this study. Fig. 5 shows the effect of addition of 10 wt% Ca-bent, Na-bent, Mg-bent, and quartz on the flexural strength of the mortar. The results indicate that the 7-d flexural strengths of samples with 10 wt% addition of Ca-bent, Na-bent, Mg-bent, and quartz increased by 42.1%, 54.5%, 41.2%, and 21.8%, respectively, while the 28-d flexural strength increased by 47.9%, 54.5%, 52.2%, and 13.6%, respectively, compared to those of the blank sample. Thus, Na-bent had the most obvious effect on the flexural strength of the mortar. Moreover, the flexural strengths of the Mg-bent and
46mm 8mm
6mm
Fig. 3. Water swelling of bentonites.
4
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
Fig. 5. Effect of different bentonites on the flexural strength (10 wt% addition).
that of the Ca-bent mortar. This indicates that Mg-bent can more effectively promote the development of the late flexural strength of the mortar. The effect of different bentonite and quartz contents on the compressive strength of the mortar is shown in Fig. 6. The results show that the influence of bentonite addition on the compressive strength of mortars is similar to that of the flexural strength. With increasing bentonite content, the compressive strength of mortar also exhibits an increasing trend, while the amount of quartz added has a lesser effect on the compressive strength. The effect of different bentonites on the compressive strength of the mortar is shown in Fig. 7. When 10 wt% Ca-bent, Na-bent, Mgbent, and quartz are added to the mortar, the 7-d compressive strengths can be increased by 46.1%, 63.8%, 49.2%, and 20.3%, respectively, while the 28-d compressive strength can be increased by 62.2%, 77.5%, 71.6%, and 9.3%, respectively compared to the blank sample. This indicates that the bentonite has a positive effect on the development of the later strength of the mortar, and the effect of Mg-bent is most obvious. At the early age, the increase in the compressive strength with Mg-bent is close to that with Ca-bent; however, at 28 days, the strength of the Mg-bent mortar is significantly higher than that of the Ca-bent mortar. Of the three types of bentonites, Na-bent also has the most significant effect on improving the mortar compressive strength.
3.3. Impermeability of the cement mortar
Fig. 4. Effect of different cement replacement amounts on the flexural strength of mortar: (a) Ca-bent; (b) Na-bent; (c) Mg-bent; (d) quartz.
Ca-bent mortars are similar at 7 days; however, at 28 days, the flexural strength of the Mg-bent mortar is significantly higher than
The cement mortars were measured with a step-by-step pressure method to determine the waterproofing ability and impermeability of the mortars. According to the requirements of Chinese building materials industry standard JC 474-2008, mortar test blocks with dimensions of 70 mm 80 mm 30 mm were prepared for the mortar impermeability tests, and the impermeability pressure was tested after 28 d of standard curing. The impermeability pressure values for the mortar samples with the addition of bentonites and quartz are listed in Table 2. The results indicate that the impermeability pressure of the mortar increases by 9.8%, 19.6%, 54.9%, 68.6%, and 101.9% with the addition of 2, 4, 6, 8, and 10 wt% Ca-bent, respectively. Correspondingly, with the addition of Na-bent the impermeability pressure increases by 17.6%, 35.3%, 56.9%, 76.5%, and 115.7%, respectively, and with the addition of Mg-bent, the impermeability pressure increases by 35.3%, 43.1%, 80.4%, 115.7%, and 137.3%, respectively. In general, the effects of the three types of bentonite on the impermeability of the mortar after 28 d of curing are very apparent. The greater the amount of
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
5
Fig. 7. Effect of different bentonites on the mortar compressive strength (10 wt% addition).
