1. School of Civil and Environmental Engineering, University of Technology Sydney, Sydney NSW 2007, Australia
2. School of Civil Engineering, Central South University, Changsha 410075, China
3. Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA 50011, USA
wengui.li@uts.edu.au
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History+
Received
Accepted
Published
2019-09-09
2019-12-29
2020-12-15
Issue Date
Revised Date
2020-11-17
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(5942KB)
Abstract
Self-consolidating concrete (SCC) with manufactured sand (MSCC) is crucial to guarantee the quality of concrete construction technology and the associated property. The properties of MSCC with different microlimestone powder (MLS) replacements of retreated manufactured sand (TMsand) are investigated in this study. The result indicates that high-performance SCC, made using TMsand (TMSCC), achieved high workability, good mechanical properties, and durability by optimizing MLS content and adding fly ash and silica fume. In particular, the TMSCC with 12% MLS content exhibits the best workability, and the TMSCC with 4% MLS content has the highest strength in the late age, which is even better than that of SCC made with the river sand (Rsand). Though MLS content slightly affects the hydration reaction of cement and mainly plays a role in the nucleation process in concrete structures compared to silica fume and fly ash, increasing MLS content can evidently have a significant impact on the early age hydration progress. TMsand with MLS content ranging from 8% to 12% may be a suitable alternative for the Rsand used in the SCC as fine aggregate. The obtained results can be used to promote the application of SCC made with manufactured sand and mineral admixtures for concrete-based infrastructure.
Concrete has become a ubiquitous building material worldwide because of its wide-ranging properties, applicability, and simplicity in local operations. Conventional concrete, even with high fluidity, cannot fill in the reinforced-concrete formworks with tightly arranged steel bars. Under these circumstances, the application of self-consolidating concrete (SCC) has brought about several advantages to construction, including eliminating compaction, shortening construction time, reducing noise, improving homogeneity, and optimizing external quality [1,2]. SCC is a type of high-fluidity concrete that does not segregate and can extend into place, filling the tightly arranged reinforced-concrete formworks with no application of mechanical vibration [3,4]. In other words, as an extraordinary concrete known for its high fluidity, SCC contains complex components and needs careful design and preparation procedures to meet the corresponding performance requirements.
In SCC, the fine and coarse aggregates account for 60%–70% of the total volume [5,6]. The appropriate selection of aggregates has a notable effect on the fresh and hardened concrete performance. The shapes of fine aggregates are more critical than those of coarse aggregates in concrete production, in addition to the effects of the granular texture [7]. Ordinary sand from river banks is normally utilized as fine aggregate in concrete production, but because of the increasing requirements for river sand (Rsand), there is no guarantee of the availability of high-quality sand that satisfies the needs of the construction industry [8]. Manufactured sand (Msand) is receiving considerable attention currently used as a substitute for Rsand and fine aggregate in cement-based composites. Msand, produced by mechanical crushing of the parent rock, has additional angular and cubic-shaped grains. Msand is quite distinct from Rsand in shape, grade, component, and stone powder content [9]. Therefore, the distinguishing feature of Msand and Msand particle has numerous effects on the performances of the fresh and hardened cement-based composites. Shen et al. [10] examined the distinguishing feature of Msand, such as grain shape, granular texture, and behavior in cement-based composites. Li et al. [11,12] investigated the effects of Msand’s distinguishing features (such as micro-fine rock, external roughness, smashing value and different types of rock) on the mechanical properties and abrasion resistance of cement-based composites manufactured with Msand. They also studied the influence of micro-fine rock content in Msand on the durability of cement-based composites manufactured with Msand. These investigations demonstrated that the properties of Msand were not worse than that of Rsand in cement-based composites.
Among the different characteristics of Msand and Rsand, the micro-fine rock generated with the production of Msand plays a vital role in changing the characteristics of Msand. Owing to the existence of highly micro-fine rock content, the Msand has a notable effect on water demand and fluidity of the cement-based composites [9]. The highly micro-fine rock content in Msand primarily strengthens the yield stress of the cement-based composites because of the increased interparticle friction, leading to an increase in plastic viscosity. The effects of Msand on the performances of cement-based composites, for the most part, lie on the paste volume of cement-based composites. The adverse effects of negative gradients and shaped aggregates could be eliminated or distinctively decreased by improving the micro-fine rock content in cement-based composites [13]. Besides, it has been suggested that the application of Msand can visibly promote the mechanical performance and durability of cement-based composites, in particular, when the Msand is prepared from granite sources. In contrast, the use of dolomite and sandstone sources do not yield such results [14]. For SCC, highly micro-fine rock content is indispensable to produce a high-quality fresh concrete. However, owing to the high water requirement of Msand, the applicability of using fine rock powder in SCC production is questionable.
