Slag is the by-product in pig iron, steel, lithium carbonate, phosphor, or copper plants and their primary oxide components are CaO, SiO2
, and Al2
, although the proportions are not the same among different sources [72
]. The effects of slag on UHPC compressive strength, flowability, and shrinkage are summarized in Table 2
, Table 3
and Table 4
, respectively. They also include information such as the water to binder (w/b) ratio, the curing method, specimen size, the superplasticizer to binder (SP/b) ratio, the aggregate to binder (Agg/b) ratio, the mixture type, and other solid ingredients. The same types of tables are used for other SCMs in the following subsections.
Slag tends to decrease the compressive strength of UHPC at an early age because of its low reactivity. Slag has hydraulic properties and reacts with water [73
] and the hydration product of slag is calcium silicate hydrate (CSH) [43
]. Slag is chemically activated by calcium hydroxide (Ca(OH)2
) and gypsum in cement, but its reaction speed is slow, whereas SF reacts with Ca(OH)2
first in UHPC because of its fineness [44
]. Researchers have also reported that slag decreases the heat of hydration [39
]. As a result, slag tends to decrease the compressive strength of UHPC at 3 days or earlier. Most of the research revealed that the compressive strengths at 3 days of the UHPCs with slag decrease by around 4.8–18.1% from the reference specimens. Pyo and Kim revealed that the addition of GGBS decreases the compressive strength at 1 day and 3 days by 39% and 18%, respectively [40
]. They claimed that GGBS slow down the hydration process and setting time, which causes low early strength. Li et al. reported that SSP improves the workability even though it degrades the mechanical properties and durability of UHPC [12
]. The 15% replacement of cement with SSP results in 8.7% lower compressive strength at 1 day than that of the reference specimen. As the SSP has a low hydration activity as well as a retarding effect on the cement hydration, the activity of SSP is lower than OPC and it slowed down the early age hydration. He et al. replaced SF with the LTS that is a by-product in the process of lithium carbonate [50
]. The 10% of the LTS to the total binder results in a 4.8% lower compressive strength at 3 days compared to the control mix with SF only. The addition of LTS decreases the early age compressive strength because the pozzolanic reaction of LTS is slower than SF.
Slag could enhance the late age compressive strength of UHPC; the secondary pozzolanic reaction between slag and Ca(OH)2
in the pore solution produces additional CSH, which increases the packing density of the UHPC [44
]. Liu et al. found that compressive strength increases up to 9% when GGBS content increased to 40% of the binder because the secondary pozzolanic reaction of GGBS is accompanied by consumption of Ca(OH)2
and the densification of the hardened paste [44
]. Abdulkareem et al. reported that the use of GGBS can accelerate the hydration reaction of cement and also improve packing density because its fineness is between those of cement and SF [45
]. When GGBS content increased to 23.6% of binder, the compressive strength increased by 3.3% at 7 days because of the improved packing density and higher cement hydration due to the addition of GGBS. Gupta used GGBS with 35% calcium oxide content that contributes to CSH formation, and thus, improves strength development [39
]. The 60% replacement of cement with GGBS resulted in a 5.2% increase in the UHPC’s compressive strength at 28 days. Yu et al. reported a 10% increase in 28-day compressive strength by replacing 30% of the cement with GGBS [43
]. He et al. successfully replaced 10% of cement with LTS and the compressive strength increases by 2.8% and 6.8% at 28 days and 90 days, respectively [50
]. The fineness of LTS is between those of cement and SF, which can improve the packing density of UHPC and the pozzolanic reaction of LTS also contributes to the late age compressive strength development. Peng et al. used PS that has a similar glass structure to GGBS [48
]. When PS content increased from 30% to 35%, the compressive strength increased by 3.7% because the addition of PS increased both the degree and speed of the pozzolanic reaction, which results in more hydration products in the paste and low porosity. The PS used by Yang et al. has relatively lower reactivity than ordinary slag because of the lower Al2
in the PS can also retard hydration at an early age. The PS of 27.4% of the binder was found to increase the compressive strength at 28 days by 6.3%. The PS can yield small pores in microstructure at an early age and fill those by the hydration product from the long-term pozzolanic reaction. Edwin et al. proved that CS can be used as an SCM in UHPC increasing the amount of CSH [51
]. CS is a by-product of the copper metal smelting process. When CS content increases up to 16% of the binder, the compressive strength of UHPC increases by 3.1% at 90 days. Ahmadm et al. studied the effect of using industrial waste materials like PSS to replace parts of SF [49
]. When replacing 20% of SF, the compressive strength of UHPC can still reach 161 MPa at 28 days, which is the same as that of the reference specimen with SF only and flow diameter slightly decreased, but it was still within the acceptable range.
