The flowability and high-temperature resistance of manufactured sand concrete: An exploration for high-rise buildings

The depletion of natural concrete aggregates, e.g., river sands, is a gradual process, and hence, manufactured sand concrete (MSC) is widely used in various construction projects. The flowability and high-temperature resistance of MSC directly determine the transport of fresh concrete and the fire resistance of high-rise buildings. In this study, MSC with different superplasticizer contents and sand ratios was prepared and its flowability and high-temperature resistance were studied. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were used to characterize the microstructure and porosity of MSC. The flowability of MSC with higher than 0.75% superplasticizer content or lower than 43% sand ratio is suitable for super high-rise buildings according to GB/T 50081-2019. The mechanical properties of other MSC meet the C30 requirements except for the MSC with a sand ratio of 48%. And the relatively high superplasticizer content or low sand ratio can make the denser structure and lower porosity of MSC. In addition, the MSC with relatively high superplasticizer content and low sand ratio exhibits better resistance to high temperatures due to a denser structure. This study provides theoretical guidance for using MSC in high-rise buildings and studying fire performance.


Introduction
Concrete is widely used in various fields such as roads, houses, water conservancy projects, and so on [1,2].The volume of sand, i.e., an essential component of concrete, can be up to about one third.Given that the unending exploitation and depletion of river sand caused a series of environmental problems, it is a trend to replace river sand in concrete production with manufactured sand which has significant angular characteristics with higher silt and mud contents.However, the flow performance of manufactured sand concrete (MSC) is relatively poor [3,4].With the development of urban society, the technologies for building construction are improved and super high-rise buildings are increased, but the available lands are continuously decreased.Super high-rise buildings have strict requirements on the flowability of concrete, promoting researchers to study the flowability of machine-made sand concrete in-depth [5,6].
The workability of concrete is mainly determined by the water-cement ratio and aggregate properties, including the shape, surface roughness, and particle size distribution of aggregate [7,8].River sand with a smooth surface and rounded shape due to water erosion performs a higher workability than manufactured sand with a rough surface and angular/slender shape.These properties of manufactured sand reduce the workability of concrete while enhancing the mechanical strength of concrete [9,10].Therefore, the workability of manufactured sand concrete becomes the first problem restricting the large-scale use of manufactured sand.
Shen et al found that the workability of concrete could be improved by adjusting the fineness modulus of manufactured sand, and medium sand gave the best flowability to concrete [11].Jadhav et al found that the flowability of mortar became worse with the increase of flat long grain content and particle size in manufactured sand [12].Superplasticizers, as the most commonly used additive to improve the flowability of concrete and mortar, have also been found to be used in workability improvement studies of MSC [13].Nevertheless, superplasticizer the high dosages will affect the strength of concrete or mortar.The workability of MSC adjusted by superplasticizer requires to be optimized according to aggregate characteristics and strength grade of concrete, so as to achieve the workability adjustment on the premise of ensuring the mechanical properties of concrete [14,15].Han et al found that the flowability of concrete would affect its compactness and the degree of initial defects.The degree of initial defects is closely related to the strength of concrete [16].The sand ratio also affects the flowability and mechanical strength of concrete [17,18].
It was reported that building fires performed a serious impact on the mechanical properties of concrete, resulting in the collapse of buildings [19,20].High-rise building fire requires building materials with better high-temperature resistance, so as to reduce casualties and economic losses.The research shows that concrete with iron tailing has better high-temperature resistance than traditional concrete [21].The concrete with a desert sand ratio of 40% has the best mechanical properties under high temperature [22].And the admixtures also have different degrees of influence on the high-temperature resistance of concrete [23].However, different kinds of manufactured sand are on the market, and the high-temperature resistance of different MSCs requires further study separately.
In this study, C30 MSC was prepared by using limestone mine manufactured sand.The workability, basic physical-mechanical parameters, and high-temperature resistance of MSC with different superplasticizer content (0.5%, 0.75%, 1.0%, 1.25%) and sand ratio (38%, 40.5%, 43%, 45.5%, 48%) are investigated.The microstructure and pore characteristics are used to characterize the effects of superplasticizer and the sand ratio on MSC.This study provided a theoretical guidance for solving the shortage of natural river sand and the technical problems of high-rise building construction and fire prevention.  1 shows.The manufactured coarse aggregate of limestone mine was made of 4.75 ∼ 13.2 mm.The measured properties of manufactured sand were: apparent density 2602 kg m −3 , loose density 1488 kg m −3 , fineness modulus 3.05, and stone powder content 5.05%.The GK-3000 polycarboxylate superplasticizer procured from Shijiazhuang Chang'an Yucai Building Materials Co., Ltd is used in this study, whose water reduction rate was 25.0%.