In addition, the performance of cement-based composites and the decreased greenhouse gas emissions could be enhanced by using mineral admixtures such as fly ash (FA) and silica fume (SF) as a replacement for ordinary Portland cement (OPC). FA, which is utilized as a pozzolan in cement-based composites, can be utilized to a small extent as a substitute for 15% to 35% of the OPC in its composition without compromising the strength [15,16]. SF, produced by the silicon metal production industry, has pozzolanic characteristics, which have a significant impact on the mechanical performance and durability of cement-based composites. SF-based composites with a considerable decrease in porosity and chloride permeability have been presented in [17,18]. Moreover, fine grains can exist in Msand, SF, or FA. Notably, the fine grains in FA with spherical and glassy shapes behave like ball bearings and improve the composites accumulation, thereby reducing the voids and the chloride permeability of SF-based composites [19–21]. However, compared with FA, SF is a type of pozzolan with a huge surface area, which in turn increases water requirements [18,22].
Previous investigations on SCC with Rsand entirely replaced by Msand [23,24] and with Rsand containing mineral admixture [17,25,26] are insignificantly related to the characterization of SCC with micro-fine rock and partial substitution of Rsand by Msand incorporated with mineral admixtures. Therefore, a series of experiments were carried out in this study to examine the characterization of SCC with microlimestone powder (MLS, sieving from limestone Msand) and at different substitute levels of retreated Msand (TMsand) incorporated with FA and SF. The influence of MLS on the workability, durability, and mechanical characteristics of TMSCC, incorporated with a certain proportion of mineral admixtures, was investigated. The effect of different Msand replacements of MLS on the hydration reaction and the microscopic characterization of the cement-based paste were detected by the measurement of electrical resistivity and analysis using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). It is considered that the experimental results can be applied to sustain the utilization of SCC made with Msand containing mineral admixtures.
Experimental program
Raw materials
The materials employed for this investigation were grade ASTM type I OPC, ASTM Class C FA, and high-quality commercial-grade SF. The gravel with a nominal aggregate size of 20 mm at maximum was utilized as the coarse aggregate with an apparent density of 2670 kg/m3 and loose bulk density of 1530 kg/m3. The fine aggregates employed for this study were Rsand and limestone Msand. The physical and chemical performances of the aggregates and the particle size distributions of the materials are displayed in Table 1 and Fig. 1, respectively.
In order to appropriately perform the experiment, Msand was treated using the following steps: first, the Msand was sieved to remove the MLS with particle size less than 75 mm; then, the surface-attached mud was washed with water; and finally, the sand was dried. The treated Msand is defined as TMsand. The MLS with particle size less than 75 µm used in this study was obtained by sieving the Msand as per the aforementioned steps. X-Ray Fluorescence was utilized for the chemical composition analysis of OPC, FA, SF, and MLS. Tables 2 and 3 display the physical performance and chemical compositions of the cement, FA, SF, and microlimestone. The particle size distributions of OPC, FA, and MLS are shown in Fig. 1.
Mix proportion
To examine the effect of microlimestone on the performance of SCC cast with TMsands, the microlimestone was used as a partial replacement of TMsand, and the TMSCC was prepared by incorporation of MLS/TMsand with FA and SF in the ratios of 0/100, 4/96, 8/92, 12/88, 16/84, and 20/80 wt.%. In all the TMSCC mixes used in this experiment, the total amount of cementitious materials (cement, FA, and SF) was about 470 kg/m3, and the water to binder (W/B) ratio was approximately 0.32. SCC made with Rsand (RSCC) was fabricated as the control sample. A commercial polycarboxylic superplasticizer (SP) with a water-reducing rate of 30% was adopted, and the dosage was 1.2%. To determine the effect of MLS incorporated with mineral admixtures on the microscopic characterization, six paste mixtures with the same solid binder (including OPC, FA, and SF) added with different percentage of MLS (MLSP) and the SP dosage of 0.6% were designed in this study. The paste and concrete mix proportions utilized are illustrated in Table 4.
