Use of magnesia sand for optimal design of high performance magnesium potassium phosphate cement mortar

Construction and Building Materials 153 (2017) 385–392

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Use of magnesia sand for optimal design of high performance magnesium potassium phosphate cement mortar Yan-Shuai Wang, Jian-Guo Dai ⇑ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China

h i g h l i g h t s
Magnesia sand increased the M/P ratio of MKPC paste particularly at the ITZ.
MKPC mortar with magnesia sand to replace quartz sand achieved a higher strength.
High strength is due to the strong bonding capacity of paste/aggregate interface.
Failure mode of this mortar was on-site traced by X-ray CT.
Excellent volume stability of this mortar was confirmed.

a r t i c l e

i n f o

Article history: Received 22 April 2017 Received in revised form 5 July 2017 Accepted 10 July 2017

Keywords: MKPC mortar Magnesia sand Mechanical properties ITZ

a b s t r a c t Magnesium potassium phosphate cement (MKPC) mortar as a construction repair material has been studied and applied for several years. In this paper, reactive magnesia sand is employed to replace conventional inert quartz sand as the fine aggregate of MKPC mortar, to adjust the M/P (i.e. MgO-to-KH2PO4) molar ratio of cement paste during mortar formation, as well as to improve the mortar performance though strengthening the bonding capacity of the interfacial transition zone (ITZ) (i.e. the paste to aggregate interface). Experimental results showed that the compressive strength of the MKPC mortar with reactive magnesia sands could achieve a mean value of 36.65 MPa at 12 h, and outperformed about 27.30% than that with inert quartz sand at 28-day. The interfacial transition zone (ITZ) development in the MKPC mortar with magnesia sands was investigated using the on-site X-ray computed tomography (CT). The failure modes of specimens subjected to uniaxial compression were analyzed and visualized by means of image processing technique. In addition, the chemical interaction between the paste and the aggregate was theoretically and experimentally analyzed, explaining well the excellent volume stability of the MKPC mortar with reactive magnesia sands. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction High temperature processing techniques, e.g., sintering, firing, melting and thermal shock, are very common in the field of traditional ceramics, while chemically bonded ceramics (CBCs), terminologically coined by Roy’s research group [1], are polycrystalline inorganic bodies or monoliths synthetized through chemical reactions instead of heat treatment [2,3]. Magnesium potassium phosphate cement (MKPC) is a typical chemically bonded phosphate ceramic (CBPC) with excellent performances, such as clinker-free, quick-setting, strong bonding strength and high early strength, volume stability, and deicer-scaling resistance [4–8]. The occurrence of MKPC is based on a through-solution chemical reaction

⇑ Corresponding author. E-mail address: [email protected] (J.-G. Dai). http://dx.doi.org/10.1016/j.conbuildmat.2017.07.099 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

(Eq. (1)) undergoing the dissolution of MgO and KH2PO4 to the crystallization of potassium magnesium phosphate hexahydrate (i.e. MgKPO46H2O, struvite-K) [9]. Theoretically, KH2PO4 is firstly dissolved in water to create an acid environment where the magnesium ions can be extracted from MgO. The generation of magnesium ion is synchronized with the increase of pH environment due to the consumption of hydrogen ions. When the solution pH is approaching 6.0, crystal growth of struvite-K is initiated to form a cementing cluster and arrives at the optimum at a pH value of about 9 [10]. According to the standard enthalpies of formation of the reactants and resultant (shown under Eq. (1)) [11], this process is an exothermic reaction and theoretical reaction heat is 124 kJ/mol when 1 mol potassium dihydrogen phosphate (KDP) is completely reacted with magnesia.

MgO þ KH2 PO4 þ 5H2 O ! MgKPO4  6H2 O DHf ðkJ=molÞ  602  1568  286  3724 DH ¼ 124