minimum impermeability pressure of the mortar can be increased to more than double that of the reference group. Of the bentonites, Mg-bent has the best water resistance, followed by Na-bent, and finally Ca-bent. A linear regression analysis between the bentonite content and the impermeability pressure reveals that there is a good linear relationship between these parameters. As shown in Fig. 8, the linear relationship for Mg-bent is the strongest, with R2 = 0.9780; followed by Na-bent, R2 = 0.9699; and finally, calcium-based bentonite, R2 = 0.9437. These linear correlations provide good quantitative relationships between the amount of bentonite added and the impermeability of the mortar. The slope of the fitted line between the Mg-bent content and impermeability pressure is k1 = 0.0702; the slope of the fitted line for Na-bent is k2 = 0.0565; the slope of the fitted line for Ca-bent is k3 = 0.0525. This indicates that for the same proportion of bentonite added to the mortar, the impermeability of the mortar is increased to a certain extent, and the required amount of bentonite to achieve the same increase in impermeability follows the order Ca-bent > Nabent > Mg-bent. Therefore, the use of a smaller amount of Mgbent allows the mortar to achieve superior water resistance. For the reference group, the addition of quartz powder provides a certain enhancement of the impermeability of the mortar, but its effect is weaker than that of the bentonites. This indicates that the improvement in the impermeability of the bentonite mortars is not only due to the filling of pore spaces by the particles, but also due to the properties of the bentonite itself.
3.4. Microstructure of the cement mortar
Fig. 6. Effect of the bentonite or quartz content on the compressive strength of mortars: (a) Ca-bent; (b) Na-bent; (c) Mg-bent; (d) quartz.
bentonite added, the greater the improvement in the impermeability will be. When 10 wt% bentonite is added to the mortar, the
Scanning electron microscopy and EDS analyses were used to observe the morphology of bentonite in the hydration products and its influence on the morphology of the hydration products. It can be seen from Fig. 9 that there are many needle-like phases on the surface of Na-bent. EDS analysis reveals that these needlelike products may be a C-S-H gel, and the amorphous product on the surface of the bentonite is C-A-S-H gel, which has a higher content of Al and Si. The surface of bentonite has many unsaturated chemical bonds (Al-O or Si-O), and these chemical bonds can serve as origin sites for the growth of hydration products during the hydration process of cement, thus promoting the growth of hydration products there. The hydration products have an obvious refinement effect on the pores, and the needle-like hydration products on the surface can be oriented and grow along the direction of
6
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
Table 2 Impermeability pressure of mortars (MPa).
0% 2% 4% 6% 8% 10%
a
Ca-bent
Na-bent
Mg-bent
Quartz
0.51 0.56 0.61 0.79 0.86 1.03
0.51 0.60 0.69 0.80 0.90 1.10
0.51 0.69 0.73 0.92 1.10 1.21
0.51 0.53 0.55 0.56 0.61 0.63
Area2
Spot1 Area3 Area1
Fig. 8. Linear relationship between the bentonite content and impermeability pressure.
the pores. In addition, there is a phenomenon of overlapping with the surrounding hydration products, which has a specific effect on the strength of the mortar. It can be seen from Figs. 10 and 11 that there is no formation of needle-like hydration products on the surface of the Ca-bent or Mg-bent mortars; only amorphous hydration products can be observed on their surfaces. Owing to the larger ionic valences of Ca2+ and Mg2+, the attraction effect on the crystal layer is also significant, making it difficult for the crystal layers to be stretched, thus limiting the growth of hydration products there. In addition, it can be seen that the bentonite particles are located at the large pores in the hydration product, allowing the particles to effectively fill the pores, thereby hindering the transport of water in the pores and enhancing the impermeability and strength of the mortar. Fig. 12 shows that the quartz particles can also act to fill the pores in the mortar matrix. As the quartz is inert, it will not participate in the hydration process of the cement, and thus there is no formation or enrichment of the hydration product, and the improvement in the mortar strength and impermeability is also weak. For the needle-like products around quartz particles, the results of a large number of literature investigation and preliminary experiments show that it is mainly ettringite formed by cement hydration [33–36]. 3.5. Pore structure of the cement mortars The impermeability and strength properties of cement-based materials are mainly related to the pore volume and pore structure characteristics of the material. MIP was used to determine changes in the pores inside the mortar. The size of the pores and the pressure at which the mercury is injected can be calculated with Eq. (1); the volume of mercury injected can represent the volume of the pores.