Mixed design
The complex shape, high stone powder content, and strong absorption of the superplasticizer of manufactured sand will adversely affect the workability and strength of concrete [24].To overcome this problem, two groups of mix ratios were designed to explore the influences of superplasticizer content and sand ratio on the performances of MSC as shown in tables 2 and 3.

Samples preparation
As shown in figure 2, cement, fly ash, manufactured sand, and coarse aggregate were firstly mixed and stirred for 2 min.Then, same amounts of water and superplasticizer were added into the mixture simultaneously and stirred continuously for 1∼2 min.Finally, the remaining water and superplasticizer were added to the mixture simultaneously and stirred quickly for 1 min.The slump of the fresh concrete was examined according to GB/T 50080-2016 [16].The mixture was then placed in the molds and vibrationally compacted on a shaking table.The samples were removed from molds after standing indoors for 24 h.The samples were placed in the curing box (20 ± 2 °C and 65% relative humidity) for curing. .Samples with the sizes of 100 mm × 100 mm × 100 mm were utilized to access the compressive and splitting tensile properties (GB/T 50080-2016).In the compressive tests, the loading rate was set as 2400 N s −1 .In the flexural and splitting tensile tests, the loading rate was set as 50 N s −1 .All tests results were taken from the average of the three groups of  samples.Fragments of about 1 cm × 1 cm × 1 cm were used for microstructural and pore characteristics tests by Scanning Electron Microscope (SEM) and Mercury Intrusion Porosimetry (MIP), respectively.The sample fragments were collected from a 28 d compressive strength test.The high-temperature resistance test of MSC was carried out in a muffle furnace (figure 3(d)).The test process was divided into 5 min of preheating (100 °C) and 2 h of constant temperature (800 °C).After natural cooling, the sample was taken out for a compressive strength test.

Results and discussions
3.1.Flowability Figure 4 shows the effect of superplasticizer content and sand ratio on concrete slump.The increase of superplasticizer content enhanced the flowability of concrete.On the contrary, the increase in sand ratio reduced the flowability of concrete.When the content of the superplasticizer was lower than 0.75%, the flowability of concrete was poor, due to the absorption of superplasticizer by manufactured sand containing fine powder [25].Superplasticizers with high dosages inhibited the absorption of fine powder in manufactured sand, thus enhancing the flowability of concrete.The increase in sand ratio improved the adsorption of superplasticizer and moisture, increaseing the friction between aggregates and reducing the flowability of concrete [26].Based on GB/T 50081-2019, concrete with a slump value higher than 180 mm belongs to high flowability, which can meet the flowability requirements of concrete for super high-rise buildings.Therefore, for limestone mine aggregate, concrete with superplasticizer content higher than 0.75% and sand ratio less than 43% can be used in super high-rise buildings.

Compressive strength
Figure 5 shows the compressive strength of MSC with different superplasticizer content and sand ratios at 7 and 28 days.The compressive strength of concrete increased first and then decreased with the increase of superplasticizer content.Especially, the maximum compressive strength at 28 d was 38.73 MPa when the superplasticizer dosage was 1.0%.It was same as the phenomenon reported by Li [27], in which, an appropriate amount of superplasticizer could promote the leaching of Si 4+ in sand, thus improving the degree of hydration.Meanwhile, an appropriate superplasticizer content could make the porosity of concrete lower and the interfacial transition zone (ITZ) denser.On the contrary, the compressive strength of concrete was decreased with increasing sand ratio.Specifically, the 28-day compressive strengths were 43.96MPa, 38.98 MPa, 37.59 MPa, 34.53 MPa, and 29.09 MPa for 38%, 40.5%, 43%, 45.5%, and 48% sand ratios, respectively, because the increase in sand ratio would introduce more fine powder.Fine powder with a high specific surface area and fine particle size absorbed a large amount of water, thus reducing the amount of hydration products [9,28].
The failure mode of MSC under different superplasticizer content and sand ratio is shown in figure 6.All the failure modes of concrete under uniaxial compression belonged to X-type shear failure, which was accompanied  by a large number of cracks, indicating that the content of the superplasticizer and sand ratio did not affect the failure mode of concrete.The difference was that the number of cracks produced in the concrete with higher strength was relatively small after failure.