ð1Þ

Y.-S. Wang, J.-G. Dai / Construction and Building Materials 153 (2017) 385–392 fluorescence spectroscopy (AXS GmbH, Bruker), and the results are displayed in Fig. 1. KDP and borax were analytical grade chemicals provided by Tianjin Damao Chemical Reagent Factory (China). The quartz and magnesia sand were sieved with a range of 0.6–2.36 mm. The mix proportions of the MKPC mortar are given in Table 1. An M/P molar ratio of 4.0, with a water/powder ratio of 0.2 and a powder/sand ratio of 1.0, was set for the MKPC mortar preparation. It should be noted that the powder mentioned here denotes a sum of DBM, KDP and borax. Borax, as a retarder, was introduced with a 4% mass ratio of DBM powder. Cubic specimens with a dimension of 20  20  20 mm, cylindrical specimens with a dimension of U10  20 mm and prismatic specimens with a dimension of 25  25  285 mm were cast for compressive strength measurement, X-ray CT test and length deformation evaluation, respectively. After being demoulded, they were cured at room temperature with a relative humidity of 50 ± 5%. Volume change of the mortars was determined according to ASTM C157 [25], where the prismatic specimens were cast with end-face gauge plugs. The length at 12 h after casting was the initial criterion. Compressive strength was measured at 0.5, 1, 7, 28 and 45-day by a material testing machine (Testometric CXM 500-50kN) with a loading rate of 0.3 mm/min according to BS EN 12390-3:2009 [26]. Microscopic and spectroscopic investigations of the MKPC mortars were conducted after 28 d curing. Micromorphological features for the MKPC mortars were characterized by scanning electron microscopy (JEOL, JSM-6490) equipped with energy dispersive X-ray spectroscopy detector (SEM-EDX). Typical mortars were treated by desiccation, polish and gold-sputtering before the SEM-EDX measurement. The powder mineralogical compositions of the mortars were determined by a high resolution X-ray diffractometry (Rigaku SmartLab) with a 9 kW Cu-Ka radiation source and a scanning step of 0.02°. The spatial geometry of the MKPC mortar with reactive magnesia sands was traced by means of X-ray computed tomography (CT, Xradia, XCT-400, USA). The X-ray energy was allocated to 65 kV and 123 mA according to the sample geometry and material attributes (e.g., density). The optical magnification was 0.399, generating a high resolution of 1024  1024 pixels with a pixel size of 10.668 mm. The acquisition in each pixel, represented by the material specific X-ray absorption coefficient, was normalized to 8-bit gray values. Region of interest was focused in a U10  10 mm cylinder in the central part to reflect the internal structure of the MKPC mortar, and a sub-volume was extracted from this cylinder to reduce surrounding abnormalities. Such a process is illustrated in Fig. 2. A series of slice images acquired from the X-ray CT were reconstructed, and the pore, crack, paste and magnesia aggregate phases were isolated respectively. Thus, the desired information could be observed, calculated, and analyzed. Actually, the data collected were mathematically treated as a four-dimensional set, including the Cartesian coordinates (X, Y, and Z) and the corresponding material attributes. In each 8-bit slice image, there are 256 possible gray levels (from 00000000 to 11111111) according to the binary computation. When a grayscale image is transformed to a binary image, a pixel will be allocated to a value of 1 in the target region or 0 out of this region. Thus, the features of the region of interest can be extracted from the whole data set based on the gray threshold value, and a series of binary images can be combined into a 3D matrix of voxels, which can be labeled and quantified [27]. The volume of the target region was calculated as in Eq. (2):

V CT ¼ N v ox  Z v ox

ð2Þ

where, VCT , cm3, the labelled volume; Nvox , – the voxel count of labelled region; Zvox , cm3, the spatial voxel size (10.668 mm in this case).

Median particle diameter: 5.553 µm Mean particle diameter: 8.294 µm Specific surface: 0.692 m2/g

5

100

80

4 Chemical composition (wt.%) MgO: 88.9% SiO2: 4.2%

3

60

Fe2O3: 3.19% CaO: 2.37% Other: 1.34%

2

40

1

20

0

0

Cumulative (%)