D¼
4ccosh P
ð1Þ
Fig. 9. Micromorphology of Na-bent in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
where D is the equivalent pore diameter, c is the surface tension of mercury, h is the contact angle between the mercury and solid, and P is the pressure required for mercury injection. The MIP tests were carried out after the samples had cured for 28 d. The pore size distribution is shown in Fig. 13, and the porosity of the mortars is listed in Table 3. Fig. 13(a) shows that Ca-bent decreases the integral curve of the mortar pore volume with the pore diameter, and the greater the amount of Ca-bent added, the more obvious this downward trend will be, which indicates a reduction in the pores in the mortar and a smaller pore size. When the content of Ca-bent was 4 wt% and 8 wt%, the porosity of the mortar decreased from 27.16% to 25.63% and 24.59%, respectively. The main peak of the differential curve is shifted slightly to the left, which also indicates that the pore structure of the mortar has been refined to some extent. From the above analysis of the microstructure of bentonite in the mortar, it can be found that the surface of Ca-bent can generate more amorphous hydration products, which reduces the pore space in the mortar, and thus the strength of the mortar containing Ca-bent is higher than that of the reference group. Fig. 13(b) shows that when Na-bent is added to the mortar, the integral curve of the pore volume with pore size also exhibits a downward trend, and the extent of the decrease is larger than that with Ca-bent, indicating that Na-bent can more effectively reduce the porosity of the mortar and refine the pore size. The porosity of
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
a
Fig. 10. Micromorphology of Ca-bent in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
the mortars mixed with 4 wt% and 8 wt% Na-bent decreased from 27.16% to 24.87% and 22.58%, respectively. At the same time, the leftward shift of the main peak of the differential curve is much larger with Na-bent than Ca-bent. This also indicates that Na-bent has a stronger effect on the pore refinement of the mortar. From the analysis of the micromorphology, Na-bent in the mortar matrix can not only form amorphous hydration products on the mortar surface, but can also form acicular hydration products oriented toward the pores. This causes the porosity of the Na-bent mortar to decreases more significantly than that of the Ca-bent mortar, and thus the effect of Na-bent on the mortar strength is also greater than that of Ca-bent. Fig. 13(c) that the variation in the integral curve of the pore volume for Mg-bent mortar is similar to that of the Na-bent mortar, indicating that Mg-bent can also effectively reduce the porosity of the mortar and refine the pore structure. When 4 wt% and 8 wt% Mg-bent is added to the mortar, the porosity of the mortar can be reduced from 27.16% to 25.11% and 22.81%, respectively, which is equivalent to the results obtained with addition of Nabent. Analysis of the micromorphology of Mg-bent in the mortar indicates that the microstructure of Mg-bent is similar to that of Ca-bent; hydrated products are only formed on the surface, and there is no formation of acicular hydration products along the pores to reduce the porosity of the mortar further. However, Mg2+ between the Mg-bent layers forms Mg(OH)2 crystals around
7
a
Fig. 11. Micromorphology of Mg-bent in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
the bentonite through ion exchange, and ability of these crystals to fill the pores causes the Mg-bent mortar to also have a low porosity[37,38]. This is a reasonable explanation for the above trends in the effects of different types of bentonite on the mortar strength and impermeability. The effects of Mg-bent and Na-bent on the porosity of the mortar are similar, and thus their strength properties at the 28-d age are also similar. However, at 7 d of aging, the compressive strength and flexural strength of Mg-bent are similar to that of Ca-bent and significantly weaker than that of Nabent, which may be due to the slow exchange between Mg2+ in Mg-bent and Ca2+ in the hydration product pore solution. There is not a large amount of Mg(OH)2 crystal formation at 7 d of aging. In addition, because the surface of Mg-bent is similar to that of Cabent, the formation of hydration products is less than that of Nabent, and the porosity is larger than Na-bent; thus, the strength of the Mg-bent mortar is lower at 7 d. In the later process, with the progress of ion exchange and the formation of Mg(OH)2 crystals, the porosity of the mortar containing Mg-bent decreases rapidly, while the strength performance in the later stage increases rapidly. Although the porosity of the mortars containing Mg-bent and Na-bent is similar, the impermeability pressure of the Mgbent mortar is higher than that of the Na-bent mortar, mainly due to the formation of flake-like Mg(OH)2 crystals, which can increase the degree of tortuosity of the water permeation path in the pores of the mortar.
8
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
a
Fig. 12. Micromorphology of quartz in the mortar after 28 d of hydration: (a) micromorphology; (b) EDS analysis.