Flexural strength
Figure 7 shows the flexural strength of MSC with different superplasticizer content and sand ratios at 7 and 28 days.Similarly, the variation trend of flexural strength of concrete with superplasticizer content and the sand ratio was similar to that of compressive strength with superplasticizer content and sand ratio.The flexural strength of concrete was the maximum when the content of the superplasticizer was 1.0% or the sand ratio was 38%.Especially, the maximum flexural strength at 28 d was 7.32 MPa and 7.94 MPa when the superplasticizer dosage was 1.0% and the sand ratio was 38%, respectively.In the previous studies, the flexural strength of concrete was mainly affected by porosity and mortar bonding [29,30], indicating that improper superplasticizer content and high sand ratio would affect the internal porosity of MSC.

Splitting tensile strength
Figure 8 shows the splitting tensile strength of MSC with different superplasticizer content and sand ratio at 28 days.It was observed that the tensile properties of MSC generally revealed the same trend as the compressive and  flexural properties.The maximum flexural strengths at 28 d were 3.64 MPa and 3.88 MPa when the superplasticizer dosage was 1.0% and the sand ratio was 38%, respectively.

High-temperature resistance
The compressive strength variation of MSC with different superplasticizer content and sand ratio after hightemperature treatment at 800 °C is shown in figure 9. Compared with the initial compressive strength at 20 °C, the compressive strengths of MSC from 0.5% to 1.25% superplasticizer content decreased by 44.8%, 41.6%, 41.2%, and 51.8%, respectively, indicating that the strength loss of MSC was mainly determined by the initial strength of the samples.In addition, the higher the initial strength led to the lower the strength loss rate.Meanwhile, compared with the initial compressive strength at 20 °C, the compressive strengths of MSC from 38% to 48% sand ratio decreased by 49.9%, 45.8%, 41.6%, 37.9%, and 33.2%, respectively.The results demonstrated that the high-temperature resistance of MSC increased with the increase in sand ratio, due to an increase in the content of sand with good thermal insulation properties and a decrease in the content of coarse aggregates with poor thermal insulation properties [19,31].
Figure 10 shows the SEM images for the microstructure of MSC with different superplasticizer content and sand ratio after treatment at 800 °C temperature.S1, S2, and S4 represent MSC with superplasticizer content of 0.5%, 0.75%, and 1.25%, respectively.It was found that improper dosage of superplasticizer would reduce the  strength and the high-temperature resistance of MSC.The microstructure showed that more macropores appeared in the structure of MSC with poor high-temperature resistance after high-temperature treatment.MS1, MS2, and MS3 represent MSC with sand ratios of 38%, 43%, and 48%, respectively.Different from MSC with different superplasticizer content, MSC with different sand ratios did not appear obvious cracks after hightemperature treatment.Nevertheless, the roughness of the mortar was significantly increased, indicating that the adhesion of the mortar is reduced, which would also lead to the deterioration of the mechanical properties of MSC.

Microscopic characteristics
4.1.Microstructure It is commonly well-known that the ITZ between aggregate and cement paste is a weak spot, whose performance directly affects the mechanical and durability properties of the concrete.Therefore, the ITZ of MSC under different superplasticizer content and sand ratio were studied.As shown in figure 11, obvious cracks was obtained in the ITZ and the surface of the mortar was rough when the content of superplasticizer was relatively small (0.5%), indicating that the hydration degree of mortar was relatively poor at 0.5% content of superplasticizer, which led to the low strength of concrete.The density of ITZ and mortar could be enhanced significantly with the increase of superplasticizer content, thus enhancing the mechanical properties of concrete.
As shown in figure 12, the ITZ and mortar were denser when the sand ratio was 38%, which was consistent with the results of concrete mechanical properties.With the increase in sand ratio, there was no obvious crack in ITZ, while the roughness of mortar increased significantly, because the fine powder in the sand could hinder the hydration of the cement, but it also performed the effect of filling the pores [32].