Over the past decades, MKPC has been branded as Ceramicrete [12] by Argonne National Laboratory (ANL) for speedy rehabilitation of bridges and runways [13,14]. As a patch repair material, it is expected to hold a mechanical strength higher than that of the substrate material. It is therefore imperative in practice to slow down the reaction rate (mainly to reduce the sharp reaction heat) to facilitate a well-formed crystalline phase, a reasonable working period and a compact matrix in the end. In engineering application, MKPC mortar (usually with quartz sand) is more widely used compared with its paste, especially when considering the economic benefits (e.g., as a low-cost extender) and the mechanical and transport properties (e.g., high elastic modulus, conductivity and low shrinkage of mortar) [15,16]. However, properties of this mortar (e.g., strength and permeability) are largely dominated by the properties of MKPC paste, and the paste is governed by the W/C mass ratio and the M/P molar ratio [17]. It is well-recognized that a low W/C for MKPC paste will may to a low porosity, but a short setting time, bad workability and low reaction degree. When a constant W/C ratio is given, decreasing the M/P ratio (e.g., Ref. [1–4]) can overcome these above weaknesses to some extent. Recently, Xu and Ma [17,18] reported that an M/P molar ratio of 4.0 in the MKPC paste could create an optimal chemical environment, which led to a maximum amount of reaction products (i.e. struvite-K) and a low void space. As a result, the strength of MKPC paste could be increased and low permeability and thermal conductivity could be achieved because of the higher volume ratio of the struvite-K to the void space (namely, struvite-K-to-space ratio). However, such an M/P ratio is not normally adopted in the MKPC-based rehabilitation of civil engineering due to poor thermal stability caused [18,19]. Instead, the M/P molar ratios ranging from 6 to 14 are more commonly used in practice to obtain good engineering properties [7,11,20–22]. This paper proposes to utilize the reactive magnesia sand as fine aggregates in MKPC mortar to adjust the M/P ratios of the cement paste. Instead of the quartz sand, which is a non-renewable resource, the magnesia can be extracted from the sea water or highly concentrated brine wells, which is more environmentally friendly [23]. It was expected that the magnesia sand can react with some KDP to achieve a high performance MKPC mortar through increasing the M/P ratio of the MKPC paste during hardening to satisfy the engineering requirements. In the meantime, the ideal conditions (e.g., M/P ratio) of MKPC paste for a high performance MKPC paste are met. Moreover, the chemical interaction between the cement paste and the aggregate in this proposed solution is expected to overcome the adverse effect of their interfacial transition zones (ITZs) due to the material compatibility between them. At the same time, the magnesia sand may serve as reinforcement to strengthen the load-bearing capacity of the cement mortar. During the preparation of MKPC mortar with inert quartz sand, ITZs, which appear inevitably around the aggregates as those in conventional Portland cement mortar, may compromise the mechanical strengths. Indeed, cracks in mortar or concrete typically initiate in the immediate vicinity of the aggregates. If the aggregate-to-paste bond is strong enough, a higher compressive strength will be achieved due to some diverted transversal forces taken up by the aggregate [24]. The test program in the paper was designed to validate experimentally the proposed solution and to analyze the ITZs in the formed MKPC mortar, in which reactive magnesia sands were used.

Differental (%)

386

2. Materials and methods The starting materials used in this work were dead-burnt magnesia (DBM) powder, potassium dihydrogen phosphate (KDP), borax, standard quartz sand and magnesia sand. DBM powder with fine granularity was obtained from Jimei Refractory Co. Ltd (China). Its particle size distribution and chemical composition were respectively checked by laser particle analyzer (BT-9300ST, Bettersize) and X-ray

0.1

1

10

100

Diameter (µm) Fig. 1. Particle size distribution and chemical composition of DBM powder.

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Y.-S. Wang, J.-G. Dai / Construction and Building Materials 153 (2017) 385–392 Table 1 Mix proportions (mass ratio) for the MKPC mortar. Series

DBM

KDP

Borax

Magnesia Sand

Quartz Sand

Water

MQ0 MQ25 MQ75 MQ100

0.558 0.558 0.558 0.558

0.422 0.422 0.422 0.422

0.02 0.02 0.02 0.02

1 0.75 0.25 0

0 0.25 0.75 1

0.2 0.2 0.2 0.2

Focus region by X-ray CT

10 mm

20 mm

Note: M/P molar ratio = 4; Water-to-binder = 0.2; Binder-to-sand = 1.0.

900 slices

CT Slice

CT Slice Extraction

Extraction

Subvolume

6.2 mm

CT Slice 10 mm

Fig. 2. Region of interest and sub-volume focused by X-ray CT.

3. Results and discussion 3.1. Characterization of MKPC mortar As shown in Fig. 3, the compressive strength development of a series of MKPC mortars at five different curing times was compared. During the curing period, the compressive strength of these MKPC mortars kept a similar tendency: MQ100 (quartz sand only) and MQ0 (magnesia sand only) held the lowest and highest compressive strength, and the compressive strengths of MQ25 and 70

MQ100 MQ75 MQ25 MQ0

Compressive strength (MPa)

60 50

6.62

9.65

6.35 1.38

40 30

13.21

20 10 0

0.5

1

7

28

45

Curing time (Day) Fig. 3. Compressive strength development of the MKPC mortars at different curing periods.