Fig. 13(d) shows that after incorporating quartz with a similar particle size to bentonite, the pore volume of the mortar changes little with the change in the pore diameter, indicating that the influence of quartz particles on the porosity of the mortar is small. After the incorporation of 4 wt% and 8 wt% quartz, the porosity of the mortar is 27.54% and 26.72%, respectively, which is very close to the 27.16% of the blank group. At a quartz content of 4 wt%, the porosity even increased slightly, although this increase of only 0.38% may be the result of instrumental errors. It can be seen from the differential curve that quartz has a certain refinement effect on the pore structure of the mortar. The permeability resistance of cement-based materials is determined not only by the overall porosity, but also by the characteristics of the pore structure. The addition of bentonite leads to the refinement of the pore structure, but is represented by a differential curve of the pore volume as a function of pore size. There is no specific data to describe this phenomenon. To understand the refinement effect of bentonite on the pore structure of the mortar more clearly, the full pore size range in the mortar (the pore size range of the MIP instrument used in the analysis is 6 nm to 360 lm) is divided into four different pore sizes categories: 6– 50 nm are classified as harmless or less harmful; 50–200 nm are harmful pores; 0.2–1 lm and 1–360 lm are also harmful pores. The volume fraction of pores (c) is also used to express the proportion of pores with different pore sizes in the whole pore size range.
Fig. 13. Pore size distribution of mortars: (a) Na-bent; (b) Ca-bent; (c) Mg-bent; (d) quartz.
The variation in c is used to understand the effect of bentonite on the refinement of mortar pores. The effects of different types of
9
M. Liu et al. / Construction and Building Materials 241 (2020) 118015 Table 3 Mortar porosity (%). Adding content (wt%)
Ca-bent
Na-bent
Mg-bent
Quartz
0 4 8
27.16 25.63 24.59
27.16 24.87 22.58
27.16 25.11 22.81
27.16 27.54 26.72
bentonite and similarly sized quartz particles on the volume fraction (c) of pores with different pore sizes in the mortar is shown in Fig. 14. Fig. 14 indicates that the effect of bentonite on the refinement of the pores of the mortar is mainly reflected in a decrease in macropores and increase in small pores. When 8 wt% Ca-bent, Na-bent, Mg-bent, and quartz were added to the mortar, the pore volume fraction, c6–50 nm, of the less harmful or harmless pores in the mortar increased by 12.3%, 17.5%, 11.5%, and 10.8%, respectively, at 28 d of curing; the pore volume fraction of harmful pores, c50–200 nm, changed little; the pore volume fraction of the multipore pores with sizes of 0.2–1 lm, c0.2–1 lm, increased by 6.4%, 13.3%, 18.1%, and 0.1%, respectively; and the volume fraction of the multi-pore pores with sizes of 1–360 lm, c1–360 lm, decreased by 17.5%, 29.6%, 28.2%, and 9.1%, respectively. The pores in the cement hydration products are caused by the evaporation of free water during the hydration process. Because bentonite has a strong water absorption capacity, its water absorption during the mortar mixing process reduces the amount of free water in the fresh mortar, thereby reducing the gap between the cement particles during the hydration process. As a result, the pores in the hydration product are smaller, and the data shows an increase in the c6–50 nm fraction and a decrease in the c1–360 lm fraction. In addition, through the above description of the micromorphology of bentonite in the mortar, it can be seen that the bentonite will release absorbed water during hydration of the mortar to form pores of approximately 1 lm at the junction of the bentonite and the matrix. Thus, the analysis of the change in the pore volume fractions of different pore sizes has also been effectively verified. With the incorporation of bentonite, the c0.2–1 lm fraction exhibits a certain increase. Na-bent has the best water absorption capacity, and its effect on the refinement of the pores of the mortar is also the most significant. The pore volume fraction of the less harmful or harmless pores, c6–50 nm, is the largest in the Na-bent mortar, and thus
Fig. 14. Volume fraction of pores with different pore sizes in the mortars (8 wt% addition).
