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Effects of MgO-based expansive additive on compensating the shrinkage of cement paste under non-wet curing conditions
Abstract
Expansive additives are widely used to compensate the drying shrinkage of cement-based materials to avoid cracking. However, the expansion of conventional ettringite-bearing expansive additive depends strongly on wet curing and is mainly generated at early age, and hence it may not work well in concretes without sufficient water supply or exhibit long-term shrinkage. MgO-based expansive additive, for which less water is needed for the formation of Mg(OH)2 in comparison to ettringite, was prepared and its compensating effect on the autogenous shrinkage and late age thermal shrinkage of Portland and fly ash cement pastes at low water-to-cement ratio was investigated. The tests were conducted under sealed condition, so that the moisture exchange with the environment was prevented. Results show that, even under the non-wet curing condition, the shrinkages of cement pastes can be compensated effectively. Microstructure analysis by scanning electron microscope indicates that the macro-expansion of cement pastes is probably caused by the locally restrained expansion of MEA due to the hydration of MgO.
... Currently, CaO-based and MgO-based expansive agents are the two common agents for expansive concrete fabrication. The expansion is generated by the formation of expansive crystals (i.e., Ca(OH) 2 or Mg(OH) 2 ), when Portland cement hydrates [1][2][3]. After setting, the expansive crystals fill the voids in concrete, resulting in the primary expansion of concrete [1][2][3]. ...
... Currently, CaO-based and MgO-based expansive agents are the two common agents for expansive concrete fabrication. The expansion is generated by the formation of expansive crystals (i.e., Ca(OH) 2 or Mg(OH) 2 ), when Portland cement hydrates [1][2][3]. After setting, the expansive crystals fill the voids in concrete, resulting in the primary expansion of concrete [1][2][3]. ...
... The expansion is generated by the formation of expansive crystals (i.e., Ca(OH) 2 or Mg(OH) 2 ), when Portland cement hydrates [1][2][3]. After setting, the expansive crystals fill the voids in concrete, resulting in the primary expansion of concrete [1][2][3]. Expansive concrete is particularly efficient in restraining the shrinkage-induced cracks in mass concrete constructions like dams and tunnels [3]. Recently, the combined effect of short structural fibers (including steel fibers and synthetic fibers) and expansive agents in concrete material properties has also been studied. ...
... Since the 1970s, MgO has been increasingly used in various concrete structures. The reaction between MgO and water produces Mg(OH) 2 crystals [18], the growth of which could generate expansion and compensate for the shrinkage of concrete [1]. ...
... The reactivity and dosage of MgO are the two main factors affecting the expansion properties of concrete [19]. So far, there have been many studies concerning the effects of MgO reactivity and dosage on the shrinkage behavior, strength and microstructure of concrete as well as the cement hydration [16,[18][19][20][21][22][23]. For instance, the reactive MgO reacts with water rapidly and produces a large expansion at an early age, therefore it was suitable for compensating the early rapid shrinkage of thin concrete structures [16,[20][21][22]. ...
... In contrast, weak reactive MgO reacts with water more slowly and produces a slower expansion at an early age but produces a larger expansion at a later age than the reactive one, thus it is more beneficial in compensating for the shrinkage of massive concrete structures at a later hydration age [16,20,22]. Generally, the shrinkage compensation effect of MgO enhances with the increase in MgO dosage [18,19]. In addition to shrinkage and strength, concrete durability is closely related to the safety and service life of concrete structures. ...
Currently, the MgO expansion agent is widely used to reduce the cracking risk of concrete. The influence of MgO reactivity (50 s and 300 s) and dosage (0, 4 wt.% and 8 wt.%, by weight of binder) on the air void, pore structure, permeability and freezing–thawing (F–T) resistance of concrete were studied. The results indicate (1) the addition of 4–8 wt.% reactive MgO (with reactivity of 50 s and termed as M50 thereafter) and weak reactive MgO (with reactivity of 300 s and termed M300 thereafter) lowers the concrete’s compressive strength by 4.4–17.2%, 3.9–16.4% and 1.9–14.6% at 3,
28 and 180 days, respectively. The increase in MgO dosage and reactivity tends to further reduce the concrete strength at all hydration ages. (2) Permeability of the concrete is closely related to the pore structure. M50 can densify the pore structure and lower the fraction of large capillary pores at an early age, thus it is beneficial for the impermeability of concrete. In contrast, M300 can enhance the 180-day impermeability of concrete since it can densify the pore structure only at a late age. (3) The influence of MgO on F–T resistance is minor since MgO could not change the air void parameters. (5) MgO concretes exhibit obvious fractal characteristics. The fractal dimension of the pore surface (Ds) exhibits a close relationship with the permeability property of concrete. However, no correlation can be found between F–T resistance and Ds.
... (3) Adding an expansion component to concrete by adding expansion sources to concrete can compensate for the shrinkage deformation of concrete through the volume expansion generated by the expansion component during hydration, thereby alleviating or avoiding the generation of shrinkage cracks [22][23][24]. ...
... Additionally, how to make the fiber uniform distribution in the concrete is also a problem to be solved [26]. Relatively speaking, adding an expansive agent to concrete is an economical, simple and effective control method [23]. ...
Using the volume expansion generated by the hydration of the MgO expansive agent to compensate for the shrinkage deformation of concrete is considered to be an effective measure to prevent concrete shrinkage and cracking. Existing studies have mainly focused on the effect of the MgO expansive agent on the deformation of concrete under constant temperature conditions, but mass concrete in practical engineering experiences a temperature change process. Obviously, the experience obtained under constant temperature conditions makes it difficult to accurately guide the selection of the MgO expansive agent under actual engineering conditions. Based on the C50 concrete project, this paper mainly investigates the effect of curing conditions on the hydration of MgO in cement paste under actual variable temperature conditions by simulating the actual temperature change course of C50 concrete so as to provide a reference for the selection of the MgO expansive agent in engineering practice. The results show that temperature was the main factor affecting the hydration of MgO under variable temperature curing conditions, and the increase in the temperature could obviously promote the hydration of MgO in cement paste, while the change in the curing methods and cementitious system had an effect on the hydration of MgO, though this effect was not obvious.
... Mo et al. [10] stated that in the hydration reaction of MgO brucite, Mg(OH)2, is obtained as the end product (Equation (2)). ...
... The smaller specific surface area causes more clumping of the particles in a sample, which leads to lower hydration. Jin and Al-Tabba and Mo et al. [5,10] reached the same conclusions. ...
In this research, the durability performance of sustainable concrete with the incorporation of reactive magnesium oxide (MgO) and fly ash (FA) was evaluated. The partial replacement of cement with these two materials is an appealing solution for the construction sector due to sustainability benefits and shrinkage reduction. The incorporation of FA by partial replacement of cement was carried out at 0%, 15% and 30%. The incorporation of MgO in concrete was carried out at 0%, 5%, 10% and 20%. Two types of MgO were used, one from Australia and another of Spanish origin. These two materials were evaluated in terms of their individual incorporation, and then an evaluation was carried out when the two were simultaneously used. In terms of durability, performance losses between 3% and 95% were obtained in all tests (water absorption by capillarity and immersion , carbonation depth and resistance to chloride penetration). However, over time, the difference in performance relative to the reference concrete tends to decrease due to the slow hydration that characterizes these two alternative materials. It was found that, in most of the tests, no overlapping of the negative effects occurred. In other words, the simultaneous incorporation of MgO and FA caused performance losses lower than the sum of the losses of their individual incorporation.
... Studies showed that when the percentage of expansive cement is increased, the hydration process of the cement is greatly affected and the setting time of the concrete mix is increased [6]. When the shrinking ability of the concrete is reduced, nevertheless, by incorporating shrinkagehindering agents and fibers into the concrete, the concrete mix tends to perform better in aspects of crack development behavior [7]. ...
... When employed as reinforcing material, PVA fibers help to improve the strength of concrete while somehow reducing the amount of tensile reinforcement required [4]. As per the investigative outcomes, adding more expansive cement hinders the hydration process of the cement, which increases the time considered necessary for the concrete mix to set [7]. On the contrary, when the shrinkage of the concrete is minimized as a consequence of the inclusion of shrinkage-hindering agents and fibers in the concrete, the concrete mix generally performs more effectively in crack development behavioral aspects [8,11]. ...
Expansive cement is a unique type of cement that, when mixed with water, produces a paste which expands in volume significantly more than standard Portland cement. This expansion nullifies the deficit of shrinkage that arises during concrete hardening. This paper presents the outcomes of two phases of experimentation. Phase I of the paper provides a summary of the performance of expansive cement concrete mixes prepared with various proportions of expansive cement, which partially supplements the ordinary Portland cement, and which is infused with varying amounts of PVA fibers of 0.5, 1, 1.5, 2, and 2.5%, as a form of reinforcement under compression, tension, and flexure. Concrete strength, curing effect, and PVA fibers are the variables in Phase I of the study. The Phase II findings provide the buckling behavior of the self-stressed concrete columns reinforced with PVA fibers when the optimum concrete mix obtained from the Phase I investigation was poured inside steel tubes of varying thicknesses of 2 mm and 2.5 mm to restrict the expansion of the concrete, thereby making it self-stressed concrete. The D/t ratio, inclusion of expansive cement, and PVA fibers are the variables for Phase II of the study. The self-stressed columns with 2% PVA fibers showed better performance than the other columns.
... Specifically, the high σ and low T c vaules imply good cracking resistance. Other studies reported that the high σ c,max value favors the compensation of the thermal stress [18,58]. of TSTM curves of the CT0, CSM8, CSP4 and CSM4P4 concretes is given in Figure 7. It can be seen from Figure 7 that, these four concrete specimens present similar thermal stress behavior, i.e., the compressive stresses raised as the internal concrete temperature increased, until the maximum temperature Tmax was reached. ...
... This experimental result is supported by other study [25], in which the compacted pure MgO powders with a reactivity of 71 s produced a great amount of expansion stress when it touched water. The high σ c,max value in concrete is beneficial to eliminate more thermal stress during cooling period, in other words, to bear high cracking tensile stress σ and large temperature declines before cracking [18,58]. Besides, CSM4 and CSM8 concretes exhibited σ and T c vaules of 0.94 MPa and 0.98 MPa, 11.5 • C and 9.3 • C, which are 6% and 11% larger, 2.3 • C and 4.5 • C lower than those of CT0 one, respectively, illustrating that the MgO addition enhanced the cracking resistance of concrete during the temperature drop. ...