Specimen preparation
Concrete specimens were manufactured by a laboratory tilting drum mixer. TMsand, added with different proportions of MLS used as fine aggregate, was prepared before casting. Both coarse and fine aggregates present in a saturated-surface dried situation, and the binders (OPC, FA, and SF) were dry-mixed entirely in the mixer for 2 min. Next, water with 1.2% SP was gently and evenly poured into the mixer over 1 min. Then, the mixing was sustained for further 3–5 min to guarantee uniform mixing and the absence of dry residuals in the mixer. Paste samples were prepared by a 10 L planary-high-shear mixer. To begin with, the binders (OPC, FA, SF, and MLS) were dry-mixed entirely in the mixer for 2 min. Next, water with 0.6% SP was gently and evenly poured into the mixer for 1 min. After that, the mixture was mixed for 2–3 min. The fresh concrete was poured into f100 mm × 200 mm cylindrical molds for compressive strength test, and the fresh paste was poured into 70 mm × 70 mm × 70 mm cubes molds. All specimens were placed in the casting condition at a temperature of approximately 23°C for 24 h. The specimens were finally demolded and cured in a standard-curing chamber.
Test setup
The bulk density, void ratio in bulk, and tap density of TMsand with different MLS replacements were measured according to ASTM C39/C39M-18 (Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens) [27]. The flowability of fresh concrete was evaluated by measuring the slump flow and mini V-funnel flow time (T500) in conformity with ASTM C 1611M (Standard Test Method for Slump Flow of Self-Consolidating Concrete) [28]. After 7, 28, 60, 90, and 150 d of curing, concrete cylinders were measured under axial compression on the basis of ASTM C39/C39M-18 (Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens) [29]. At 60 d after casting, a 50 mm thick specimen was sectioned from the mid-height of the cylinder. The specimen was air-dried before the epoxy resin was applied to the surface of the circumference, and then vacuum saturation was performed in a vacuum dryer. Rapid chloride permeability test (RCPT) was recorded in accordance with ASTM C1202 (Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration) [30]. The chloride permeability of the SCC was examined by the amount of electric charge passing through the specimen in 6 h.
The electrical resistivity of the fresh paste was recorded by a non-contact electrical resistivity equipment, as displayed in Fig. 2. The transformer theory is explained in Ref. [31], and the assessment of the electrical resistivity was carried out for 24 h at data recording intervals of 1 min, under a temperature of 22°C±2°C. The hydrated pastes for TGA and XRD were arranged by preventing hydration using acetone at 28 d and the samples were then heated in a vacuum oven with temperature of 40°C for 24 h. Paste samples at 28 d were crushed and polished for SEM analysis. Some details of the analysis techniques adopted are summarized in the following texts.
TGA was carried out by applying Netzsch’s STA449. The test temperature was elevated from 20°C to 1000°C at a rate of 10°C/min in a nitrogen environment [32]. The weight loss results were acquired from the thermogravimetric (TG) curve and differential thermogravimetric analysis (DTA) curve was adopted to identify the different types of hydrates products. The principle of quantitative analysis of Ca(OH)2 content by thermogravimetric is that the Ca(OH)2 crystal in the solid powder is thermally decomposed around 400°C to 500°C, releasing water vapor and reducing the mass of the powder (water vapor production amount). The formula for calculating the solid powder content is as shown in the Eq. (1).
where is the mass of Ca(OH)2, mg; is the molar mass of Ca(OH)2, 74.10 g/mol; is the molar mass of H2O, 18.02 g/mol; m0: sample weight, 15 mg; m1–m2 is the mass loss rate.
XRD analysis was carried out by applying the Bruker D8 Discover diffractometer using CuKa radiation of wavelength 1.54 Å. Diffraction results were recorded between 5° and 70° 2q with a step size of 0.02θ [33,34]. The Jade 6.5 software was employed to analyze the diffraction results. SEM analysis using Zeiss Supra 55VP SEM with Raith Elphy was performed.