the permeability resistance of the mortar can improve significantly. Ca-bent has a weak water absorption capacity, and thus its ability to refine the pores of the mortar is also weak. The increase in the c6–50 nm fraction is only 12.3% with Ca-bent, and the increase in the corresponding c0.2–1 lm fraction is also low, only 6.4%. The water absorption capacity of Mg-bent is comparable to that of Ca-bent, and thus the increase in the c6– 50 nm fraction of the Mg-bent mortar is also only 11.5%. However, the c0.2–1 lm fraction of the mortar doped with Mg-bent is larger than that with the other types of bentonite, indicating that a 2– 3 lm sheet of Mg(OH)2 crystals is formed in the macropores around the bentonite, and thus the macropores are refined into pores with sizes between 0.2 lm and 1 lm. Quartz is inert during the hydration process of the mortar, but because its particles can fill the pores in the mortar, it can convert a small number of harmful pores with sizes of 1–360 lm into less harmful or harmless pores with sizes of 6–50 nm, thereby increasing the impermeability pressure of the mortar. It can be seen that the bentonite particles of the same size also have the function of filling the pores in the mortar to achieve a degree of refinement. In addition, it is worth noting that the pore volume fraction of the harmless or less harmful pores of the mortar, c6–50 nm, after adding quartz, Ca-bent, and Mg-bent with equal grain sizes is 25.4%, 26.8%, and 26.0%, respectively; the three are almost equal. However, the impermeability pressure pressures of Ca-bent and Mg-bent are much higher than that of quartz with an equal particle size. In particular, the highest impermeability pressure of Mg-bent is 1.92 times that of the quartz mortar sample with the same dosage.
3.6. Mechanism for the effects of bentonite on the cement mortar Through the discussion of a series of macroscopic properties of bentonite mortars combined with the influence of bentonite on the microstructure of mortar hydration products, the following mechanism by which bentonite improves the waterproofing effect of cementitious materials has been proposed. This section will explain the waterproofing mechanism of bentonite from two perspectives: the influence of bentonite on the hydration process of cement-based materials, and the effect of bentonite when water passes through the pores. Fig. 15 shows the effect of bentonite on the hydration of the mortar and the infiltration of water into the pores. As Fig. 15 shows, the bentonite in the fresh mortar can absorb some of the free water, thus expanding the volume of the bentonite. At the same time, the gap between the unhydrated cement particles and the amount of free water are also reduced relative to the unblended bentonite, which leads to smaller pore sizes in the formed hydration products, expressed as the harmless or less harmful pores(Fig. 14). In addition, the surface of the bentonite in the cement hydration products can generate a large number of amorphous or needle-like hydration products, which can reduce the pore space in the matrix, thereby enhancing the impermeability to water of the cement-based material. The water-expanded bentonite gradually releases the absorbed water as the humidity in the mortar decreases, causing the volume of the bentonite to shrink, and forming micron-sized harmful pores between the bentonite and the matrix. However, when external water penetrates
10
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
(a)
(b)
Pore
Free water between particles
Cement granule (d)
(c)
Outside water
Hydrated slurry (e)
(f)
Quartz particles
(g)
(h)
Bentonite surface hydration product
(i)
Swelling bentonite Bentonite in hydration products
Bentonite expands again with water
Fig. 15. Waterproofing mechanism of bentonite: (a) (b) (c) reference mortar; (d) (e) (f) mortar mixed with quartz; (g) (h) (i) mortar mixed with bentonite.
(a)
(b)
(c)
Mg(OH)2 Fig. 16. Waterproofing mechanism of modified bentonite: (a) fresh mortar; (b) mortar mixed with Mg-bentonite; (c) water penetration process in pores.