Abrasion resistance and cracking resistance are two important properties determining the normal operation and reliability of hydropower projects that are subjected to erosion and abrasive action. In this study, polyvinyl alcohol (abbreviated as PVA) fiber and magnesium oxide expansive agents (abbreviated as MgO) were used together to solve the problems of cracking and abrasive damage. The effects of PVA fiber and MgO on the mechanical property, abrasion and cracking resistance, pore structures and fractal features of high-strength hydraulic concrete were investigated. The main results are: (1) The incorporation of 4–8% Type I MgO reduced the compressive strength, splitting tensile strength and the abrasion resistance by about 5–12% at 3, 28 and 180 days. Adding 1.2–2.4 kg/m3 PVA fibers raised the splitting tensile strength of concrete by about 8.5–15.7% and slightly enhanced the compressive strength and abrasion resistance of concrete. (2) The incorporation of 4–8% Type I MgO prolongs the initial cracking time of concrete rings under drying by about 6.5–11.4 h, increased the cracking tensile stress by about 6–11% and lowered the cracking temperature by 2.3–4.5 °C during the cooling down stage. Adding 1.2–2.4 kg/m3 PVA fibers was more efficient than adding 4–8% MgO in enhancing the cracking resistance to drying and temperature decline. (3) Although adding 4% MgO and 1.2–2.4 kg/m3 PVA fibers together could not enhance the compressive strength and abrasion resistance, it could clearly prolong the cracking time, noticeably increase the tensile stress and greatly lower the racking temperature; that is, it efficiently improved the cracking resistance to drying and thermal shrinkage compared with the addition of MgO or PVA fiber alone. The utilization of a high dosage of Type I MgO of less than 8% and PVA fiber of no more than 2.4 kg/m3 together is a practical technique to enhance the cracking resistance of hydraulic mass concretes, which are easy to crack. (4) The inclusion of MgO refined the pores, whereas the PVA fiber incorporation marginally coarsened the pores. The compressive strength and the abrasion resistance of hydraulic concretes incorporated with MgO and/or PVA fiber are not correlated with the pore structure parameters and the pore surface fractal dimensions.
... The first inflexion point occurs when the expansion due to MgO hydration exceeds the value of autogenous shrinkage due to cement hydration. The similar inflexion points due to MgO hydration have been also reported by others [36,87,94]. About 12.4 × 10 −6 and 20.1 × 10 −6 of autogenous shrinkage of LC4M50 and LC8M50 can be compensated at 5 days, respectively. ...
... The addition of 4% M300 produces a final autogenous shrinkage strain of −7.4 × 10 −6 , while adding 8% M300 produces a 360-day expansion of 14.3 × 10 −6 . Overall, this enhanced compensation effect with MgO dosage is in good aggreement with the results obtained from cement pastes [25,87,94], mortars [34,89], and concretes [27,31,95]. ...
Currently, low heat Portland (LHP) cement is widely used in mass concrete structures. The magnesia expansion agent (MgO) can be adopted to reduce the shrinkage of conventional Portland cement-based materials, but very few studies can be found that investigate the influence of MgO on the properties of LHP cement-based materials. In this study, the influences of two types of MgO on the hydration, as well as the shrinkage behavior of LHP cement-based materials, were studied via pore structural and fractal analysis. The results indicate: (1) The addition of reactive MgO (with a reactivity of 50 s and shortened as M50 thereafter) not only extends the induction stage of LHP cement by about 1–2 h, but also slightly increases the hydration heat. In contrast, the addition of weak reactive MgO (with a reactivity of 300 s and shortened as M300 thereafter) could not prolong the induction stage of LHP cement. (2) The addition of 4–8 wt.% MgO (by weight of binder) lowers the mechanical property of LHP concrete. Higher dosages of MgO and stronger reactivity lead to a larger reduction in mechanical properties at all of the hydration times studied. M300 favors the strength improvement of LHP concrete at later ages. (3) M50 effectively compensates the shrinkage of LHP concrete at a much earlier time than M300, whereas M300 compensates the long-term shrinkage more effectively than M50. Thus, M300 with an optimal dosage of 8 wt.% is suggested to be applied in mass LHP concrete structures. (4) The addition of M50 obviously refines the pore structures of LHP concrete at 7 days, whereas M300 starts to refine the pore structure at around 60 days. At 360 days, the concretes containing M300 exhibits much finer pore structures than those containing M50. (5) Fractal dimension is closely correlated with the pore structure of LHP concrete. Both pore structure and fractal dimension exhibit weak (or no) correlations with shrinkage of LHP concrete.
... The effect of expansive admixtures was also investigated by Nagataki and Gem [16]; they observed that these admixtures compensate early-age shrinkage. Using MgO-based expansive admixture in low w/c can effectively reduce shrinkage even in dry curing condition [17]. Using type K of expansive agent can decrease drying shrinkage [18]. ...
... The effect of various additives on the shrinkage of high and ultrahigh performance concrete [25][26][27][28][29][30] and self-compacting concrete [9,31,32] has been widely evaluated by many researchers [12][13][14][15][16][17][18][19][20]. Recently, nanomaterials and fibers have been used in various fields such as strengthening, improving material properties and etc. [33,34]. ...
Shrinkage is the volume change of concrete without influence of external forces. Drying shrinkage, which occurs after the concrete hardening, is one of the most important causes of the cracking in concrete members. In addition to adverse effects on the appearance of concrete, cracks may reduce the strength and durability by increasing the permeability of concrete and facilitate the entry of aggressive agents into it. Free and restrained drying shrinkage are the most important types of shrinkage. The present study investigated the effect of two admixtures—including damp proofing and expansive materials— on compressive strength, tensile strength, electrical resistivity, modulus of elasticity, unrestrained and restrained shrinkage and water absorption of concrete specimens. In addition, the effect of admixtures on crack properties was investigated. The cracks properties, which occurred in unrestrained shrinkage test, such as average and maximum crack width and total cracks area, were computed by image processing. The results showed that the use of damp proofing additive, (i.e. calcium stearate) reduced maximum free drying shrinkage by 42.4%, maximum restrained drying shrinkage by 22.8%, maximum crack width by 51% and final crack area by 21%. Moreover, the use of expansive additive (i.e. aluminum powder) almost eliminated the free shrinkage, but on average, it increased the restrained drying shrinkage by 18% and reduced the maximum crack width and the final crack area by 2.67 and 27%, respectively.
... In addition, some studies have found that adding a certain amount of nano-MgO into the material can effectively fill the voids in the cement mortar, thus improving the strength and toughness of the material, and making its microstructure denser than ordinary cement-based materials [26,27]. At the same time, nano-MgO reduces the self-shrinkage of cement mortar, which is beneficial in enhancing its stability [28]. ...
Nano-metallic oxide particles have been found to be potentially effective microstructural reinforcements for cement mortar and have become a research hotspot in recent years for nano-modification technology of building materials. However, different conclusions have been obtained due to various researchers used different research methods, which have resulted in a deficiency for the performance comparison between different nano-metallic oxide particles. In the present study, the effects of five kinds of nano-metallic oxide particles, namely nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO, on the performance of cement mortar at 28 days and 730 days in terms of mechanical, durability, microstructure, and pore size distribution properties by performing different experiments were investigated. Test results show that the dosage of nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO is 2%, 1%, 1%, 1%, and 2%, respectively, where they can significantly prove the compressive and flexural strengths, decrease the porosity, drying shrinkage, and permeability, and refine the pore size distribution of cement mortar. It can be seen through SEM analysis that nano-metallic oxide particles can promote cement hydration, and also refine the size and distribution of Ca(OH)2 crystal, but the specific principles are different. The analysis concluded that the five kinds of nano-metallic oxide particles can play a filling role in cementitious materials to improve the denseness and surface activity role to promote the hydration of cement particles, thus improving the mechanical properties, durability, and pore size distribution of cementitious materials, with the order of their modification effect on cement-based materials being nano-ZrO2 > nano-MgO > nano-Al2O3 > nano-ZnO > nano-CuO.
... The shrinkage deformations arising from corrosion can lead to the formation of cracks and other defects in the structure of building materials, reducing the service life of these materials [34]. Therefore, in order to increase the durability of concrete, a number of additives that reduce or affect shrinkage are introduced into the composition of concrete [34][35][36][37]. ...
The reliability of concrete structures is closely related to the durability of the concrete materials stable under external environmental conditions. The present study is aimed at analysing the effect of a prospective hardening additive containing calcium alumoferrites and calcium sulfate (AFCS) as a substitute (5–15%) for Portland cement. The hardened cement pastes were characterized by water absorption, shrinkage, strength and corrosion resistance. It was shown that replacing a part of Portland cement with the AFCS additive results in an increase in the strength of fine-grained concrete and in the water resistance grade of concrete. The use of the AFCS additive in the mixed cements reduces the shrinkage of cement stone, resulting in shrinkage-free fine-grained concretes. The increased corrosion resistance of the hardened cement paste is caused by a chemical (saturation) equilibrium between corrosive medium and a cement stone. Penetration of sulphate ions from corrosive solution into the hardened cement paste is much lower, unlike Portland cement. Following saturation of the hardened cement paste with sulphate ions, their further penetration into the cement stone does not occur. Based on the results of the study, recommendations were developed for the use of the hardening alumoferrite-gypsum additive to Portland cement, which allows to improve the mechanical and corrosion characteristics of concrete.
... Aggregate not only restrains the deformation of concrete mixed with MgO but also causes changes in the water content of the concrete via water absorption. The expansive strain in concrete specimens is about 10%-30% of the strain in the mortar or paste specimens with the same MgO content (by weight of cement) and w/b ratio [20,26,[39][40][41]. The addition of aggregate leads to the fact that the amount of MgO in concrete is lower than that in mortar or paste. ...