MQ75 followed successively below that of MQ0 except at the 45 d curing where almost the same strength value was observed for MQ25 and MQ75. Besides, the compressive strengths of all MKPC mortars increased with the curing age because the main hydration product, MgKPO46H2O (struvite-K), was accumulated to reach a higher strength by cementing unreacted substances and sealing micropores. Taking the curing ages of 7 and 28-day as an example, the average compressive strength at 28-day increased, compared to that at 7-day, by 9.65 MPa, 6.62 MPa, 6.35 MPa and 1.38 MPa for MQ0, MQ25, MQ75 and MQ100, respectively. Furthermore, it was noteworthy that 12 h after casting the MKPC mortar with complete reactive magnesia sands (MQ0) as fine aggregates achieved a desirable compressive strength (mean value = 36.65 MPa), which was 13.21 MPa higher than MQ100, indicating that the application of reactive magnesia sands as fine aggregates enabled to meet a higher engineering requirement, especially as a patch repair material. Fig. 4 plots the XRD diffractograms of MKPC mortars with different proportions of magnesia and quartz aggregate after 28-day curing, revealing that the characteristic peaks of magnesia, quartz and MKP hexahydrate were located at around 43, 27 and 21° (2theta), respectively. As observed in previous publications [11,15], other peaks assigned to MKP hexahydrate (e.g., 16.5, 22, 26, 28, 31, 33. . .) were clearly displayed regardless of the mix proportion. More importantly, with the increase of magnesia sands, stronger characteristic peaks (e.g., 21 and 22° of 2-theta position) of struvite-K were exhibited. The compressive strength of MKPC mortars was closely relevant to the hydration degree, namely the crystallinity of MKP hexahydrate, and the peak intensity of struvite-K in four types of MKPC mortars coincided well with the compressive strength results. The change of peaks intensity (i.e., quartz and periclase) should be attributed to the different proportions as indicated in Table 1.

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P - Periclase S - Struvite-K Q - Quartz

P

Q P MQ0

SS

SS S

MQ25

S SSP

S

S

MQ100 10

Q

S

MQ75

0

S

20

30

Q 40

50

60

70

2-Theta (Deg.) Fig. 4. XRD diffractograms of four types of MKPC mortars at 28-day.

Typical microstructure morphologies for the MKPC mortars (MQ0 and MQ100) at 28-day curing are showcased in Fig. 5, where (a) and (b) display the aggregate embraced by the MKPC paste, and (c) and (d) are the enlarged images for the corresponding interfacial transition zone between the paste and the aggregate. A clear contour line can be found in Fig. 5(b), highlighting the difference between the quartz aggregate and the paste. However, in the MKPC mortar with magnesia aggregate, the paste and the aggregate seemed to present as an integral whole except the slightly different colors. Besides, the element analysis by EDX spectra is plotted in Fig. 6(a), where no phosphorus was detected in the spectrum 2. The elementary composition of struvite-K is shown in the spectrum 1 of Fig. 6(a), revealing that the ratio of Mg: P: K is around 1:1:1. The other visible difference was the degree of crystallization. The MKPC paste around the aggregate in the MQ0 presented a well-crystalized struvite-K shape that can be described as bladed prism (see Fig. 5(c)), while the ill-formed or poor crystalline phases occurred in its opponent (MQ100) (see Fig. 5(d)). This result may be due to the incorporation of magnesia sand, which improved the M/P ratio of the paste in the vicinity of the magnesia sand through consuming KDP. Thus, the interfacial bonding of the paste and aggregate was chemically enhanced. Despite the morphological difference of struvite-K crystallinity, the EDX spectra shown in Fig. 6 (Spectrum 1 and 4) revealed both MKPC pastes held the similar elementary composition. In addition, the naturally formed quartz sand (Spectrum 3) was much denser than the artificially prepared magnesia sand (Spectrum 2) whose porosity is related to the calcination degree. 3.2. Failure mode of MKPC mortar with magnesia sands From the results of compressive strength development, it was found that the MKPC mortars with reactive magnesia sands exceeded that with inert quartz sands at all the curing time. In order to clarify the reasons behind, a comprehensive study on the ITZ development in the MKPC mortar with magnesia sands was conducted by the on-site monitoring of X-ray computed tomography (CT). The 2D slice images and corresponding 3D reconstructed images of the specimens are displayed in Fig. 7, where the tomography results of the specimen at 1-day curing, and the specimens at 28-day curing before and after the uniaxial compression, are visualized. According to the qualitative observa-