the matrix, the bentonite can re-expand to fill the surrounding pores, thereby effectively blocking the passage of water through the pores. As mentioned above, Ca-bent can significantly improve the waterproofing performance of the mortar compared to the quartz particles with an equal diameter. However, by comparing the volume fractions of pores with different pore sizes in the quartz and Ca-bent mortars, it can be found that the pore volume fraction of the harmless or less harmful pores is similar. The difference is that the 0.2–1 lm pores in the Ca-bent mortar increase, and because these pores are classified as harmful pores, the improvement in the penetration resistance of the cement-based material does not have a positive effect or even a negative effect. Therefore, the waterproofing mechanism described above for the reexpansion of bentonite in the mortar provides a more appropriate interpretation of the observed phenomena. From this, it can be concluded that the improvement in the waterproofing and impermeability of cement-based materials by bentonite is mainly caused by the swelling of the bentonite itself. For Na-bent, because of its stronger water absorption (Fig. 3), it has a better refining effect on the pores after hydration and can convert more harmful macropores into harmless or less harmful pores. In addition, owing to the looser surface of Na-bent particles, more nucleation sites can be provided for the growth of surface
hydration products; the more hydrated products are formed, the lower the porosity of the matrix will be(Fig. 9). These two factors cause Na-bent to have a better influence on the waterproofing and impermeability of mortar than Ca-bent. In addition, because quartz is inert and cannot expand, it can only enhance the impermeability of the mortar by the act of particles filling the pores (Fig. 12). For Mg-bent, its swellability is similar to that of Ca-bent, but its improvement in the anti-penetration performance of the mortar is the best among the three bentonites (Table 2, Fig. 8). The effect of Mg-bent on the hydration of cement-based materials and the penetration of water into the pores is shown in Fig. 16. Owing to the ion exchange of Mg2+ with Ca2+ in the pore solution of the hydration products, Mg2+ is exchanged between the layers of Mg-bent. Thus, Mg(OH)2 crystals are formed under highly alkaline conditions, which generally occur in the form of flakes near the bentonite to create the micromorphology shown in Figs. 16(b), 11. When external water passes through the pores, the Mg-bent not only has the water-swelling and waterproofing properties described above, but the formed Mg(OH)2 crystals can also reduce the pores and increase the tortuosity of the water permeation path. Therefore, mortar mixed with Mg-bent can achieve excellent water resistance under the dual action of Mg-bent and the formed Mg
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
(OH)2 crystals. Recently, studies have also shown that there is a pozzolanic reaction between bentonite and calcium hydroxide [39–41]. The effects of bentonite on the cement mortar were not only the waterproof mechanism mentioned but also the pozzolanic reaction consumes part of calcium hydroxide, which generated more C-S-H and made the matrix more compact. 4. Conclusion By comparing the effects of different types of bentonite and quartz on the workability, strength properties, and impermeability of cement mortars, combined with consideration of the changes in the microstructure and pore structure of the mortars, the following conclusions could be drawn: (1) Bentonite has a significant influence on the workability of the mortar. The main result is that the consistency and fluidity of the fresh mortar decrease with increasing bentonite content; the loss of mortar fluidity is mainly caused by the absorption of water by bentonite. (2) The addition of bentonite or quartz with an equal grain size can increase the flexural strength and compressive strength of the mortar; the larger the added amount of bentonite or quartz, the more obvious the increase will be. (3) The mortar made with Mg-bent has the best impermeability, and the maximum impermeability pressure can reach 2.37 times that of the blank group. The impermeability pressure of the mortar generally increases with increasing bentonite content and exhibits a strong linear relationship. (4) Bentonite can refine the pore structure of the mortar; the increase in micron-sized pores between 0.2 lm and 1 lm is a characteristic unique to the incorporation of bentonite. It is worth noting that although Na-bent can provide the mortar with better impermeability, it has a significant influence on the workability of the fresh mortar, and Mg-bent can achieve an excellent waterproofing effect with small fluid loss of the mortar. Author contributions Lai Zhenyu developed the idea for the study, Liu Mengliang and Hu Yang analysed most of the data, and wrote the initial draft of the paper. The other authors contributed to refining the ideas, carrying out additional analyses and finalizing this paper. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the projects supported by the National Key Research and Development Plan (2016YFC0701004), Sichuan Science and Technology Program (No.2019ZDZX0024), Open Fund of Southwest University of Science and Technology (17FKSY0108, 19FKSY05). References [1] J.Y. Yoo, H.S. Lee, Y.J. Kim, Experimental study on the water penetration into mortar under water pressure condition, Key Eng. Mater. 385–387 (2008) 681–684. [2] Y. Farnam, C. Villani, T. Washington, M. Spence, J. Jain, W. Jason Weiss, Performance of carbonated calcium silicate based cement pastes and mortars exposed to NaCl and MgCl2 deicing salt, Constr. Build. Mater. 111 (2016) 63– 71.