In China, MgO-based expansive agent (MEA) has been used for concrete shrinkage compensation and cracking control for over 40 years. The expansive behavior of MEA in cementitious materials could be manipulated to some extent by adjusting the calcination process of MEA and influenced by the restraint condition of the matrix. It is key to investigate the factors related to deformation and cracking resistance so that the desired performance of MEA in certain concrete structures could be achieved. This paper reviews the influence of key parameters such as hydration reactivity, dosage, and calcination conditions of MEA, the water-to-binder ratio, supplementary cementitious material, aggregates, and curing conditions on the deformation and cracking resistivity of cement paste, mortar, and concrete with an MEA addition. The numerical simulation methods and deformation prediction models are then summarized and analyzed for more reasonable estimations.
... The performance of this expansive cement can substantially be improved by reinforcing them with PVA fibers. The intrusion of expansive cement in concrete has found to have the capacity to provide a number of increased benefits, including resistant to shrinkage (Mo et al., 2012), improved cement paste-aggregate interface and more densified and uniform material. This was considered to be one of the most significant advances in concrete technology (Toshiyuki et al., 2021). ...
As part of this piece of research, the buckling behavior of columns made of expansive cement concrete and reinforced with PVA fibers was investigated. The variables in this investigation included the percentage of expansive cement, the percentage of PVA fibers, and the grade of the concrete. The load–strain plots that were produced as a result of this investigation demonstrated a significantly enhanced load carrying capability and a significantly delayed fracture development in comparison to other conventional columns. During the course of this work, observations were made regarding the failure modes of the expanding cement concrete columns that contained PVA fibers. The results of the tests demonstrated that M50 grade concrete columns made with 10% expansive cement and 1.5% PVA fibers had a load carrying capacity that was 1.5 times greater than that of conventional concrete of the same grade, and that M40 grade concrete columns made with 10% expansive cement and 2% PVA fibers had a load carrying capacity that was 1.48 times greater than that of conventional concrete of the same grade. Both of these types of concrete columns exhibited improved crack resisting behavior.
... The use of expanding agents such as magnesium oxide (MgO), calcium oxide (CaO), superabsorbent polymers, and saturated aggregates can help mitigate the effects of shrinkage and inherent debonding of cement at the interface (Jafariesfad et al., 2017c;Mo et al., 2014;Saito et al., 1991). The successful use of MgO as an expansive agent in cement has been acknowledged by several researchers (Mo et al., 2012;Sherir et al., 2017) who have reported the self-healing ability of the MgOenhanced cement systems. To produce sound cement for use in real well operations, the recommended volume of MgO should be limited to ≤6% (ASTM, 2001). ...
Wellbores constructed in high-temperature environments require more reliable cement systems to overcome complicated near-borehole loads. In this study, we numerically evaluate the effect of temperature on the interfacial bond integrity of two cementitious materials - an industrial class expansive cement and a rock-based geopolymer based on their experimental triaxial test properties. The results from the triaxial test indicate that the test temperature did not have a significant impact on the mechanical properties of both types of materials except for the confined compressive strength. Geopolymer showed higher flexibility, higher Poisson’s ratio, and lower compressive strength than the industrial class expansive cement at both temperatures. From the numerical analysis, the results show that the most likely cement failure location is at 90° to the maximum in-situ stress on the casing-cement interface. Without considering pore pressure effects, cement crushing resulting from excessive compressive stress is the main reason for the inner interfacial debonding. The test temperature has a significant influence on hoop stress. High test temperature results 13 – 45% increase in the hoop stress. Radial stress was insignificantly influenced by the test temperature. When test temperature changes from 30 to 90°C, geopolymer experiences an 80% increase in hoop stress while the industrial class expansive cement only experiences a 1% increase.
1. INTRODUCTION
Cement is one of the major barrier elements for maintaining well integrity. It is conventionally used for in primary, remedial and plug and abandonment operations. The loss of well integrity can lead to environmental risks, economic losses for the operator and even safety issues. Several factors can compromise cement integrity during its placement, these include but are not limited to poor cement displacement, fluid loss, mud contamination, etc. Despite the completion of a good cement job, fissures, cracks and microannuli may still develop long after well completion.
... At the same time, they proved by microscopic experiments that nanomagnesium oxide can promote the early hydration reaction of highvolume fly ash cement mortar. 20 Mo et al. 21 found that magnesium oxide-based additives may be more appropriate for concrete that has low permeability and insufficient external water curing. Therefore, it is of great significance to study the role of nano-magnesium oxide in cement-based composites with a low water-binder ratio. ...
Nano-magnesium oxide (MgO) can be used as an expansion agent for cement-based materials, but little is known about the influence of nano-magnesium oxide on the mechanics, microstructure and expansion stability of cement materials. The influence of nano-magnesium oxide on the consistency, fluidity, porosity, dry and wet densities, mechanical properties and microstructures of cement-based materials was researched in this study. The results show that the compressive strength and flexural strength of the cement mortar test block containing nano-magnesium oxide particles increase and are higher than those of the control cement mortar test block at any time. The addition of nano-magnesium oxide increases the mechanical strength of the cement mortar block, and the strengthening effects are most obvious in the early stage of maintenance. When the curing age is 28 days and the nano-magnesium oxide content is 1.5%, the compressive strength and flexural strength of the modified cement mortar test block are the best. Fourier transform infrared spectroscopy and scanning electron microscopy show that the expansion effect of nano-magnesium oxide makes the microstructure of cement-based materials denser and more uniform. This paper also discusses the influence mechanism of nano-magnesium oxide on cement-based materials, which provides a reference for the application of nano-magnesium oxide in cement-based materials.
... At the same time, it has brought some problems: the concrete of the subway underground station faces various constraints and loads, early cracks are hard to avoid [1], and cracking and leakage have caused the failure of underground concrete structure, resulting in huge engineering losses. The traditional anti-cracking method for concrete is a single mineral admixture or admixture [2][3][4][5][6], and mostly ordinary concrete, while the concrete of the underground subway station is mostly high-performance concrete (HPC). ...
In view of the easy cracking of the high-performance concrete (HPC) of the subway underground station floor, the effects of fly ash, basalt fiber, expansive agent, and water reducer on the compressive strength, initial crack time, through-crack time, and crack area of the HPC on a subway underground station floor at different ages by orthogonal experiment are examined. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP) are used to further analyze the microstructure and product composition of the optimal ratio HPC and reference concrete. The results show that with the increase in the content of fly ash and expander, the 7 d and 28 d compressive strength of the HPC gradually decreased. However, as the content of basalt fiber increased, the 7 d and 28 d compressive strength of the HPC gradually increased. The 7 d and 28 d compressive strength of the HPC increased and then decreased with the increase in water-reducer content. When the content of fly ash, basalt fiber and expander increased, the initial crack and through-crack time of the HPC delayed gradually, and the crack area gradually decreased. When the fly-ash content reached 30%, the cracking area accounted for 65.1% of the concrete with 15% fly-ash content. When the basalt fiber content reached 0.4%, the cracking area accounted for 56.5% of the concrete with 0.1% basalt fiber content. When the expander content reached 10%, the cracking area accounted for 60.5% of the concrete with 4% expander content. With the increase in the content of water reducer, the initial crack and through-crack time of the HPC gradually advanced, and the crack area gradually increased. When the water-reducer content reached 1.3%, the cracking area accounted for 105.7% of the concrete with 1.0% water-reducer content. The addition of fly ash and expander can produce a large number of crystalline products to fill the pores, and the disordered distribution of the added basalt fibers increases the compactness of the structure; moreover, the internal micro-pores increase, and the macro-pores decrease, thus improving the crack resistance.
... The length change of the specimens consists of drying shrinkage and volumetric expansion (Gu et al., 2021b). The drying shrinkage was largely attributed to the water evaporation (Saliba et al., 2011), while the expansion induced by the conversion of periclase to brucite was a typical characteristic of magnesia-based cement (Mo et al., 2012). Fig. 11 shows the effect of acid water on length change of MOS cement at air curing stage. ...
Phosphate tailings (PT) and acid water, by-products of phosphorus chemical industrial chain, are usually identified as waste because they can hardly be reused in cement production. This paper proposes an approach of recycling solid waste PT and acid wastewater in preparing a high value-added magnesium oxysulfate (MOS) cement. Results show that the phosphorus in acid water promotes the formation of 5Mg(OH)2·MgSO4·7H2O (517) phase, thereby improving compressive strength of MOS cement. MOS cement prepared with undiluted acid water present excellent resistance to water and acid. The incorporation of calcined PT lowers the strength of MOS cement but improves the volume stability. The phosphorus and fluorine in acid water are effectively immobilized by the cement matrix. Moreover, recycling of PT and acid water to produce MOS cement not only largely consumes the waste and benefits the environment, but also reduces the cost of waste disposal and cement production.
... Ma [9] suggested that for the same dosage incorporation of MEA, the concrete under higher curing temperature exhibited faster expansion and reached greater ultimate expansion during a shorter period. In addition to the study of MEA to improve the concrete cracking resistance [10], Zhang and Deng [11] further conducted an in-depth study on the expansion mechanism of the cement paste incorporating MEA. It was found that the expansion was caused by the generation and growth of Mg(OH) 2 crystals. ...
The creep of face-slab concrete in a rockfill dam is critical for determining the restrained stress and cracking resistance of the concrete at an early age. In this article, the mix proportion of the face-slab concrete for a rockfill dam under construction without any cracking resistance additive was taken as the reference concrete (called JC) mix proportion. The carbon nanotubes (CNTs) and magnesium oxide expansion agent (MEA) were incorporated into JC to prepare face-slab concretes called NC and PC, respectively. The temperature–stress tests under temperature matching curing (TMC) and constant temperature curing (CTC) modes were conducted on these three kinds of concretes to investigate the effects of CNTs and MEA on the early-age creep properties of the face-slab concrete under variable stress conditions. The results showed that the creep performance of NC concrete under CTC mode was lower than that under TMC mode. Combined with mercury intrusion porosimetry test results, the mechanism of the effect of CNTs and MEA on creep was analyzed. The results showed that the temperature change may lead to the CNTs debonding from the cementitious matrix or matrix cracking for the NC concrete. The incorporation of CNTs can increase the early-age creep and improve the cracking resistance of concrete.