Pore characteristics
The pore characteristics (including cumulative pore content, pore size distribution, pore volume, and total porosity) of MSC were tested through MIP experiments.As can be seen in figure 13(a), the cumulative pore contents of concrete with 0.5%, 0.75%, and 1.25% superplasticizer content were 0.063 ml g −1 , 0.051 ml g −1 , and 0.052 ml g −1 , respectively.The cumulative pore contents of concrete with 38%, 43%, and 48% sand ratios were 0.046 ml/g −1 , 0.051 ml g −1 , and 0.066 ml g −1 , respectively.These trends indicate that the increasing superplasticizer content and the decreasing sand ratio can reduce the cumulative pore content of concrete.
It was reported that pores with the size larger than 200 nm were defined as harmful pores [33,34].The proportion of harmful pores in concrete with 0.5%, 0.75%, and 1.25% superplasticizer content reach 48.11%, 50.64%, and 61.46%, respectively, because the high dosage of superplasticizer would bring more air bubbles into   the concrete.And the proportion of harmful pores in concrete with 38%, 43%, and 48% sand ratios reach 47.87%, 50.64%, and 60.17%, respectively, indicating that the high content of sand ratio would lead to a lower hydration degree of mortar and increase porosity.

Conclusions
In this study, the flowability of the MSC of limestone mine aggregate used in super high-rise buildings was regulated by the content of superplasticizer and sand rate.The effects of superplasticizer content and sand ratio on mechanical properties and microscopic characteristics of concrete are studied.The following conclusions were extracted.
(1) The flowability of the MSC of limestone mine aggregate could be enhanced by increasing the content of the superplasticizer and reducing the sand ratio.When the content of the superplasticizer reaches 0.75% and the sand ratio was lower than 43%, the concrete slump could meet the construction requirements of super high-rise buildings.
(2) The mechanical properties of concrete first increased and then decreased with the increase of superplasticizer content, but gradually decreased with the increase of sand ratio.Except for the concrete with the sand ratio of 48%, the rest of the concrete met the strength grade of C30 concrete.
(3) The results of SEM and MIP tests demonstrated that the internal porosity of concrete was high and cracks in ITZ are obvious under the condition of low superplasticizer content.The increase of sand ratio would increase the porosity of concrete and there were no obvious crack in ITZ, indicating that the increase of sand ratio could fill the large pores and reduce the weakening degree of cement mortar.
(4) The compressive strength of MSC with superplasticizer content of 0.75%∼1.0%or sand ratio of 38%∼45.5% was still above 20 MPa after high-temperature treatment at 700 °C, which had a good hightemperature resistance.

Figure 1 .
Figure 1.Illustration of the CAs and FSs used in this study.

Figures 3 (
a)-(c) illustrates the mechanical property tests of the specimen after curing.Samples with the sizes of 40 mm × 40 mm × 160 mm were utilized to access the flexural properties (EN 196-1)

Figure 3 .
Figure 3.The mechanical property and high-temperature resistance test of MSC.

Figure 4 .
Figure 4. Effect of superplasticizer content and sand ratio on the concrete slump.

Figure 5 .
Figure 5.Effect of superplasticizer content and sand ratio on compressive strength.

Figure 6 .
Figure 6.The failure mode of concrete under different superplasticizer content and sand ratio.

Figure 7 .
Figure 7. Effects of superplasticizer content and sand ratio on flexural strength.

Figure 8 .
Figure 8. Effects of superplasticizer content and sand ratio on splitting tensile strength.

Figure 9 .
Figure 9. Compressive strength of MSC after high temperature.

Figure 10 .
Figure 10.The microstructure of MSC with different superplasticizer content and sand ratio after treatment with 800 °C temperature.

Figure 11 .
Figure 11.The ITZ of MSC under different superplasticizer content.

Figure 12 .
Figure 12.The ITZ of MSC under different sand ratio.
China) were used in this study.The chemical components of cement and fly ash (FA) are shown in table 1.Both coarse aggregate (CA) and fine sand (FS) in concrete were crushed by limestone mine aggregate in a quarry in Sichuan, China, as figure

Table 1 .
Chemical compositions of cement and fly ash (% by weight).

Table 3 .
Mix ratio under different manufactured sand ratios/kg.