11
[3] J.M. Loche, A. Ammar, P. Dumargue, Influence of the migration of chloride ions on the electrochemical impedance spectroscopy of mortar paste, Cem. Concr. Res. 35 (9) (2005) 1797–1803. [4] B. Meng, Q. Guo, X. Men, S. Ren, W. Jin, B. Shen, Preparation of modified bentonite by polyhedral oligomeric silsesquioxane and sodium dodecyl sulfate in aqueous phase and its adsorption property, Mater. Lett. 253 (2019) 71–73. [5] Additives EPo, Products or Substances used in Animal F. Scientific Opinion on the efficacy of Bentonite (dioctahedral montmorillonite) for all species. EFSA J 2011;9(6):2276. [6] M. Segad, B. Jonsson, T. Akesson, B. Cabane, Ca/Na montmorillonite: structure, forces and swelling properties, Langmuir 26 (8) (2010) 5782–5790. [7] M.H. Köster, L.B. Williams, P. Kudejova, H.A. Gilg, The boron isotope geochemistry of smectites from sodium, magnesium and calcium bentonite deposits, Chem. Geol. 510 (2019) 166–187. [8] G. Sposito, K.M. Holtzclaw, L. Charlet, C. Jouany, A.L. Page, Sodium-calcium and sodium-magnesium exchange on wyoming bentonite in perchlorate and chloride background ionic media, Soil Sci. Soc. Am. J. 47 (1983) 51–56. [9] L. Liu, Prediction of swelling pressures of different types of bentonite in dilute solutions, Colloids Surf., A 434 (2013) 303–318. [10] M. Muzakky, C. Supriyanto, Modification of three types of bentonite with zirconium oxide chloride (ZOC) of local products using intercalation process, Indonesian J. Chem. 16 (2016) 14–19. [11] R. Fernandez, J. Cuevas, U.K. Mader, Modelling concrete interaction with a bentonite barrier, Eur. J. Mineral. 21 (1) (2009) 177–191. [12] S. Andrejkovicˇová, C. Alves, A. Velosa, F. Rocha, Bentonite as a natural additive for lime and lime–metakaolin mortars used for restoration of adobe buildings, Cem. Concr. Compos. 60 (2015) 99–110. [13] P. Leroy, A. Revil, D. Coelho, Diffusion of ionic species in bentonite, J. Colloid Interface Sci. 296 (1) (2006) 248–255. [14] H.H. Lee, C.W. Wang, experimental study on cement mortar with bentonite, Adv. Mater. Res. 671–674 (2013) 1741–1744. [15] A.K. Parande, B. Ramesh Babu, M. Aswin Karthik, K.K. Deepak Kumaar, N. Palaniswamy, Study on strength and corrosion performance for steel embedded in metakaolin blended concrete/mortar, Constr. Build. Mater. 22 (3) (2008) 127–134. [16] M. Li, Z. Wu, Preparation, characterization, and humidity-control performance of organobentonite/sodium polyacrylate mortar, J. Macromol. Sci. Part B 51 (8) (2012) 1647–1657. [17] J. Mirza, M. Riaz, A. Naseer, F. Rehman, A.N. Khan, Q. Ali, Pakistani bentonite in mortars and concrete as low cost construction material, Appl. Clay Sci. 45 (4) (2009) 220–226. [18] S. Ahmad, S.A. Barbhuiya, A. Elahi, J. Iqbal, Effect of Pakistani bentonite on properties of mortar and concrete, Clay Miner. 46 (1) (2018) 85–92. [19] J.P. Gonçalves, L.M. Tavares, R.D. Toledo Filho, E.M.R. Fairbairn, Performance evaluation of cement mortars modified with metakaolin or ground brick, Constr. Build. Mater. 23 (5) (2009) 1971–1979. [20] N. Mesboua, K. Benyounes, A. Benmounah, S.K. Shukla, Study of the impact of bentonite on the physico-mechanical and flow properties of cement grout, Cogent Eng. 5 (1) (2018). [21] I. Garcia-Lodeiro, N. Boudissa, A. Fernandez-Jimenez, A. Palomo, Use of clays in alkaline hybrid cement preparation. The role of bentonites, Mater. Lett. 233 (2018) 134–137. [22] F. Raúl, A.I. Ruiz, J. Cuevas, Formation of C-A-S-H phases from the interaction between concrete or cement and bentonite, Clay Miner. 