... Hydration of MEA generates Mg(OH) 2 to compensate the shrinkage of concrete and even causes micro expansion at early curing age, leading to degradation of performance such as mechanical properties [24][25][26][27]. Mo et al. [28] reported that MEA should be added to the concrete as an admixture. Cao et al. [29] proved that the optimal water-to-cement (w/c) ratio of MEA was 0.4, too high or low w/c ratio would affect the expansion or the hydration rate of MEA. ...
The addition of Magnesia Expansion Agent (MEA) to reduce shrinkage of concrete has attracted attention gradually. In this study, three types of MEA with different activity, i.e. R, M, and S were added into concrete by 5% weight of cement. The effects of MEA on hydration, mechanical properties, shrinkage and durability of concrete were comprehensively evaluated. The results indicated that MEA has a certain delaying effect on hydration of cement and the effect is related to its activity. The addition of MEA exhibites a larger porosity at early curing age leading to negative effects on the mechanical properties of concrete. Later, the microstructure of concrete is refined due to the micro-expansion of MEA. As a result, the mechanical strength of concrete increases 5–20 % at 84 d. Meanwhile, the autogenous shrinkage of concrete with MEA of type R, M, and S decreases by 47.6 %, 49.7 %, and 23.8 % at 90 d, respectively. Because of the smaller MPPD and lower porosity, concrete with MEA of higher activity presents better resistance to chloride penetration. In addition, the models of shrinkage and chloride diffusion coefficient of concrete with MEA of different activity were proposed and verified, which can be used in the shrinkage and service life prediction of concrete with MEA. Furthermore, with the increase of MEA activity, the frost resistance of concrete decreases. On the contrary, the lower the MEA activity, the lower the sulfate resistance of concrete.
... The use of expanding agents such as magnesium oxide (MgO), calcium oxide (CaO), superabsorbent polymers, and saturated aggregates can help mitigate the effects of shrinkage and inherent debonding of cement at the interface (Jafariesfad et al. 2017c; Saito et al. 1991;Mo et al. 2014). The successful use of MgO as an expansive agent in cement has been acknowledged by several researchers (Mo et al. 2012;Sherir et al. 2017) who have reported the self-healing ability of the MgO-enhanced cement systems. To produce sound cement for use in real well operations, the recommended volume of MgO should be limited to Ä 6% (ASTM 2001). ...
A fundamental understanding of the mechanical properties of zonal isolation materials is important for predicting well integrity during well operation conditions. Conventionally, the mechanical properties of zonal isolation materials are tested at ambient temperature using uniaxial testing. This study examined the mechanical properties of alternative zonal isolation materials such as rock-based geopolymer, thermosetting resin, and an industrial class expansive cement under realistic well conditions by triaxial testing. Mechanical properties such as Young’s modulus, Poisson’s ratio, cohesive strength, friction angle, and compressive strength of these materials at 30 and 90°C were compared. The effect of confining pressure on the mechanical properties of the materials was also examined. The findings of this study show that all selected materials possess compressive strength at 30 and 90°C and that the compressive strength of all the selected materials is strongly impacted by temperature and confining pressure. The Young’s modulus of all the selected materials was unaffected by confining pressure, while only the Young’s modulus of thermosetting resin was sensitive to temperature. The influence of temperature on the Poisson’s ratio varied from one material to another. In addition, when the test temperature increased, the friction angle of neat Class G and geopolymer decreased.
... This observation was consistent with the previous observation that the self-desiccation was mostly determined by the degree of alkali activation [39]. It should be noted that the C1M4 binder exhibited slight expansion after 28 days of ambient curing, probably because the excessive addition of Mg(OH) 2 acted as the expansive agent in the binder to mitigate the volume shrinkage [40]. The drying shrinkage originates from the moisture loss in the hardened binder suffered the external evaporation. ...
Delayed strength development and long setting times are the main disadvantageous properties of Na2CO3-activated slag cements. In this work, combined auxiliary activators of Ca(OH)2 and Mg(OH)2 were incorporated in one-part Na2CO3-activated slag binders to accelerate the kinetics of alkali activation. The properties and microstructure evolution were investigated to clarify the reaction mechanism. The results showed that the additions of auxiliary activators promoted the hardening of the pastes within 2 h. The 28 days compressive strengths were in the range of 39.5–45.5 MPa, rendering the binders practical cementitious materials in general construction applications. Ca(OH)2 was more effective than Mg(OH)2 in accelerating the kinetics of alkali activation. The dissolution of Ca(OH)2 released more OH− and Ca2+ ions in the aqueous phase to increase alkalinity in the aqueous phase and promote the formation of the main binding gel phase of calcium-aluminosilicate hydrate (C-A-S-H). An increase in the Ca(OH)2/Mg(OH)2 ratios increased autogenous shrinkage and decreased drying shrinkage of the binders. The formation of a compact pore structure restricted the water evaporation from the binders during the drying procedure.
... [13,14]. Due to the action of the expansive agent, the volume of grout will expand during the hydration process [15,16]; the constrained stress generated by the volume expansion of the grout causes the fractured rock masses in the grouting area to squeeze together, and then a complete stone mass is formed under the bonding action of the expansive grout, resulting in a "squeezing before bonding" grouting reinforcement effect, which improves the overall reinforcement strength [14,17,18]. ...
Adding an expansive agent to ordinary grout can cause an expansion in volume, but also reduces its strength. In order to improve the strength of expansive grout, quartz sand is used as the strength enhancement additive. In this study, the expansion behavior and mechanical properties of the expansive grout with quartz sand are explored, through expansion development monitoring, uniaxial compression strength (UCS), acoustic emission (AE), SEM and XRD test methods. The results showed that: (1) The final expansion ratio and expansion development of the samples are related to the use of an expansive agent, but not affected by quartz sand. With the increase in expansion agent content, the average expansion ratios of the samples are 0.03%, 0.16%, 0.67%, 1.06% and 1.48%; (2) The UCS of the samples decreases with the increase in expansive agent content but increases with the increase in quartz sand content. Compared with no quartz sand, and with the increase in quartz sand content, the average strength of the samples increased by 10.51%, 29.88%, and 37.92%; (3) Quartz sand does not effectively participate in the hydration reaction, but it can effectively enhance the strength of the expansive grout without affecting its volume expansion, which makes it an ideal expansive grout strength enhancement additive.
... The thermal expansion capitalizes on higher temperature exceeded with development of autogenous shrinkage simultaneously. The pastes shrank rapidly due to the decreasing temperature as a result the hydration of MgO also contributed to the deformation [35]. ...
Magnesium oxide (MgO) based cements finds its way active in current researches where diverse range of applications and characteristics such as production process, reactivity and physical properties are essentially focus with the perception of individual expansion objectives. In general, relativity between distinctive MgO characteristics is examined in conjunction with the impact of MgO embodiment on the resources of cementitious materials is further considered. MgO is a key to develop the construction industry thereby mechanical strength and durability performance of cement paste, adhesive and concrete composites impose of MgO needs to be explored. Subsequently, this research paper explicitly defines the investigation of MgO cement composites in terms of compressive and flexural behavior, toughness, tensile and durability performances, flexibility, water susceptibility, porosity, carbonation, chloride ion diffusion, shrinkage and degree of hydration. In this regard, to application of magnesia-based cement products is influenced by various factors such as raw material, composition, performance. The review provides a detailed information of current research available related to magnesia-based cement products based on its properties.
... Figure 8 shows that an appropriate mixed amount of nano-MgO significantly improved the expansion performance of the cement paste after boiling and autoclaving, which reflected the paste's stability. Nano-MgO reacted slowly with water, but it had a very fine particle size and high surface area [3,34], properties that increased its reaction and made for a more uniform expansion in the cement paste [2]. Its expansion after boiling and autoclaving was amplified with an increased amount of nano-MgO because the MgO hydration rate was greatly affected by temperature. ...
Many scholars are concerned about the effect of nano-MgO as an expansion agent on the performance of cement-based materials at an early age, but over a long period less attention is paid to expansion stability and mechanical properties. This article examines the influence of nano-MgO on the long-term consistency, fluidity, expansion stability, hydration, and mechanical properties of 30% fly ash cement-based materials and improves research into nano-MgO as an expansion agent. Expansion performance, flexural and compressive strength, and stability after boiling and autoclave treatment were tested for specimens mixed with a 2, 4, 6, 8 and 10% cementitious material mass of nano-MgO. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were employed to study their hydration process and microstructure. The results showed that nano-MgO had an obvious effect on the consistency, fluidity and expansion performance of cement paste. After curing in water for 365 days and autoclaving thereafter, the hydration of nano-MgO was relatively complete. The volumetric expansion pressure of the magnesium hydroxide (Mg(OH)2) crystals and the crystallization pressure generated after their continuous precipitation were the main reasons for the expansion of the slurry. Nano-MgO improved the microstructure of cement paste and significantly enhanced its long-term flexural strength and compressive strength. When the content of nano-MgO was less than 10%, the cement with 30% fly ash had good long-term stability with the potential to compensate for the shrinkage of large-volume concrete.
... To maintain the volume stability of concrete or reduce drying shrinkage (or thermal contraction), expansive hydraulic cement containing expansive agents or an expansive component was studied and utilized over the past half-century; in particular, shrinkage-compensating concrete is used worldwide [2][3][4][5][6][7][8][9][10]. e three common expansive portions in expansive hydraulic cement are based on calcium sulfoaluminate (4CaO·3Al 2 O 3 ·SO 3 ), calcium oxide (CaO), and magnesium oxide (MgO). ...
e expansion mechanism of magnesium oxide expansive hydraulic cement as a novel expansive hydraulic cement was reviewed. Anisotropic crystallization results in crystal growth pressure, causing volume expansion while also increasing the porosity of the whole system. e theoretical relationship between porosity and expansion was analyzed. A basic method is given for predicting the expansion rate considering the expansive agent content in MgO expansive hydraulic cement. A concise equation is proposed for calculating the ultimate expansion. A theoretical relationship between porosity and expansion is presented. e compressive strength and durability of magnesium oxide expansive hydraulic cement were analyzed considering porosity changes and compared with hydraulic cement. If the expansion rate exceeds 0.8%, the mechanical properties and durability changes caused by porosity should be considered. If magnesium oxide expansive concrete is used with restraining in real structure, extra compressive stress is generated and the porosity decreases, compared with that during free expansion. In particular, for strain-hardening cementitious composites, expansion confined with the fibers present in the composite is beneficial for refining cracks and improving the self-healing ability of these materials whenever exposed to humid environments. is paper describes the expansion mechanism and properties of magnesium oxide expansive hydraulic cement for engineering applications.