The particle size of slag can be a critical factor in the compressive strength of UHPC. Randl et al. studied the effect of GGBS and FGGBS on the UHPC compressive strength [42
]. The GGBS and FGGBS have the Blaine values of 4790 and 5620 cm2
/g, respectively. When 38.5% of cement is replaced with FGGBS, it decreases the compressive strength by 1.5%, whereas GGBS decreases the strength by 16.1% because FGGBS results in a higher packing density than GGBS. The study concluded that the packing density of UPHC is an important factor even more than the hydraulic reactivity of slag. Pyo and Kim compared slags with different particle sizes [40
]. The median particle size of FGGBS and GGBS are 2.69 μm and 14 μm, respectively. They reported that the usage of FGGBS increases the compressive strength increased by 7.4% at 3 days, while GGBS decreases the compressive strength with the same dosage of FGGBS. From the hydration heat measurement, they concluded that FGGBS plays a significant role in the early hydration process, which results in a higher compressive strength than the reference specimen.
Some slags may decrease the porosity of UHPC. Liu and Guo studied the effect of SSP on the compressive strength of UHPC [46
]. The particle size distribution of SSP is similar to that of cement. It was found that the compressive strength decreased rapidly when the SSP content is high. With the 16.9% replacement ratio of cement with SSP, the compressive strength decreased by 10.3% compared to the reference specimen that contains SF only because SSP increases the proportion of the pores larger than 50 mm by 33%.
The effect of slag on the UHPC shrinkage possibly depends on the type of slag. Table 4
summarizes the effect of slag on the UHPC shrinkage. It has been proved that the addition of GGBS increases the shrinkage of conventional concrete because slag increases the self-desiccation by consuming pore solution (calcium hydroxide) in a small capillary pore structure [76
]. However, different effects of different types of slags on the UPHC shrinkage have been observed in some studies in the literature. Li et al. found that the total shrinkage of UHPC incorporating SSP is lower than the UHPC without SSP [12
]. As the amount of SSP increases, hydration of cementitious materials decreases at early ages, water consumption is reduced, and, thus, the self-desiccation of UHPC becomes weaker. Yang et al. indicated that as the cement replacement ratio with PS increases, the hydration can be slowed down and the cement dilution effect can improve the UHPC volume stability [47
]. In other words, the high volume of PS can reduce the autogenous shrinkage of UHPC. On the other hand, Liu et al. found that the addition of GGBS can increase autogenous shrinkage [44
]. They insisted that the secondary pozzolanic reaction of GGBS increases the consumption of calcium hydroxide, which increases water consumption. The higher water consumption results in more self-desiccation. They concluded that the secondary pozzolanic reaction of GGBS increases the autogenous shrinkage due to the refined pore structure and the increased depletion of water.
Slag increases the UHPC flowability because of its lower water absorption compared to cement having a slippery surface [79
]. Table 3
summarizes the effect of slag on the UHPC flowability. Pyo and Kim reported that the addition of FGGBS and GGBS increases the UHPC flowability by 11.6% and 4.1%, respectively, compared to the reference specimen with SF only [40
]. Yang et al. reported that the use of PS can significantly improve the flowability of UHPC. The flowability of UHPC can increase by 17.2% when the cement replacement ratio with PS increased up to 34.2% because it reduces the water absorption [47
]. Furthermore, the addition of PS provides the cement dilution effect, which increases the water to cement ratio of UHPC with PS indirectly. Abdulkareem et al. reported that the workability is improved by the addition of GGBS [45
]. In the paper, with the increase in GGBS, the dosage of superplasticizer should be reduced to achieve the same level of slump flow. Liu et al. used GGBS to improve the flowability of UHPC. With up to 30% replacement of cement with GGBS, the slump flow can increase by 6.3% because of the smooth surface and lower water absorption of the slag compared to cement [44
]. Liu and Guo proved that the addition of SSP can improve the flowability of UHPC. The 16.9% replacement ratio of cement increases the flow diameter by 26.8% because the activity of SSP is lower than that of cement and the water requirement of SSP is also less than that of cement [46
]. However, Li et al. found that the 15% replacement of cement with the different SSP source slightly decreases the flowability of UHPC because of the higher specific area than that of cement, which caused higher water demand [12
]. Therefore, it should be pointed out that a higher specific area of different sources of slag could decrease the flowability of UHPC. Randl et al. found that the specific area of GGBS and FGGBS can improve the flowability of UHPC [42
]. The slump flow of UHPC increased by 1.8% with 38.5% GGBS replacement and the slump flow of UHPC increased by 10.7% with 38.5% FGGBS replacement.