51 (2) (2018) 223– 235. [23] J. Wei, B. Gencturk, Hydration of ternary Portland cement blends containing metakaolin and sodium bentonite, Cem. Concr. Res. 123 (2019) 105772. [24] M.A. Fadaie, M. Nekooei, P. Javadi, Effect of dry and saturated bentonite on plastic concrete, KSCE J. Civ. Eng. 23 (8) (2019) 3431–3442. [25] A.T. Amlashi, S.M. Abdollahi, S. Goodarzi, A.R. Ghanizadeh, Soft computing based formulations for slump, compressive strength, and elastic modulus of bentonite plastic concrete, J. Cleaner Prod. 230 (2019) 1197–1216. [26] I. Plecas, R. Pavlovic, S. Pavlovic, Leaching behavior of 60Co and 137Cs from spent ion exchange resins in cement–bentonite clay matrix, J. Nucl. Mater. 327 (2) (2004) 171–174. [27] I. Sato, K. Maeda, M. Suto, M. Osaka, T. Usuki, S. Koyama, Penetration behavior of water solution containing radioactive species into dried concrete/mortar and epoxy resin materials, J. Nucl. Sci. Technol. 52 (4) (2015) 580–587. [28] M. Katsioti, N. Katsiotis, G. Rouni, D. Bakirtzis, M. Loizidou, The effect of bentonite/cement mortar for the stabilization/solidification of sewage sludge containing heavy metals, Cem. Concr. Compos. 30 (10) (2008) 1013–1019. [29] H. Yang, D. Long, L. Zhenyu, H. Yuanjin, Y. Tao, H. Xin, et al., Effects of bentonite on pore structure and permeability of cement mortar, Constr. Build. Mater. 224 (2019) 276–283. [30] C.H. Yu, G.H. Li, J.L. Gao, B. Lan, Q. Wei, D.Z. Xu, Effect of bentonite on the performance of the limestone manufactured-sand mortar, Appl. Mech. Mater. 357–360 (2013) 1374–1378. [31] A. Kaci, M. Chaouche, P.A. Andréani, Influence of bentonite clay on the rheological behaviour of fresh mortars, Cem. Concr. Res. 41 (4) (2011) 373– 379. [32] B. Abu-Jdayil, Rheology of sodium and calcium bentonite–water dispersions: effect of electrolytes and aging time, Int. J. Miner. Process. 98 (3) (2011) 208– 213. [33] R. Komatsu, N. Mizukoshi, K. Makida, K. Tsukamoto, In-situ observation of ettringite crystals, J. Cryst. Growth 311 (3) (2009) 1005–1008.
12
M. Liu et al. / Construction and Building Materials 241 (2020) 118015
[34] S. Luo, M. Liu, L. Yang, J. Chang, Effects of drying techniques on the crystal structure and morphology of ettringite, Constr. Build. Mater. 195 (2019) 305–311. [35] P.K. Mehta, Scanning electron micrographic studies of ettringite formation, Cem. Concr. Res. 6 (2) (1976) 169–182. [36] K. Ogawa, D.M. Roy, C4A3S hydration, ettringite formation, and its expansion mechanism: II. Microstructural observation of expansion, Cem. Concr. Res. 12 (1) (1982) 101–109. [37] T.S. Qureshi, A. Kanellopoulos, A. Al-Tabbaa, Encapsulation of expansive powder minerals within a concentric glass capsule system for self-healing concrete, Constr. Build. Mater. 121 (2016) 629–643. [38] T. Qureshi, A. Kanellopoulos, A. Al-Tabbaa, Autogenous self-healing of cement with expansive minerals-I: impact in early age crack healing, Constr. Build. Mater. 192 (2018) 768–784.
ˇ e˙sniene˙, A. Baltušnikas, I. Lukošiu te˙, K. Brinkiene˙, R. Kalpokaite˙-Dicˇkuviene˙, [39] J. C Influence of organoclay structural characteristics on properties and hydration of cement pastes, Constr. Build. Mater. 166 (2018) 59–71. [40] J. Calabria-Holley, S. Papatzani, B. Naden, J. Mitchels, K. Paine, Tailored montmorillonite nanoparticles and their behaviour in the alkaline cement environment, Appl. Clay Sci. 143 (2017) 67–75. [41] J. Jiang, Z. Lu, J. Li, Y. Xie, K. Luo, Y. Niu, Preparation and properties of nanopore-rich lightweight cement paste based on swelled bentonite, Constr. Build. Mater. 199 (2019) 72–81.