... In this study, it was considered that calcium oxide and calcium sulfoaluminate react violently in a highly alkaline environment and are difficult to control; it is thus difficult to achieve good shrinkage compensation effects. The concept of improving mass concrete volume stability in water conservancy projects was used, and MgO was adopted to compensate for GI shrinkage [25][26][27][28][29]. ...
Geopolymers (GI) possess excellent seawater erosion resistance because of stable hydration products and compact microstructures, and therefore, they have broad application prospects in marine engineering if their large volume shrinkage behavior can be mitigated. In this study, active MgO was used to compensate for different GI shrinkage stages and a scheme was proposed for staged compensation. Autogenous shrinkage was controlled by adjusting the content, activity, and combinations of active MgO with particle sizes of 1–100 µm. Based on the effects of MgO on shrinkage, GI paste composition and microstructure, the GI reaction process and mechanism of MgO-compensated volume shrinkage were examined and summarized. The results showed that the GI paste shrinkage decreased with MgO addition, and the higher the MgO activity, the smaller was the paste shrinkage in the early stages of hardening. Late-stage shrinkage of hardened paste was improved with the addition of low-activity MgO. Combining low and high activity MgO addition effectively ameliorated the paste hardening shrinkage at different stages. In the high-alkalinity environment of the liquid-phase GI reaction, the Mg(OH)2 product presented microcrystalline structures, and it was dispersed in the cement matrix, which resulted in uniform volume expansion. Meanwhile, other reaction products containing magnesium were produced, filling pores in the hardened paste and effectively compensating for volume shrinkage during the hardening process. Based on the effects of MgO on the composition structure, reaction process, and volume shrinkage performance of GI slurries, this study proposes a mechanism for MgO to compensate for the volume shrinkage of GI.
... In research and practice, during the dam concrete preparation process, an appropriate amount of magnesium oxide that has been burned under high temperatures can be added to make the concrete expand, which can compensate for shrinkage deformation [4,5] during a temperature drop. In combination with regular auxiliary measures (e.g., surface heat preservation and maintenance), MgO can be added as an expansion agent for mass concrete to control self-generated volume deformation and thereby reduce and eliminate dam concrete cracks. ...
The low swelling property of magnesium oxide concrete is a significant feature that can be used to control the cracking of mass concrete. Based on the characteristics of the chemical reaction, this work proposes a coupled hydro-thermo-mechanical model that can be implemented with the finite element method for predicting the autogenous volumetric deformation of magnesium concrete. By introducing the degree of the hydration reaction of magnesia and the degree of the hydration reaction of cementitious materials as intermediate variables of the chemical reaction system, a prediction model of the concrete temperature and chemical fields is established, and using this model, the effect of the temperature on the reaction rate can be considered in real time. In addition, by combining the relationship between the degree of the hydration reaction of magnesium oxide and the comprehensive expansion of concrete, a mathematical model for calculating the expansion stress of magnesia concrete was established. The algorithms were derived by mathematical equations, and the simulation results were compared to the experimental temperature and autogenous volumetric strain curves, which showed that the hydration model provides a relatively high accuracy. The model was also applied to an arch dam, and the coupled thermo-chemical-mechanical responses of mass concrete during construction were investigated. Simulation results show that the increase in temperature (hydration of cementitious material) and expansion volumetric deformation (hydration of MgO) of the concrete on the upstream and downstream surfaces lags obviously behind that of the inner regions. Quantitative analysis for differences of internal and external expansion is worthy of further attention and study on a basis of further experimental data as well as monitored data.
... As described in the Materials and Methods section (Section 2), this shell contains some unreacted magnesium oxide. When mixed with fresh concrete, magnesium oxide reacts with water, forming a stable defensive covering of magnesium hydroxide [25]. The formed hydroxide crystals that are basically insoluble could effectively seal the pores of EC preserving the bacterial spores. ...
One of the biggest challenges in the development of a biological self-healing concrete is to ensure the long-term viability of bacteria that are embedded in the concrete. In the present study, a coated expanded clay (EC) is investigated for its potential use as a bacterial carrier in biological concrete. Eight different materials for coatings were selected considering cost, workability and accessibility in the construction industry. Long-term (56 days) viability analysis was conducted with a final evaluation of each coating performance. Our results indicate that healing efficiency in biological concrete specimens is strongly related to viable bacteria present in the healing agent. More viable bacteria-containing specimens exhibited a higher crack closure ratio. Our data suggest that the additional coating of EC particles improves long-term bacterial viability and, consequently, provides efficient crack healing in biological concrete.
... For the MgO-based cement, the expansion was largely attributed to the hydration of MgO to Mg(OH) 2 [24,36], and the formation of hydration products restrained this expansion. Expansive stress occurred during the hydration process, and reflected in the macro-expansion of mortar specimens [37,38]. Shrinkage also needed to be noticed when the specimens were cured in air. ...
A new magnesium-calcium oxysulfate cement (MCOSC) was proposed by replacing MgSO4 in magnesium oxysulfate cement (MOSC) with untreated flue gas desulfurization gypsum (FGDG) at levels of 25%, 50%, 75% and 100%. The synergistic effect of FGDG and chemical additives, including citric acid, ammonium citrate tribasic and ammonium dihydrogen phosphate on the mechanical performance and hydration mechanism of MCOSC was investigated. The results showed that three additives all promoted the formation of 5 Mg(OH)2·MgSO4·7H2O (5·1·7) phase and hindered Mg(OH)2 precipitation, improving mechanical properties of MCOSC. FGDG incorporation induced no new crystalline phase, but gypsum crystal provided the space for nucleation of 5·1·7 phase. FGDG also reacted with MOSC to form amorphous phase, which was identified as a magnesium-sulfide-calcium-hydrate gel. Moreover, it affected the morphology of 5·1·7 phase and lowered the temperature of endothermic peaks in the paste during the heating process. The additive-incorporated specimens with 25% FGDG replacement presented a superior compressive strength, water resistance and volume stability. This confirmed the effective application of FGDG in preparing MCOSC, and it also provided an approach for recycling waste gypsum.
... The Mg cation can lead to degradation of aggregates and reduction of soil hydraulic conductivity. However, nano-Mg has a slow reaction with water (MgO + H 2 O → Mg(HO) 2 ) (Mo et al. 2012;Polat et al. 2015), a very fine particle size, and high specific surface area (Polat et al. 2015). These properties increase its reactivity, improve aggregation and increase soil hydraulic conductivity. ...
The aim of this study was to investigate the long-term effect of magnesium and iron nanoparticles on solute transport and its parameters, which has not been investigated to date. Different concentrations of two types of nanoparticle metal oxides, MgO and Fe3O4, were mixed with a loamy soil and their effects on solute transport and its parameters after three years were investigated. Nanoparticles improved soil structure through increased macro pores compared to the control, by 2.3% in the 3% concentration of MgO to 15.3% in the 3% concentration of Fe3O4. Immobile water content of the soil decreased from 0.42 in the control to 0.05-0.24 cm³ cm⁻³ in the treatments. The mass transfer coefficient was decreased from 0.48 in the control to 0.005-0.37 in the nanoparticle treatments. Moreover, nanoparticles increased the breakthrough curve peak concentration and decreased discharge time of chloride in the extract from 0.41 and 88.8 min in the control to 0.62-0.76 and 24.7-87.9 min in the treatments, respectively. The results of this study showed that the nanoparticles affected the solute transport and its parameters in the soil. Therefore, nanoparticles could be used to simplify the nutrients transport and leaching of salt and contaminants.
... MgO-based expansive cement (Mo et al. 2012;Liwu Mo et al. 2014) and ZY-type™ expansive cement (Miao et al. 2019) are used to compensate shrinkage cracks in China. MgO expansive agent was applied to the wall structure to minimize the shrinkage cracks (Yu et al. 2019;Li et al. 2018). ...
This paper investigates the strain behavior of normal concrete and expansive concrete used in three different newly constructed reinforced concrete structures, i.e., slabs, water tank walls, and pavements. Expansive additives with different chemical compositions were used for expansive concrete structures. Strain gauges and thermocouples are installed in different locations in each of the structures to study the effects of restraints from reinforcement, adjacent structural member, base friction, as well as effects of environment, such as moisture loss due to drying and sunlight. The results indicated that expansive concrete could reduce contraction strain due to shrinkage at long term. Environmental conditions, such as ambient temperature and relative humidity, causing drying of the concrete surface resulted in non-uniform strain along the thickness of the structure. Reinforcement as an internal restraint is also a significant factor for the expansion and shrinkage strains of the structures. Moreover, the level of expansion and shrinkage strains can also be affected by external restraints, such as base friction, provided by the subbase materials, adjacent walls, and base slab, etc. The characteristics of strains in concrete and reinforcing bars, with various mentioned restraint and environment conditions can be observed in this study.
Traditional grouting reinforcement has been well reported only to provide a bonding effect to a jointed rock mass. Nowadays, the expansive grout technology has been touted as an alternative to provide squeezing reinforcement effects to a jointed rock mass. Still in infancy, there is a lack of state-of-knowledge on its fundamentals and applications to enhance stability in the surrounding rock. This review article aims to define, characterize, and summarize the mechanism and behavior of cement/concrete expansion additives, cement foaming additives, and high soundless cracking agents (HSCA). Further, this review discussed the characterization of the expansion behavior of cementitious materials, and highlighted the potential applications of expansive grout in grouting reinforcements. This review is focused on determining the optimum selection of suitable expansive materials needed for expansive grout to improve its reinforcement efficiency, in addition to examining technical studies on the expansion mechanism and behavior of distinct expansive cementitious materials. The expansive cementitious materials were analyzed based on four mechanical criteria in grouting and concrete structures: volume expansion ratio, expansion development time, uniaxial compressive strength, and expansion stress. This overview is expected to enrich the current knowledge of expansive cementitious materials and aid researchers in advancing its applications in future global grouting reinforcement projects in jointed rock mass and steeply inclined layered rock mass support systems.