Slag is possibly more helpful than FA to improve the UHPC’s compressive strength. Wu et al. reported that when the cement replacement ratio is same, slag exhibits a higher compressive strength of UHPC than FA [41
]. The 30% replacement ratio of cement with GGBS results in 5.7% and 4.0% lower at 3 days and 28 days, respectively, compared to the reference specimen containing SF only, while the 30% replacement with FA results in 13.5% and 8.0% lower at 3 days and 28 days, respectively.
Although slag is apt to decrease the early compressive strength of UHPC, many studies have demonstrated that it successfully improves the late compressive strength. The additional CSH produced by the secondary pozzolanic reaction between slag and Ca(OH)2 increases the density of the matrix and the compressive strength, which happens at the late age because of the slow hydration of slag. The disadvantage of the low early compressive strength is assumed to be overcome by adopting heat treatment as it increases the pozzolanic reactivity. The particle size of slag is also an important factor to increase the compressive strength of UHPC; slag of a finer particle size exhibits higher compressive strength. However, the extra grinding work increases the material cost and, therefore, finding reactive slag material seems a more efficient option. Slag decreases the water demand of UHPC because of its lower water absorption compared to cement. It is another critical factor to increase the compressive strength of UHPC because a lower w/c ratio can increase the compressive strength.
3.2. Fly Ash (FA)
FA is a by-product of power plants and is collected during the process of coal combustion. The chemical composition and particle size of FA are different from plant to plant, but it is generally a fine spherical powder, which increases the workability of conventional cementitious material. As a pozzolanic material, it is known that FA increases the late age strength of conventional cementitious material. The usage of FA can reduce CO2
] and decrease the production cost and energy of concrete [41
]. In recent years, many researchers have focused on developing new UHPC mixtures with locally available FA because substituting cement and/or SF with FA can reduce environmental impacts.
The effect of FA on UHPC compressive strength is summarized in Table 5
. The compressive strength data reported show around 95 MPa at 3 days, 110–185 MPa at 28 days, and 152–202 MPa at 91 days with a 10–20% replacement of binder materials. It has been shown that the UHPC with FA exhibits a lower compressive strength than those of reference specimens. Ahmad et al. replaced a part of SF with FA, and found that using FA to substitute the SF to up to 11.8% of the binder slightly decreases the compressive strength by 1.9% compared to the reference specimen at 28 days [49
]. Although FA degrades the compressive strength of UHPC, the value is higher than the minimum requirement of 150 MPa and the usage of FA can reduce the cost of UHPC. It has been proved that FA can improve many characteristics of high strength mortar. However, Pyo et al. found that replacing 12.8% cement with FA decreased the compressive strength by 48.9% and 6.1% at 1 day and 3 days, respectively, because of the high crystallinity of FA [40
]. Wu et al. also investigated the effect of FA as an SCM for concrete, and concluded that the FA has negative effects on the compressive strength of UHPC [41
]. The 15% replacement ratio of cement with FA results in 13.5% and 8% lower than those of reference specimen at 3 days and 28 days, respectively. Alsalman et al. adopted that FA can be used as an SCM for UHPC to reduce the cost of UHPC [53
]. It was found that adding FA up to 15% of the binder significantly decreases the compressive strength by 33.7% at 1 day because the addition of FA delayed the strength development at early ages. The compressive strength of UHPC becomes similar to that of the reference specimen after the normal curing of 7 days or longer. Randl et al. found that the 38.5% replacement ratio of cement with FA decreases the compressive strength by 24.9% at 28 days compared to the reference specimen, even though the packing density is higher [42
]. Therefore, it is inferred that the slow pozzolanic reaction of FA degrades the compressive strength of UPHC. It should be noted, however, that there are occasional studies reporting different trends. Šeps et al. replaced the cement of 30% with FA and it results in 19% higher compressive strength at 28 days than that of the reference specimen containing SF only [52
Some of the FFA can improve the compressive strength of UHPC. Ferdosian and Camões introduced the method of how to optimize the UHPC mix design that satisfies the requirements of the compressive strength and the flowability using FFA of which the mean particle size is 4.48 μm [56
]. They suggested the eco-efficient mix design that releases the lowest CO2
and the cost-efficient mix design that maximizes the amount of FFA and sand as well as minimizes the amount of SF. The eco-efficient mix design results in the compressive strength being 6.8% higher than the reference samples using the FFA of 34.1% in the binder.