Experimental results
Effect of microlimestone content in TMsand
Figure 3 displays the results of bulk density, void ratio in bulk and the tap density of TMsand under the MLS effect. It was observed that the bulk and tap density gradually increased, but the void ratio in bulk gradually decreased with the increase in the MLS content. There is a more significant increase of bulk density and tap density, and a more prominent decrease of void ration in bulk for TMsand with the MLS content between 0 and 4% than other percentages of the MLS content. When the MLS content continues to increase up to 8%, the rate of this density change gradually decreases. The tap density and bulk density of TMsand with 20% MLS content are close to the maximum, compared with the void ratio in bulk close to the minimum, indicating that the TMsand has significant gradation [35,36]. In general, the MLS content from 8% to 20% has an insignificant effect on the density and void ratio in bulk.
Effect of microlimestone content in TMSCC
Flowability of fresh SCC
Figure 4 shows the results of flowability on SCC samples with the effect of MLS content and the flowability of RSCC. The slump flow and T500 of all mixes produced with different MLS range from 600 to 700 mm and 4 to 8 s, respectively, evidently differing from each other. The TMSCC with high slump flow will have a low value of T500. It can also be seen from Fig. 4 that TMSCC with MLS content greater or less than 12% has a small value in slump flow and a large value in T500. This indicates that the workability of the TMSCC is not as efficient as that of the TMSCC4 with 12% MLS content. The RSCC has a better flowability than other TMSCC mixes except for TMSCC4, displaying that the TMSCC4 with the most considerable slump flow value (660 mm) and least T500 (4.8 s) has the best workability that is even better than that of RSCC. The workability of TMSCC5 and TMSCC6 is worse than that of TMSCC with MLS content lower than 8%. In general, the MLS content has a considerable influence on the workability of TMSCC.
Compressive strength of SCC
Figure 5 displays the results of compressive strength (7, 28, 60, 90, and 150 d) of SCC samples with the effect of MLS content. The compressive strength of RSCC is also displayed in Fig. 5. It is shown that the compressive strength of both TMSCC and RSCC gradually increases with an increase in the curing ages. At 7 d of curing, the compressive strength of TMSCC with MLS is higher than that of RSCC compared with a lower compressive strength of TMSCC without MLS than that of RSCC. The compressive strength of RSCC is greater than any other TMSCC when the curing age increases to 28 and 60 d. However, the compressive strength of RSCC is more significant than that of other TMSCC except for the TMSCC2 (4% MLS) at the curing ages of 90 and 150 d. The compressive strength of TMSCC specimens, with the same curing ages and the MLS content increasing, gradually increases at first but decreases after reaching a maximum. The TMSCC3 with 8% MLS has the most considerable compressive strength than other TMSCC at 7, 28, and 60 d. In contrast, the TMSCC2 with 4% MLS has the most substantial compressive strength at 90 and 150 d.
Figure 6 presents the relative strength change (7, 28, 60, 90, and 150 d) of TMSCC in contract to that of TMSCC1 with 0% MLS content, with the positive values representing a strength increase. It was examined that there is an increase in the relative strength change for all TMSCC mixes, compared to a decrease after the strength reaches the maximum. Both TMSCC3 (8%) and TMSCC4 (12%) have the largest strength change when the curing age reaches 60 d. The TMSCC2 (4%) has the highest relative strength at 90 d, while TMSCC5 and TMSCC6 have the maximum at 28 d. In addition, the MLS content ranging from 8% to 20% has little effect on the changing of the TMSCC compressive strength change at all ages, particularly, at later ages (90 and 150 d), while there is a noticeable difference of the TMSCC compressive strength change with the MLS content from 0 to 8%. In general, the TMSCC2 with 4% MLS has the highest compressive strength in later ages while there is a minor difference in the compressive strength among all TMSCC mixes at later ages (90 and 150 d). The compressive strength change is attributed to the mineral admixture and will be explained in detail, in Section 4.
Chloride permeability of SCC
The chloride permeability is measured using the TMSCC specimens with different MLS content. Figure 7 shows the 6 h electric charge passing through result of the SCC after curing at 60 d and the chloride permeability of RSCC. It was noted that changing the dosage of MLS in TMSCC has a strong influence on the chloride permeability. The evaluation of chloride penetration resistance can be made by recording the amount of 6 h electric charge passing through the SCC. All the TMSCC with different MLS content demonstrated higher resistance to chloride migration with 6 h electric charge passing through ranging from 150°C to 310°C. The electric flux of TMSCC at 60 d decreases with the increase in the MLS content. The 6 h electric charge of TMSCC is attenuated by the first-order exponential as the MLS content increases from 0 to 20%. However, the decline of the electric flux slows down gradually when the content of MLS is greater than 16%. The 6 h electric charge passing through of TMSCC with MLS content ranging from 0 to 12% is higher than that of RSCC. Only when the MLS content reaches 16% can the 6 h electric charge passing through be lower than RSCC.