Early-age cracking of concrete is a complex problem related to temperature, restraint and curing conditions. Cracking sensitivity is hard to evaluate quantitatively and the use of expansive additives to mitigate cracking remains empirical in-situ. This study developed a Temperature Stress Testing Machine (TSTM) to evaluate early-age properties of concretes under varied engineering conditions. The general design of TSTM and the mathematical derivation of its working principle are provided. With the developed TSTM, early-age properties (free deformation, restrained stress, elastic modulus and early-age creep effect) of Calcium Sulfoaluminate (CSA) expansive concrete are tested comprehensively. Experimental results indicate that under high temperature condition, CSA concrete is not effective to compensate shrinkage, while the combination of CSA and lightweight aggregate resolves the problem. It offers us valuable insights on potential application of TSTM into mix design of crack-resistant concretes under complex engineering conditions.
MgO content is of importance to the compensation feasibility and soundness of concrete, especially for concrete with different strength grades. The rheological, deformation behavior, and mechanical changes of MgO expansive agent (MEA) (5–8 wt.%) in different strength grades of concrete were revealed. Results show that MEA replacement of cement makes the yield stress and plastic viscosity increasing because of the high specific surface and agglomeration of MEA. Although same quantity of Mg(OH)2 is generated, concretes with different strength grades display different expansion behaviors because of their different pore structures and Mg(OH)2 morphology. Mg(OH)2 that growing around MgO particles presents a colloidal morphology, while Mg(OH)2 that growing in the surrounding pores presents a hexagonal flake-like morphology. The restrained deformation of Grade 40 concrete with 6 wt.% MEA after curing in water for 14 d increases to the maximum expansion, reaching 237 × 10⁻⁶.6 wt.% MEA replacement does not affect the compressive strength and splitting tensile strength of concrete with different strength grades. However, 7–8 wt.% MEA replacement reduces the splitting tensile strength of Grade 40 concrete, and the negative impact is particularly pronounced after air curing.
The influence of MgO compound expansive agent (MCEA) on the early-age cracking failure behavior of concrete considering the influence of temperature, thermal strain, autogenous strain, and creep under the adiabatic-temperature curing mode and uniaxial restrained condition was investigated. The early-age autogenous shrinkage was effectively compensated with the addition of MCEA, and autogenous strain exhibited expansion when the dosage of MCEA was 4% and 6%. The maximum free strain, temperature drop, failure age, failure stress, and failure stress-to-axial tensile strength ratio of concrete increased with increasing dosage of MCEA when the dosage was not more than 4%. The early-age tensile creep of concrete increased with increasing dosage of MCEA. The rate of tensile stress and cracking failure potential of concrete evaluated by integrated criterion decreased with increasing dosage of MCEA when the dosage of MCEA was not more than 4%, but increased with increasing dosage of MCEA from 4% to 6%. A simplified stress-strain failure criterion based on the results of tensile strength, modulus of elasticity, and restrained cracking tests was proposed for estimating the safety of concrete under restrained condition.
MgO-series expansive agents can effectively compensate for the shrinkage and deformation of concrete structures. However, few experimental studies have been conducted on MgO expansive agents, particularly concerning the difference between and effects of submicron-MgO and nano-MgO in high-performance concrete (HPC) with a low water-cement ratio, thereby limiting their application in practical engineering. To clarify the expansion effect and expansion mechanism of MgO expansive agents in HPC, the effects of submicron-MgO and nano-MgO on the strength, toughness, and expansion characteristics of HPC were examined. The test results showed that submicron-MgO and nano-MgO continued to hydrate in the cement environment to produce Mg(OH)2, thus improving the structural compactness and structural strength of HPC. Nano-MgO concrete was found to have more stable mechanical properties and better structural deformability than submicron-MgO concrete. This study provides effective data support and theoretical reference concerning the hydration expansion mechanisms and engineering applications of nano-expanded materials.
In order to study the influence of nano-MgO on the strength characteristics of cement-reinforced waste road recycled aggregate (CRA), the California bearing ratio (CBR) test, unconfined compressive strength (UCS) test, scanning electron microscope (SEM) test, X-ray energy dispersive spectroscopy (EDS) test and X-ray diffraction (XRD) test were carried out. In the test of using nano-MgO modified cement-reinforced waste road recycled aggregates (MCRA), the cement content was fixed at 4%, considering the two ages of 7d and 28d and the four types of nano-MgO content. The mechanical test results show that the addition of nano-MgO has a certain effect of improving mechanical properties; The CBR values of MCRA specimens with nano-MgO all meet the requirements of extremely heavy and extremely heavy traffic levels of expressways and first-class highways; The unconfined compressive strength of 0.4% MCRA can meet the heavy traffic grade requirements of expressways and first-class highways; 0.4% is the optimum dosage of nano-MgO; The bivariate quadratic polynomial mathematical models of CBR value and unconfined compressive strength of MCRA specimens with nano-MgO content and age were established by curve fitting, and the fitting effect was ideal. Finally, the microscopic test was used to further analyze the internal mechanism of nano-MgO enhancing the strength of CRA.
The autogenous shrinkage of cement-based materials is difficult to solve. Some studies have found that nano-magnesium oxide (MgO) can be used as an expansive agent to improve the shrinkage performance of cement-based materials. The basic research on nano-MgO in cement-based materials has appeared, but the research on the durability of cement-based materials by nano-MgO is very rare. In this article, nano-MgO was incorporated into cement mortar, and the experimental research studies effects of nano-MgO content on permeability resistance, crack resistance, sulfate corrosion resistance, and freeze-thaw resistance of cement mortar. The results show that nano-MgO can improve the durability of mortar. The mechanism of nano-MgO is discussed by this article in improving the durability of cement mortar, and it provides a theoretical basis for the further application of nano-MgO in cement-based materials.
Hydration and hardening properties of reactive magnesia and Portland cement composite were evaluated with macro and micro property tests, in order to better understand systematically hydration and hardening mechanisms of reactive magnesia and Portland cement composite. Reactive magnesia content (10 wt%, 35 wt%, 50 wt%, 65 wt% and 90 wt%) obviously affects the hydration and hardening properties of reactive magnesia and Portland cement composite. As the main hydration product of reactive magnesia, Mg(OH)2 crystal shape affected by inner alkaline environment and Mg(OH)2 crystal amount influence the macro volume expansion performance. The Mg(OH)2 crystal size in hardened reactive magnesia and Portland cement composite pastes can be bigger than 2 μm and is big enough to cause micro crack in hardened RMPC pastes, and at the same time to heal micro crack in moist curing condition (20 ± 2 °C and relative humidity ≥ 95%). The loose cluster structure of Mg(OH)2 crystals leads to low binding property of RMPC pastes and causes the increases of the amount of pores in range of 0.1–1 μm in hardened RMPC pastes. The findings will be beneficial for mixture proportion design, manufacture control and properties optimization of reactive magnesia and Portland cement composite concrete.
Ethylenediamine tetra-acetic acid (EDTA) and its disodium salt derivative (EDTA-Na) (0.5% weight of MgO powder) were incorporated into MgO–MgSO4–H2O ternary system to prepare magnesium oxysulfate (MOS) cement. The single effect of EDTA/EDTA-Na as well as the synergistic effect of EDTA/EDTA-Na and citric acid (CA) on the hydration mechanism, phase composition, microstructure and mechanical properties of MOS cement were investigated. The results showed that EDTA and EDTA-Na addition induced no new phase, but the magnesium-EDTA complexes formed by EDTA⁴⁻ and Mg²⁺ ions acted as nuclei for the hydration products, including brucite, 3Mg(OH)2·MgSO4·8H2O (318 phase) and 5Mg(OH)2·MgSO4·7H2O (517 phase). This helped to form a more homogeneous and compact microstructure, thereby improving the durability and volume stability of MOS cement in air curing condition. Moreover, the synergistic effect of EDTA/EDTA-Na and CA preferred the formation of 517 phase to that of brucite. It further improved the water resistance of MOS cement. In this study, EDTA and EDTA-Na were found to be effective additives in improving the characteristics of MOS cement.
Enhancing cracking resistance of face slab concrete is essential for the structural integrity and normal operation of concrete-faced rockfill dams (CFRDs). In this study, the effects of MgO with three reactivities and three dosages on the shrinkage and crack resistance of face slab concrete were systematically investigated by slab test, restrained drying shrinkage test and temperature stress test machine (TSTM). The results indicate that: (1) 7 %–25% of the total drying shrinkage and all (or most of) the autogenous shrinkage can be compensated by adding 5 %–10% MgO. The reactive MgO (M60) starts to compensate the drying and autogenous shrinkage at 1 day and compensates more shrinkage at early age than the moderate reactive MgO (M150) and the weak reactive MgO (M300), while M300 begins to compensate the shrinkage at about 50 days and produces larger compensation than M60 and M150 at late age. (2) M60 performs better than M150 in improving the crack resistance of face slab concrete to constraint and thermal stress. The increase in dosage of M60 and M150 from 0 to 10% prolongs the initial cracking time of face slab concrete by 10.0–27.5 h, increases the cracking strain by 9.2 %–25.7%, enhances the cracking tensile stress (σ) by 4.7 %–18.8% and lowers the cracking temperature (Tc) by 2.6–7.4 °C. Conversely, M300 weakens the cracking resistance. (3) The reactive MgO with relatively high dosage is suggested to eliminate the early shrinkage and to improve the cracking resistance of face slab concrete.