The ternary use of SCMs including FA also can be a feasible solution to reduce the amount of cement and SF in UHPC. Li found that the ternary use of FA, MK, and cement can provide better compressive strength of UHPC than the binary use of SF and cement [55
]. The 20% and 3.8% of cement were replaced with FA and MK, respectively, and the compressive strength increased by 26% at 28 days. Yazıcı et al. found that the ternary SF-FA-GGBS binder system is effective for reducing SF and water demand without sacrificing compressive strength [54
]. In the binder system, the 10% replacement ratio of cement with FA increased the compressive strength by 4.1% up to 281 MPa at 1 day under the autoclave curing condition. However, a fundamental understanding of the ternary use of SCMs improving mechanical properties of UHPC is still not clear and requires further research.
summarizes the effect of FA on the UHPC flowability, which is controversial among studies. Some researchers reported that FA can improve the workability of UHPC. Li found that the ternary use of FA, MK, and cement can significantly increase the flowability of UHPC by 47% compared to the binary use of cement and SF [55
]. Randl et al. found that the addition of FA can increase the flowability of UHPC [42
]. The 38.5% replacement ratio of cement with FA can increase the flow diameter of fresh UHPC by 3.6% compared to the reference specimen with SF only. Ferdosian and Camões focused on developing a mixing design method to minimize CO2
content and material cost with acceptable compressive strength and workability [56
]. The 34.1% FA in the cementitious binder satisfied the low limit of flowability of 190 mm. However, degradation of workability by using FA in UHPC was reported. Pyo and Kim found that the 15.7% replacement of silica powder with FA decreases the slump flow by 6.6% compared to the reference specimen [40
]. Ahmad et al. studied that the effect of FA replaces the SF in UHPC. It was found that when the use of FA as a replacement of SF and its content is increased to 11.8% of binder, the flowability of UHPC slightly decreases by 8.7% than the reference specimen with SF only but the lower flowability is still acceptable [49
summarizes the effect of FA on UHPC shrinkage. It has been shown that the FA can reduce the shrinkage of UHPC. Li et al. found that the ternary use of FA, MK, and cement can reduce the drying shrinkage of UHPC compared to the reference specimen with SF only because the ternary use can reduce water demand [55
]. Yazıcı et al. replaced cement with FA and GGBS to reduce the cement amount in UHPC [54
]. It was found that when the content of GGBS in the binder is constant, the 10% replacement ratio of cement with FA results in lower shrinkage than the reference specimen with SF only because of the lower amount of cement in UHPC.
The advantage of the usage of FA in UHPC cannot be observed in the compressive strength; most studies using FA reported degradation of the compressive strength of UHPC. The effect of FA on workability is arguable, and this might come from different characteristics of FAs from different sources. Therefore, the purpose of the usage of FA can be limited in reducing material cost or CO2 emission as discussed in the studies. Perhaps FA can increase the durability of UHPC; however, further studies are required to demonstrate it.