Effect of microlimestone content on microscopic characterization
Electric resistivity
The electrical resistivity development and development rate curves of MLSP1 (0% MLS) are plotted in Fig. 8, and the resistivity development curves for all paste mixes in 24 h are illustrated in Fig. 9. Based on the results presented in Figs. 8 and 9, the abscissa values of each characteristic point (M, T, and S) are obtained and plotted in Fig. 10. The resistivity development curve drops, almost flattens out, and could be divided into the following periods: dissolution and precipitation period (I), induction period (II), acceleration period (III), and deceleration period (IV). According to previous studies [37,38], the resistivity of the binder matrix is dominated by the resistivity of the pore solution in the period I, but dominated by porosity in period II. The ions in the cementitious materials are continuously dissolved and crystallized. A protective layer of crystallization that prevents dissolution is formed around the cement particles. Thus, for a higher MLS content, fewer ions are dissolved, strengthening the resistivity of the pore solution in the period I. In addition, after period II, a few dissolved ions result in a handful of hydration products, leading to excessive porosity, which is beneficial for ion transfer. The porosity during period III decreases as the hydration reaction proceeds, and the continuous crystal generation occurs. During period IV, the reaction begins to slow down, resulting in a decreasing growth rate of resistivity.
The four structural formation periods mentioned above can easily be determined in Fig. 9 within the confines of three peak points (M, T, and S) of Fig. 10. It can be observed from the hydration process of paste samples with different MLS content that the dissolution period (I) and the induction period (II) are prolonged at first, when the MLS content is less than 8%, but shortened with the increase in the MLS content. As mentioned in Table 4, the amount of water consumption and superplasticizer used in all paste mixes is the same. The CaCO3 in MLS is chemically inert with negligible Ca2+ dissolvability. The increasing MLS content in the binder can reduce the relative content of cement, FA, and SF. There might be a reduction in dissolution rate and an increase in the formation time of the protective layer when the MLS is lower than 12%, resulting in an increase of t1. When the MLS content is more than 12%, there is less relative content of cement, FA, and SF that needs to be dissolved, with the decreasing t1. In addition, the nucleation theory holds that the redundant ions concentration in the solution destroys the protective layer formed by wrapping the early formation of hydrates on the surface of the binder grains by increasing t2 up to 12% with the increasing MLS content. As the MLS content increases, the decreased relative water content in the equivalent volume binder results in cement mineral destruction to the protective layer, decreasing t2. Period III is prolonged with an increase in MLS, which may be possible due to the related high MLS content, which lowered the relative content of cement, FA, and SF. It can also be seen in Fig. 9 that the value of the 24 h electricity resistivity increases with the amount of increase in MLS. In conclusion, an increase in MLS content can indeed have a significant influence on the early hydration progress of all paste samples.
Thermogravimetric analysis
Figure 11 shows the TG data for all paste samples with different MLS content, indicating that the final mass loss after being heated to 1000°C is between 22% and 25%. The DTA curve is shown in Fig. 12. It can be seen that the mass loss of MLSP pastes before 700°C decreased with the increase in the MLS content. In particular, the MLSP1 with 0% MLS showed the largest mass loss before 700°C. It should also be noted that the MLSP mixes showed no considerable mass losses after approximately 800°C, indicating that no decomposition occurred after reaching this temperature. In addition, the final mass losses of MLSP pastes gradually increased with the increase in MLS content. The major reason for mass losses among all paste mixes is because of the decomposition around 600°C–800°C.