Near-neutral salt activated slag cement has better performance in environmental protection and shrinkage behavior than common sodium silicate -activated slag (AAS) cement. However, the growth of mechanical performance is too slow, which limits its application. Reactive MgO can improve early mechanical properties, while it may have different effects on the shrinkage due to changes in alkali-activators. Two near-neutral salts, sodium carbonate (SC) and sodium sulfate (SS), were selected to prepare AAS cement. The influence of reactive MgO on autogenous and drying shrinkages was studied. The hydration process and pore structure were assessed with hydration heat and nitrogen adsorption, respectively. The phase composition was evaluated by thermogravimetry (TG/DTG) and X-ray diffraction (XRD). The results indicate that the addition of reactive MgO increases the autogenous shrinkage of SC and SS activated slag. This is mainly due to accelerated reaction by reactive MgO. The drying shrinkage of AAS mortars activated with SC increases first and then decreases with the increase in reactive MgO. For SS activated slag, the more the reactive MgO used, the lower the drying shrinkage is observed. This is related to the effect of reactive MgO on pore structure and hydration products. When reactive MgO increases from 2% to 10%, the mesopore in SC activated slag increases at first and then decreases, while the SS activated slag group shows a constant decrease. Reactive MgO promotes the formation of hydrotalcite in SC activated slag, while AFt is found in SS activated slag. Hydrotalcite and AFt are crystalline products, which are beneficial to inhibit drying shrinkage.
It is possible to enhance the ability of crack control and inherent brittleness of ordinary concretes by integrating discrete fibers into concretes. Fiber-reinforced concretes are acknowledged as high-performance building materials due to their high levels of toughness under tensile and compressive loads. It is therefore broadly applied in precast structures, bridges, tunnels, and high-rise buildings. Societal demand has raised the requirements for advanced fiber-reinforced concrete composites with multifunctionality and ultra-high performance such as self-regulating, self-clearing, self-sensing, and self-healing. These special issues focus on the emerging ideas that permit the development of improved or new fiber-reinforced concrete composites and characterizations of the features of advanced fiber-reinforced concretes. Original research papers and authoritative review journals explain the present findings in the advanced fiber-reinforced concrete composite field are anticipated to cover a variety of topic. The potential topics include, but are not restricted to structural applications of advanced fiber-reinforced concrete composites, fiber-bridging behaviors, multiple micro-cracks, strain-hardening behaviors, property characterization, nanofiber reinforced concrete composites, advanced fiber-reinforced cement-free composites, ultra-high performance fiber-reinforced concretes, multifunctional fiber-reinforced concrete composites, and advanced fiber-reinforced concrete composites.
More advanced techniques have been proposed for construction purposes and improvement cement because of the global sustainability demand. Alongside the integrations of positive policies, emission reductions for all infrastructural projects can be achieved when there shall be swift scale-up in the novel cement use. The book explores techniques such as self-consolidating and advanced nano cements, green cement, and steel fiber reinforcements and how they contribute to construction cost reductions and environmental sustainability. In comparison with the traditional cement, improved multifunctional nano-engineered concretes exhibit advanced functionalities. They for example have up to 146% and 76.5% respective compressive and flexural strengths. They also have improved electrical and thermos-mechanical performances with considerable declines in absorption of waters of about 400%. Technologies of modern engineering aim at generating multifunctional and ultra-high performance concrete substances because of the increased demands for cost-effectiveness, sustainability, and durability. Such building materials are marked by long-term performances and advanced mechanical characteristics. Also, they incorporate characteristics that promote different uses making them sustainable for future applications. Advanced concrete composites are important in multifunctional uses such as in chemical and marine exposed environments because of their high corrosion resistance, affordability, high durability, and lightweight nature. Composite materials (combinations of aggregates) offer an in-built mixture of toughness and stiffness with corrosion resistance and lightweight properties. Such materials are obtained from various compositions with different physical and chemical characteristics. Combinations of such concretes give special capability that gives composite materials an advantage over other improvement methods. Major classifications are explained below:
Various strategies have been developed to mitigate the autogenous shrinkage of cement-based materials (CBMs), including internal curing and shrinkage compensation. However, the sole use of internal curing or shrinkage compensation is insufficient in many cases. In this study, magnesia-based expansive additives (MEAs) coupled with a superabsorbent polymer (SAP) were investigated as internal curing agents to mitigate the autogenous shrinkage of cement pastes. The internal relative humidity (RH), autogenous deformation and compressive strength of cement pastes containing two different types of MEAs and SAP were examined. The results showed that, with the incorporation of SAP, the RH in the cement paste was maintained at a higher level than that in cement pastes without SAP, and hence autogenous shrinkage was reduced. The combination of SAP and MEA provided very effective compensation for the autogenous shrinkage of cement paste. Moreover, the cement paste containing 8 wt% MEA and 0.2 wt% SAP produced a gentle expansion of 111 microstrain rather than shrinkage. This was attributed to the increased hydration degree of magnesia owing to the additional water supply provided by the SAP. This study provides a novel and efficient way to mitigate the autogenous shrinkage of CBMs.
This article discussed on the potential of autogenous self-healing ability of cementitious composites with magnesium oxide (MgO). Further investigation was conducted on the single blended and combination blended composites. For this purpose, 5% of silica fume and 5% MgO were used for single blended composites whilst for combination blended, both mineral admixtures were combined. A load of 80% ultimate compressive strength of 7 days was applied for cracks fabrication. The healing ability of cementitious composites was evaluated based on macro and micro tests, which included compressive strength, strength recovery, crack sealing and microstructural analysis. The result showed that the presence of silica fume offset the strength reduction by MgO. Autogenous self-healing performance of the MgO specimens was higher compared to specimens without MgO. Moreover, better autogeneous self-healing performance was observed with the inclusion of silica fume alongside with MgO. Promising results were observed in crack sealing of specimens with MgO compared to specimens without MgO.
At present, the traditional ECC material has the disadvantages of high cement content and high cost of PVA fiber, which seriously limits its application in engineering. In order to develop a sustainable building material for engineering, a low-cost and high toughness engineered cementitious composites (ECC) was prepared by using domestic polyvinyl alcohol (PVA) fiber. The effects of expansive agent (EA) and curing condition on the mechanical properties, shrinkage, early hydration degree and volume water loss rate of PVA-ECC were studied. The microstructure of PVA-ECC was analyzed by scanning electron microscopy (SEM). The test results show that EA can effectively improve the flexural and uniaxial tensile properties of PVA-ECC, as well as the shrinkage resistance, early hydration degree and volume water loss rate of PVA-ECC. Unexpectedly, the maximum uniaxial tensile strain of PVA-ECC material can reach 4.68%. In addition, it was found that PVA-ECC exhibited ultra-high tensile toughness when EA content was 10% under natural curing conditions. The above results verify the feasibility of domestic PVA fiber in ECC. This study can open up a new development direction for PVA-ECC in the future.
Previous research indicated that the expansion property of MgO-type expansive agent (MEA) depended strongly on the calcining conditions, i.e. kiln temperature and residence time. However, the intrinsic effect of calcination condition on the expansion property of MEA has not been clearly demonstrated. In the present work, the effects of calcination condition on the microstructure, hydration activity, and expansion property of MEA have been investigated, and their correlations are also studied. Results indicate that the microstructure of MEA is the intrinsic factor that controlling its expansion property, which is influenced by the calcination condition. MEA produced under higher temperature and longer residence time has less interior pores, larger crystal size of MgO, and smaller specific surface area, thus resulting in lower hydration activity and slower expansion at early age, but larger “ultimate” expansion at late age. While, a new expansion model of MEA is proposed to explain these results.
As the use of high-performance concrete has increased, problems with early-age cracking have become prominent. The reduction in water-to-cement ratio, the incorporation of silica fume, and the increase in binder content of high-performance concretes all contribute to this problem. In this paper, the fundamental parameters contributing to the autogenous shrinkage and resultant early-age cracking of concrete are presented. Basic characteristics of the cement paste that contribute to or control the autogenous shrinkage response include the surface tension of the pore solution, the geometry of the pore network, the visco-elastic response of the developing solid framework, and the kinetics of the cementitious reactions. While the complexity of this phenomenon may hinder a quantitative interpretation of a specific cement-based system, it also offers a wide variety of possible solutions to the problem of early-age cracking due to autogenous shrinkage. Mitigation strategies discussed in this paper include: the addition of shrinkage-reducing admixtures more commonly used to control drying shrinkage, control of the cement particle size distribution, modification of the mineralogical composition of the cement, the addition of saturated lightweight fine aggregates, the use of controlled permeability formwork, and the new concept of “water-entrained” concrete. As with any remedy, new problems may be created by the application of each of these strategies. But, with careful attention to detail in the field, it should be possible to minimize cracking due to autogenous shrinkage via some combination of the presented approaches.
Fundamental studies of the early-age desiccation of cement-based materials with and without a shrinkage-reducing admixture (SRA) have been performed. Studies have been conducted under both sealed and drying conditions. Physical measurements include mass loss, surface tension, X-ray absorption to map the drying profile, internal relative humidity (RH), and autogenous deformation. Interestingly, although the SRA accelerates the drying of bulk solutions, in cement paste with a water-to-cement (w/c) ratio of 0.35, it actually reduces the measured drying rate. Based on the accompanying X-ray absorption measurements and a simple three-dimensional microstructure model, an explanation for this observation is proposed. In sealed systems, at equivalent hydration times, the SRA maintains a greater internal RH and reduces the induced autogenous deformation. Thus, these admixtures should be beneficial to low w/c ratio concretes undergoing self-desiccation, in addition to their normal usage to reduce drying shrinkage.
The use of shrinkage-reducing admixtures (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. Various forms of SRA are commercially available and they may act through different mechanisms. Some SRA mainly acts on drying and weight loss leading to shrinkage. In this paper, the influence of a liquid SRA on plastic shrinkage, long-term shrinkage, mechanical characteristics as well as concrete pore structure were investigated. Samples of concrete were prepared with two water-to-cement ratios (w/c) to design ordinary and high strength concrete. The effect of the shrinkage-reducing admixture was studied by adding 1% to the total mass of binder, while keeping the other parameters constant. At early age, the results indicate that the SRA lead to the same plastic shrinkage for w/c = 0.65 while it reduced the plastic shrinkage by 25% for w/c = 0.43. Drying shrinkage was assessed from 1 day on hardened concrete. The SRA reduced the 7 day drying shrinkage for w/c = 0.65 and w/c = 0.43 concrete mixtures by up to 56%-31%, respectively, and the 70 day drying shrinkage by up to 33%-25% when the specimens were cured for 24 h then stored at relative humidity of 50%. At equal water-to-cement ratios, the SRA is seen to be more efficient in reducing drying shrinkage at early ages. These findings suggest that the SRA is most effective when internal relative humidity is relatively high or when higher porosity exists in the material. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 300 to 1000 nm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels where capillary stresses are the main cause of shrinkage. Thus, this phenomenon may contribute in the reduction of drying shrinkage that occurs when this liquid SRA is used in concrete.