3.3. Limestone Powder (LP)
The effect of LP in conventional cement and concrete is well known; the use of LP in concrete has various advantages. It can reduce the material cost and CO2
emission because of having an abundant reservoir. LP has the nucleation effect in early hydration reaction that accelerates the cement hydration [58
]. It can also physically fill the void and increase the packing density of the system [80
]. As a consequence, LP increases the compressive strength of concrete at an early age. However, it may reduce the compressive strength at a late age because it does not have a pozzolanic reaction with cement unlike other SCMs such as slag, FA, and MK, and it requires higher water demand. In this section, the effect of LP in UHPC is reviewed
The effect of FA on UHPC compressive strength is summarized in Table 8
. Three different mechanisms of how LP affects the compressive strength of UHPC were observed. First, LP enables the reduction in the amount of superplasticizer to maintain the same flowability. Huang et al. studied the effect of LP on the hydration of UHPC with different cement replacement ratios [58
]. The retardation effect caused by the superplasticizer decreases as LP enables the reduction in the amount of superplasticizer by 62.8%, and, as a result, the early compressive strength is not degraded. It was also found that the 32% replacement ratio of cement with LP results in 10.7% and 16.1% higher compressive strength at 28 days and 56 days, respectively.
Second, LP has a pozzolanic reaction with SF. Li et al. adopted that replacing cement with LP and the optimum content of 37.3% replacement ratio increases the compressive strength of UHPC by 4.3% at 28 days than that of the control mix with SF only [57
]. As UHPC with LP has a higher pozzolanic reaction with SF, which contributes to the CSH formation at late ages, strength development can be improved at late ages.
Third, the fine particle-sized LP can accelerate cement hydration. Wu et al. reported the addition of nanoparticles such as NC can improve the mechanical properties and durability of UHPC [60
]. It is found that the 3.2% replacement ratio of cement with NC increases the compressive strength of UHPC by 1.1% and 5% at 7 days and 28 days, respectively, because the use of NC accelerates the hydration of cement and makes the microstructure denser due to smaller particle size of NC compared to cement.
However, in some cases, LP can degrade the compressive strength of UHPC. Yang et al. investigated the effect of LP on the hardened properties of the UHPC that contains FA and SF of 24% and 12%, respectively [59
]. The 14% replacement ratio of cement with LP decreases the compressive strength of UHPC by 4.2% at 7 days than that of the reference specimen with SF and FA. Although the compressive strengths of UHPC at 28 days and 56 days increase compared to day 7 compressive strength, these two strengths are still 1.1% and 4% lower than the reference specimen, respectively. Even though the LP has the nucleation effect increasing the cement hydration speed, it also dilutes the cement hydration resulting in lower heat of cement hydration. As the amount of LP increases in the low cement binder, the dilution effect becomes more dominant. Ahmad et al. studied the effect of the use of locally available industrial waste material such as LP as a partial substitution of SF [49
]. The use of LP decreases the compressive strength of UHPC by 5.65% when the content of LP increases to 4% of the binder compared to the reference specimen with SF only.
LP can significantly improve the workability of UHPC as shown in Table 9
. Li et al. insisted that LP can be regarded as a mineral plasticizer that improves the flowability of the UHPC [57
]. The 37.3% replacement ratio of cement with LP results in 45.1% higher flowability than that of the reference specimen that contains SF only. The plasticization effect of LP increases the workability of UHPC because of the repulsion between OH-
group localized on the Ca2+
surface and its lower water absorption [57
]. Yang et al. found that the use of LP as a partial substitution of cement can enhance the flowability of UHPC [59
]. The 14% replacement ratio of cement with LP increases the flow diameter by 65.5% than the reference specimen with SF only. This can be attributed to the higher w/c ratio as a part of cement is replaced with LP. Ahmad et al. also found that the use of LP increases the flowability of UHPC by 10.9% when the content of LP is 4% of the binder compared to the reference specimen with SF only [49
LP can lower the shrinkage of UHPC by reducing the amount of cement in UHPC as shown in Table 10
. Li et al. found that a 57.2% replacement ratio of cement with LP can improve the total shrinkage of UHPC compared to that of the reference specimen with SF only [57
]. The study insisted that the lower amount of cement in UHPC replaced with LP slows down the hydration and reduces the hydration products, and, thus, results in the lower autogenous shrinkage. It should be pointed out, however, that the high content of LP up to 78.1% of the binder provides more free water, and, thus, drying shrinkage increases. In consequence, the total shrinkage decreases because the reduction in autogenous shrinkage is greater than the increase in drying shrinkage. Yang et al. also reported that replacing 14% cement with LP reduces the autogenous and dry shrinkage compared to the reference specimen with SF only [59
Although three different mechanisms of how LP increases the compressive strength of UHPC have been proposed, the actual performance of LP in UHPC is debatable. From the literature, it was confirmed that LP increases the workability of UPHC. Therefore, the mechanism of LP to improve the compressive strength of UHPC by reducing water content seems appropriate. The finer LP enhances the compressive strength of UHPC by accelerating the cement hydration. Some studies insisted that the addition of LP decreases the amount of cement in UHPC, which degrades the compressive strength of UHPC. However, their dosages are lower than the other studies showing higher compressive strength with LP, and, therefore, other unknown factors of LP were assumed to degrade the compressive strength.