As can be seen from Fig. 12, four primary peaks are detected for the MLSP pastes. The first and second peaks occur around 100°C. As mentioned earlier, these peaks may be associated with the elimination of evaporable water, and some parts of it can be attributed to the elimination of water from the C-S-H gel. The dissolution of ettringite could occur between 114°C and 116°C [39]; however, it can also occur at a lower temperature in accordance with the moisture circumstances [40]. Thus, the dissolution of ettringite and C-S-H can also be attributed to the first peak that occurs at approximately 100°C. The first and second peaks shown in Fig. 12 occurred at approximately 72°C to 77°C, and 95°C to 100°C, respectively. The third peak was observed at 439.3°C, 443.9°C, 453.6°C, 454.7°C, 436.6°C, and 434.6°C for MLSP1, MLSP2, MLSP3, MLSP4, MLSP5, and MLSP6, respectively. The occurrence of peaks at these temperatures can be attributed to the existence of Ca(OH)2 [41]. The peak formed at 600°C to 800°C could be attributed to the decomposition of calcium carbonate (CaCO3) [42]. From Figs. 11 and 12, it can be seen that the decomposition of CaCO3 is the main reason for the final mass losses difference among all MLSP mixes.
Figure 13 shows the calculation curve of the weight loss rate and rate of change in MLSP6. According to the TGA of the differentiated curve, the content of Ca(OH)2 obtained at the temperature of 400°C–500°C is shown in Table 5. From the table, it can be seen that the amount of Ca(OH)2 correspondingly decreases with the increase in MLS content for the same W/B ratio.The reasons for this are explained as follows. First, when the equilibrium of the phase is formed by the hydration reaction, the dissolution of calcium ions inhibits the formation of calcium hydroxide because MLS contains a higher amount of CaCO3. Next, the content of CaCO3 in samples increases with the increase in MLS content, and this causes the amount of calcium hydroxide to decrease. From the results of Ca(OH)2 content in the 12% to 20% MLS content samples, it can be observed that the effect of the increasing MLS content on the amount of Ca(OH)2 is not significant. This is probably because a larger amount of FA and SF added to the cement blinder has a significant effect on the cement hydration reaction, indicating that the amount of Ca(OH)2 is mainly affected by SF and FA rather than MLS.
X-ray diffraction
Figure 14 illustrates the XRD results of the MLSP mixes with different MLS content. In the MLSP pastes, peaks of ettringite (AFt) at 9.1° and 15.8° 2θ, quartz (SiO2) at 20.9°, 26.6°, and 48.6° 2θ, Ca(OH)2 at 18.0° and 34.1° 2θ, calcium carbonate at 23.1°, 29.5°, 36.1°, 39.5°, 43.3°, 47.1°, and 50.8° 2θ, and belite (2CaO·SiO2) at 32.2 and 43.3° 2θ have been identified. It was found that the intensity of the peak at 29.5° 2θ is the largest for the MLSP6 with 20% MLS and it decreased with the decrease in the proportion of MLS. These peaks of CaCO3 at 57.5° and 62.6° 2θ have not been identified in MLSP paste samples with the MLS content lower than 4%. Similar to TGA (in Fig. 11) of the paste samples, the peaks due to calcium carbonate increased with the increase in MLS content. This is mainly caused by the MLS content, the main component of which is CaCO3 that has nearly no reaction in the hydration process [43]. The peaks in all the mixtures, approximately at 20.9°, 26.6°, and 48.6° 2θ indicate that there is still a large amount of FA which is not included in the hydrate reaction. The peaks of AFt and Ca(OH)2 have been identified in all samples, and the intensity of these peaks showed no significant difference with the change in MLS content, indicating that MLS has a certain catalytic effect on hydration reaction in the binder with mineral admixture.
Scanning electron microscopy
A Zeiss Supra 55VP SEM with Raith Elphy was employed to observe the microstructure of MLSP samples for 28 d. The microstructure of MLSP paste samples in 1000 × and 10000 × are illustrated in Figs. 15 and 16, respectively. From Figs. 15(b) and 15(c), it can be seen that the microstructure of MLSP2 (4%) and MLSP3 (8%) is quite dense, and there are few apparent defects in these pastes. However, there are also some defects detected in the microstructure of MLS1 and MLSP4 (with small pores), MLSP5, and MLSP6 (with some porosity). The MLS powder can be used as a filler and can reduce the free interspace in the matrix [44]. Although the SF and FA can act as filler [21,35], MLS embodies a large quantity of micro-fine powder. Therefore, in the MLSP pastes, the appropriate MLS powder makes the dimensions of all hydration products of cement to turn into considerably finer paste compared to MLSP pastes with no MLS. The appropriate MLS content for MLSP pastes are 4% and 8%. When the MLS content is more than 8%, it can prevent the hydration reaction of the MLSP pastes for all paste samples with the same water content. Thus, there will be high amount of porosity in the microstructure of MLSP5 (16%) and MLSP6 (20%). In addition, the FA particles are detected in all samples, indicating that there are still unreacted FA existing in all MLSP paste samples, consistent with the XRD results, as shown in Fig. 14.