In this paper, the manufacture of MgO-based expansive agent using dolomite as raw material was studied. The decomposition of dolomite was discussed at first by DTA-TG analysis and on the crystalline view. On the characteristics of decomposition of dolomite, the authors think it is possible to use dolomite as raw material, but the silica-bearing mineral is considered to combine the CaO released from dolomite to form silicate. The phases of MgO-based expansive agent are mainly MgO, C2S and a little amount of CaO. The expansion of resulting expansive agents by autoclaving testing and hot water curing at 80 °C shows that the MgO-based expansive agents have desired expansion. If the burning temperature and burning time, dosage in cement are controlled according to the requirement of concrete construction, the MgO-based expansive agent can be used to compensate the shrinkage as designed.
The idea that MgO can be used as an expansive component in concrete came from practice. The addition of MgO is to compensate the thermal shrinkage during the period of the temperature-decreasing process in mass concrete. In this paper, the delayed expansive properties of MgO and its effects on porosity, pore structure, permeability, sulfate resistance and strength of the hardened cement paste (hcp) are discussed in detail. The experimental results show that the expansion of cement pastes takes place between age 1 day and 180 days, and ceases at near age 180 days. The free expansion of hcp increases the total porosity and the size and volume of macropores in hcp, hence increasing the permeability and reducing the sulfate resistance and the compressive strength of hcp. The restrained expansion of hcp can produce a “self-compaction” in hcp, causing the reduction of the porosity and the size and volume of macropores, which is beneficial to improving the physical and mechanical properties of hcp.
A novel approach to has been recently proposed mitigate self-desiccation, one of the foremost problems of high-performance concrete (HPC). It is based on incorporation of pre-soaked lightweight aggregate in the concrete mix. Such aggregate acts as an internal water reservoir preventing reduction of relative humidity in the cementitious matrix. This method is known as “autogenous” or “internal” curing. Recent studies demonstrated that this kind of curing could be successfully applied to obtain improved HPC with reduced sensitivity to cracking. However, the content of lightweight aggregate required to completely eliminate autogenous shrinkage was high, and this caused a reduction of compressive strength and an increase in the cost of the concrete.Recently, a work has been conducted to optimize the internal curing strategy by eliminating autogenous shrinkage while using the smallest possible amount of lightweight aggregate. The effect of grain size, pore structure and type of the lightweight aggregate was studied. The next step in this study––the effect of the properties of the cement paste matrix on the effectiveness of internal curing is discussed in this paper.
A new mechanism has been proposed to explain the delayed expansions, due to the presence of dead-burnt CaO or MgO, of Portland cement based materials. The mechanism is based on the crystal growth pressure, the solubility of hydroxide crystals, their growth habits and diffusion of Ca2+ and Mg2+ through the electrical double layer which forms round cement hydration products. In this mechanism the crystal growth pressure is the expanding agent and the others are modifiers.
Restrained autogenous shrinkage in high-strength lightweight aggregate concrete was investigated. Effects of a partial replacement of normal-weight aggregate by lightweight aggregate on autogenous shrinkage were also discussed. The concrete with saturated lightweight aggregate exhibited no autogenous shrinkage, whereas the normal-weight concrete with the same matrix exhibited large shrinkage. A partial replacement of normal-weight aggregate by 25% by volume of saturated lightweight aggregate was very effective in eliminating the autogenous shrinkage and restrained stresses of the normal-weight concrete. It should be noted that the internal supply of water from the saturated lightweight aggregate to the high-strength cement matrix caused continuous expansion, which may be related to continuous hydration.
Representative and quantitative microstructural information of cement-based materials can be obtained in the backscattered electron and X-ray modes of the scanning electron microscope (SEM). One prerequisite, of several, is to use flat specimens. Microstructures that are minimally affected by the grinding and polishing necessary to produce the flat surface can be obtained. It is essential to fill the pores of the specimen with epoxy resin prior to grinding and polishing. After hardening, the epoxy stabilizes the microstructure and enables it to withstand the stresses of grinding and polishing without alteration. In the present paper, we describe a preparation technique that we consider to have produced excellent polished specimens. The importance of epoxy impregnation is demonstrated.
The practice of using expansive agents has been recommended to manufacture shrinkage-compensating concrete provided that an adequate wet curing is carried out. On the other hand, more recently the use of shrinkage reducing admixture (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage.However, neither expansive agent nor SRA, when used separately, can definitively and safely avoid the risk of cracking caused by drying shrinkage in real concrete structures under the practical conditions of curing on many job-sites.This paper shows the advantages of the combined use of SRA and CaO-based expansive agent to produce shrinkage-compensating concrete even in the absence of an adequate wet curing.
This paper describes a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept consists of using fine, superabsorbent polymer (SAP) particles as a concrete admixture. This leads to water entrainment, i.e. the formation of water-filled macropore inclusions in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, self-desiccation and water entrainment are described and discussed. The description is based on a reinterpretation of Powers' model for the phase distribution of a hydrating cement paste. The paper forms the first part of a series. In the second part, experimental observations will be presented.
The hydration chemistry of expansive cements is described with emphasis on the components for formation of ettringites. Expansive cement concrete as ‘shrinkage-compensating concrete’ or ‘chemical prestressing concrete’ is applied to many kinds of concrete construction. This paper outlines the chemical composition of calcium sulfoaluminate and lime-based expansive admixtures and discusses the expanding mechanism, chemical prestressing, and typical properties of expansive cement concrete. Finally, research and development are introduced with respect to expansive admixtures. Furthermore, these expansive admixtures are based on cement minerals, except ‘gas forming admixture’ and ‘admixtures containing granulated iron filings’.
The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m3 of saturated LWA in HPC, providing 27 kg/m3 of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.
This investigation was carried out to study the effects of using a replacement percentage of saturated lightweight fine aggregate (LWA) as an internal curing agent on the shrinkage and mechanical behavior of Engineered Cementitious Composites (ECC). ECC is a micromechanically-based, designed high-performance, fiber-reinforced cementitious composite with high ductility and improved durability due to tight crack width. Standard ECC mixtures are typically produced with micro-silica sand (200 µm maximum aggregate size). Two replacement levels of silica sand with saturated LWA (fraction 0.59–4.76 mm) were adopted: the investigation used 10 and 20% by weight of total silica sand content, respectively. For each LWA replacement level, two different ECC mixtures with a fly ash-to-Portland cement ratio (FA/PC) of 1.2 and 2.2 were cast. In a control test series, two types of standard ECC mixtures with only silica sand were also studied. To investigate the effect of replacing a portion of the silica sand with saturated LWA on the mechanical properties of ECC, the study compared the results of uniaxial tensile, flexure and compressive strength tests, crack development, autogenous shrinkage and drying shrinkage. The test results showed that the autogenous shrinkage strains of the control ECCs with a low water-to-cementitious material ratio (W/CM) (0.27) and high volume FA developed rapidly, even at early ages. The results also showed that up to a 20% replacement of normal-weight silica sand with saturated LWA was very effective in reducing the autogenous shrinkage and drying shrinkage of ECC. On the other hand, the partial replacement of silica sand with saturated LWA with a nominal maximum aggregate size of 4.76 mm is shown to have a negative effect, especially on the ductility and strength properties of ECC. The test results also confirm that the autogenous shrinkage and drying shrinkage of ECC significantly decreases with increasing FA content. Moreover, increasing FA content is shown to have a positive effect on the ductility of ECC.
This study investigates the effects of spatial distribution of lightweight aggregates (LWAs) on internal curing of concrete. As replacements for normal aggregates, different sizes and amounts of natural pumice LWAs were used as water reservoirs to provide internal curing in mitigating autogenous deformation. Water in the pre-soaked LWAs flows into cement paste during hydration and provides internal curing to counteract the RH loss due to self-desiccation of binding paste. The results show that variations in the autogenous strain of concrete can be evaluated in terms of LWA–LWA proximity. The protected paste volume approach, previously used for air-entrained concrete, is applied to calculate the internally-cured volume of paste. The results show that the experimental rate of mitigation of autogenous strain for different series of concrete specimens, with respect to the reference concrete, gave the best-fitted values at water flow distance of 1 mm. The results indicate that the protected paste volume in internal curing can be determined by calculating the water-entrained volume using image analysis.
High-performance concrete (HPC) is characterized by its low water-to-cementitious materials (w/cm) and improved properties but also it exhibits high internal capillary tensile stress because the development of autogenous shrinkage which could result in early-age cracking risk and premature deterioration. Since the use of HPC in structural elements has gained wide acceptance in the last decades, the large magnitude of early-age autogenous strains and stresses has to be mitigated to enhance the durability of concrete structure. In this paper, internal stress development induced during the development of autogenous shrinkage strains, especially at early-age was investigated on three different types of HPC cured with a combination of two shrinkage-compensating admixtures. Binary HPC made with blended cement containing 10% of silica fume (SF) has been used with three different low (w/c + sf) of 0.15, 0.23, and 0.30. Shrinkage-reducing agent (SRA) and an expansive additive (EXA) were combined and added to the HPC mixtures to minimize autogenous shrinkage magnitude. The results indicate that the greater the autogenous shrinkage developed, the higher the induced internal tensile stress. It has been found that for the reference mixes, more than 90% of the ultimate magnitude of both autogenous shrinkage and self-tensile stress was developed during the first 24 h. However, the addition of a combination of SRA and EXA has resulted in a significant reduction and a gradual development of both autogenous shrinkage and self-tensile stress as compared to the rapid development and large magnitude in the reference concretes. Moreover, a high dimensional stability was obtained for the 0.30 and 0.23 HPC mixtures containing the combination of expansive and shrinkage-reducing admixtures. On the other hand, a slight decrease of the compressive, of the splitting tensile strengths and the modulus of elasticity was observed.
Technique of MgO concrete used in dam constrction
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BSEM image of fly ash cement paste containing 8% of MEA
- Fig
Fig. 10. BSEM image of fly ash cement paste containing 8% of MEA.
Properties of MgO-based expansive additive used in RCC of Longtan hydropower station
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