3.4. Metakaolin (MK)
MK obtained by calcining kaolin has the main chemical composition of alumina and silica, and, therefore, MK is also a pozzolanic material. Studies have reported that MK increases the durability of concrete: low permeability, high resistance against frost, and chemical attack [81
The effect of MK on the UHPC compressive strength is summarized in Table 11
. The use of MK only seems to increase the early age compressive strength of UHPC but decreases the late age compressive strength of UHPC. Li et al. found that replacing cement with MK can improve early age compressive strength but the late age compressive strength is decreased compared to UHPC with SF only [63
]. It was found that the 16.7% replacement ratio of cement with MK results in 47% higher 1-day compressive strength than the reference specimen with SF only because the use of MK improves the cement hydration at an early age. However, it decreases the 28-day compressive strength by 11.8% compared to the reference sample, of which impact is less significant compared to that of 1-day compressive strength. Tafraoui et al. found a replacement of SF of 20% with MK decreasing the 28-day compressive strength of UHPC by 26.1% with steam curing, and by 5.8% with water curing, respectively [61
]. The more significant loss of 26.1% compared to 5.8%, even with the same dosage of MK, is because of the usage of the crushed quartz that can lower the compactness by a looseness of granular stacking.
NMK may overcome the degradation of the UPHC compressive strength caused by MK. Muhd Norhasri et al. indicated that the inclusion of NMK in UHPC can achieve a similar compressive strength at early ages compared to the UHPC with MK only [64
]. NMK inclusion of 1% in UHPC can increase the compressive strength of UHPC at 28 days by 7.9% than that of the reference specimen with MK only because nano-MK provides a moderate ultra-filling effect in densifying the UHPC. The disadvantage of NMK is that it decreases the workability of UPHC; 1% NMK in UHPC decreases the slump flow by 2.4% because of the higher surface of NMK than that of MK (See Table 12
The effect of MK on the shrinkage of UHPC can be different concerning the type of shrinkage measured as shown in Table 13
. Li and Rangaraju studied the effect of MK on the shrinkage of UHPC [63
]. The addition of MK of 16.7% increases the autogenous shrinkage by 0.16%, but it decreases the drying shrinkage by 0.1%. However, no clear explanation of the different effects of MK on the different types of shrinkage is proposed.
MK can be incorporated in alkali-activated material (AAM). Wetzel and Middendorf introduced the UHPC made by AAM. Slag, MK, and SF were mixed with hydroxide solution and glass water [65
]. The specimens were cured at 60 °C and exhibit a compressive strength at 28 days over 150 MPa. The alkalinity of AAM is higher than ordinary Portland cement; the pH of AAM is usually over 14, whereas that of ordinary Portland cement is 12.6–13.5. Due to the highly alkaline environment of AAM, SF even increases the workability of UPHC and MK reduces much less than the case of ordinary Portland cement. As a result, AAM concrete shows good workability. MK creates the geopolymer network of Si-O-T (Si, Al) in AAM which increases the chemical attack resistance of UHPC.
MK tends to decrease the compressive strength of UHPC. It decreases the workability of UHPC and its beneficial effect on the shrinkage is not clear. Based on the fact that MK is not naturally stored but needs to be calcined, it also is difficult to find the merits of MK in material cost and CO2 emission compared to slag, FA, or LP. Therefore, the usage of MK in UHPC seems not suitable. However, another possible application was found; the geopolymer or alkali-activated concrete resulted in a compressive strength of over 150 MPa. As geopolymer is well known for its lower CO2 emission compared to OPC, developing geopolymer UHPC with MK can be an interesting research subject.