As can be seen in Fig. 16, the crystal of ettringite (AFt, needle-like hydrates), portlandite crystal (CH, regular hexagon), and hydrated calcium silicate (C-S-H gels, floc like hydrates) were detected in all MLSP paste samples. In the MLSP paste, the portlandite crystal and ettringite have directional growth in the interfacial transition zone of the paste. Although the SF possibly reacts with Ca(OH)2 to generate C-S-H gel in the early stage of hydration reaction [25] that is added into the MLSP pastes samples, there are still considerable unreacted Ca(OH)2 for the small SF proportion. In addition, there is a high probability that the FA can react with Ca(OH)2 to generate C-S-H gel, but only in the later stage of hydration reaction. Thus, there are still a considerable amount of undetected Ca(OH)2 in the matrix. In general, the appropriate addition of MLS, FA, and SF can reduce microstructure drawback caused by Msand, and make the microstructure of TMSCC homogeneous.
Content of microlimestone powder
When different percentages of MLS are added to the TMsand, TMSCC, and cementitious paste, the occurrences of physical and chemical transformations result in substantial changes in flowability, compressive strength, and chloride permeability. Considering the changes in the properties of fresh TMSCC and microstructure, this section explains the changes in workability, strength, and chloride permeability. In the MLSP paste samples, the cement hydration products mainly include a type of ettringite gel, which is detected at the peak of 9.1° and 15.8° 2θ from the XRD pattern (Fig. 14), around 100°C from the DTA curve (Fig. 12), and in the SEM image with the needle-like hydrates (Fig. 16). In addition, several active silica is detected at the peak of 20.9°, 26.6°, and 48.6° 2θ from the XRD pattern (Fig. 14) and in the SEM image with spherical particles (Fig. 16), which did not participate in the hydration reaction. The portlandite is also found from the XRD pattern (18.0° and 34.1° 2θ), DTA curve (around 450°C), and SEM image (regular hexagon). As shown in Figs. 9 and 14, although the microstructure of the MLS samples is different with the increase of MLS, the amplitude of the diffraction peaks of calcium hydroxide and ettringite did not change significantly with the increase of the amount of MLS, confirming that MLS has little effect on the hydration reaction. The increase in the amount of MLS also causes the diffraction peak (mainly at 29.5° 2θ) of calcium carbonate to increase continuously, which is consistent with the mass loss at about 600°C to 800°C as shown in the TG curve (Fig. 11). The reason for this is that the cementitious material in the MLSP paste samples is mixed with a large amount of FA and SF, which can effectively accelerate the hydration reaction of the tricalcium aluminate and react with Ca(OH)2. The microlimestone cannot accelerate the hydration reaction in the binder with a large amount of FA and SF. The MLS has a significant influence on the microstructure of MLSP because MLS is mainly acted as filler materials confirmed by the electric resistivity (Fig. 9) and SEM results. From the SEM image shown in Fig. 15, it can be seen that the 4% and 8% MLS used as the binder might be a good alternative for the microstructure of MLSP paste.
In the TMSCC concrete samples, the MLS content has a considerable influence on the workability, compressive strength, and chloride permeability of TMSCC. As mentioned above, the MLS has little effect on the hydration reaction in the binder with mineral admixture, indicating that the change in performance of TMSCC concrete is primarily caused by the different characteristics of TMsand at various MLS replacement levels. The schematic of the matrix filling of Rsand and TMsand with different MLS content is shown in Fig. 17. If the matrix of TMSCC could not fill the void of the packing grains, as shown in Fig. 17(b), the SCC system is not dense and has no fluidity. The TMsand particle has poor rolling ability because of significant friction between the particles. The TMsand particles have low roundness, axial coefficient, and sphericity and have high surface roughness [36]. When the MLS content is insufficient, the friction between the TMsand particles cannot be reduced to the particle size of fine cementitious material, the large specific surface area, and high water absorption. Thus, the TMSCC1 with 0% MLS has the lowest compressive strength, workability, and chloride penetration resistance among all SCC specimens.
If the matrix filled the void of the packing grains with appropriate MLS content, as shown in Fig. 17(c), the system becomes dense and has fluidity. Compared with Rsand shown in Fig. 17(a), TMsand has a rough surface and has sharp particles with many edges and corners, which requires more slurry to achieve the same flow capacity as that of Rsand. With the increase in the MLS content, the MLS absorbed on the surface of the Msand acts as a micro-ball shown in Fig. 17(c), which reduces the friction between the Msand, effectively fills the gap between the coarse aggregates, and has a lubricating effect on the coarse aggregates, and this thereby improves the flowability of the TMSCC. The workability of TMSCC increases with the appropriate increase in the MLS content, and the slump flow becomes the largest when the percentage of MLS content reaches 12%. It can be seen in Fig. 3 that the void ratio in the bulk of TMsand can be reduced by the MLS, which might enhance the compactness of the TMSCC, optimize the pore structure, and improve the bond between cement slurry and TMsand. Therefore, the compressive strength of TMSCC increases with the appropriate increase in the MLS content. Furthermore, TMSCC2, with 4% MLS content, has the largest compressive strength at the later age. When the content of MLS gradually increases, the excessive MLS affects the adhesion of the concrete aggregate and the cementitious composites [45], leading to a reduction in the strength of the TMSCC, which is consistent with the result that the MLS content from 8% to 20% has little effect on the density and void ratio in bulk shown in Fig. 3.
If the matrix overfilled the void of the packing particle, as shown in Fig. 17(d), the system becomes dense but has no fluidity [46]. With the continuous increase in the MLS content, the adsorption capacity of TMsand can exceed the increased MLS content, and the micro-ball function can be destroyed for the viscous slurry, which is caused by the excess MLS dispersed between the particles to absorb the water. In addition, the more the MLS content exceeds the optimum content, the more the flowability decreases. The chloride permeability resistance of TMSCC decreases with the increase in MLS content. In general, the MLS with smaller fineness can fill the gap in this area and slightly improve the strength of the TMSCC. However, though a large number of mineral materials are used in TMSCC, too much MLS could limit the promotion effect of mineral materials on strength development. Therefore, the mineral admixture plays a major role in determining the long-term strength of TMSCC; the long-term strength of TMSCC does not fluctuate with the MLS content. From the results of the influence of microlimestone content in TMSCC, it can be concluded that the TMsand with MLS content ranging from 8% to 12% may be a good alternative for the Rsand that can be used in the SCC as fine aggregate.
Conclusions
The influence of different Msand replacements of MLS on the flowability, compressive strength, and chloride permeability of TMSCC incorporating a certain proportion of SF and FA, and the effect of MLS content on the properties of MLSP paste were investigated. Based on the results, the following conclusions are drawn.
1) SCC with TMsand utilized as a filling layer with outstanding flowability, excellent mechanical properties and durability can be achieved by optimizing the amount of MLS replacements of TMsand incorporated with mineral admixtures.
2) Different MLS content can significantly affect the workable performance of MSCC but slightly influence the mechanical properties. The TMSCC with 12% LS content has the best workability, which is more significant than that of TMSCC with other MLS content and is even better than that of RSCC. Although the TMSCC2 with 4% MLS has the largest compressive strength among all TMSCC in the late age (150 d), the change in TMSCC compressive strength is almost negligible when the MLS content increases in late age.
3) The hydration reaction of MLSP paste was slightly affected by the increase in the amount of MLS content as the formation amount of calcium hydroxide, and ettringite did not change significantly. The MLS mainly plays a role in nucleation explained in electric resistivity and SEM results and has little effect on the hydration reaction, while SF and FA can significantly make up for this shortcoming, which could favorably ensure an efficient construction process and outstanding properties of the TMSCC.
4) The mineral admixture plays a major role in determining the long-term strength of TMSCC; the long-term strength of TMSCC does not fluctuate with the MLS content. From the results of the influence of microlimestone content in TMSCC, it can be concluded that the TMsand with MLS content ranging from 8% to 12% may be an excellent alternative for the Rsand that can be used in the SCC as fine aggregate.
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