tions, the CT results of 1-day curing and 28-day curing seemed to have no significant difference (Fig. 7a). Therefore, further quantitative analysis was carried out based on the image segmentation technique (Otsu method [28]). In the extracted MKPC mortar matrix, the porosities of the MKPC paste and the aggregate’s volume fractions were quantitatively determined, and their volume fraction distributions along the Z-axis (from the bottom to the top) at 1-day and 28-day curing were plotted in Fig. 8. The trends in such curves along the Z-axis may be closely linked to the random distributions of unreacted DBM powder, hydration products and aggregates and the casting direction and workmanship [29]. The measured porosities only involved large capillary pores and entrapped or entrained air bubbles because the pores less than 10.668 mm (the pixel size of X-ray CT measurement) were undetectable because of the offset between the micro-level image resolution and the sample dimension [27]. These detected pores were found to be reduced by about 1.05% due to the accumulation and growth of hydration products (mainly struvite-K) around the pores. At the same time, the volume fraction of reactive magnesia sands was decreased from 58.09% at 1-day curing to 55.62% at 28day curing, due to the chemical reaction between the MKPC paste and the aggregate. These changes (e.g., decreased porosity and increased hydration products around ITZs) caused by the reaction between the paste and the sand positively uplifted the struviteK-to-space ratio, which enabled the strength enhancement of the MKPC matrix. It is well known that there are three typical stages of crack propagations in Portland cement concrete during its compressive failure: (i) cracks initiation around the aggregates, (ii) bypassing the aggregates and (iii) penetrating the hardened paste in the end. The inert aggregate-induced ITZ generally exhibited a lower hardness by 15% as compared to that of the bulk cement paste matrix according to the nano-indentation measurement result [30]. The ITZ generated during matrix hardening behaves as the weakest link within the microstructure of concrete [31], where two ITZ failure modes (‘ITZ-aggregate separation’ and ‘ITZ failure’) would appear under mechanical loading [32]. However, the MKPC mortar with reactive magnesia sands proposed in this study somewhat behaved differently. From the failure morphologies of the MKPC mortar subjected to uniaxial compression, whose slice image and corresponding 3D reconstructed image are shown in Fig. 7(c), it is clearly shown that most of magnesia sands in the central part of the specimen (shown in the slice image) were crushed. Cracks both bypassed and propagated through the magnesia sands and only a small portion of the magnesia sands remained intact, particularly at the edges of the specimen. Such a phenomenon can be explained as follows: 1) the magnesia sand (5.5) is lower than the quartz sand (7) in terms of Mohs hardness, so the former is relatively vulnerable to breakage; 2) during the loading process, surface cracking and spalling occur in the beginning, leading to the reduction of effective cross section (stress-bearing area) during the loading. As a result, the aggregates in the central part would bear more stress than the edge ones (stress concentration) [33]. The reactive magnesia sand had a chemical interaction with the MKPC paste (Eq. (1)), generating a cementitious resultant, struviteK, leading to good homogenization around the ITZs. Despite its lower hardness than the quartz sand, the reactive magnesia sand exhibited a porous structure and high crystallinity due to its high temperature (1400–2000 °C) calcination, which contributed to the enhancement of the mechanical locking effect with the paste and thus the overall mortar strength [34]. Fig. 9 illustrates a schematic diagram to elaborate the role of the ITZs in the failure process of MKPC mortar. In the MKPC mortar with inert aggregates, the interaction between the aggregate and the MKPC paste mainly depends on the Van der Waals forces. However, the MKPC mortar with reactive magnesia sands achieved a stronger aggregate/paste

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Fig. 5. Micro-morphological features of the MKPC mortars with magnesia aggregate (MQ0) ((a) and (c)) and quartz sand (MQ100) ((b) and (d)) at 28-day curing.

800

600 Spectrum 1

Spectrum 3

Si

Counts

Counts

600 400 P

200 C

0 800

O

200

K

Mg

400

Au

C

O

Au

K

0 600

Mg

Spectrum 2

Spectrum 4

Counts

Counts

600 400

400 Mg

200 C

200

Au

K

K

C

0

P

Au

O

0

O

0

1

2

3

4

Energy (keV)

(a)

0

1

2

3

4

Energy (keV)

(b)

Fig. 6. EDX spectra of the MKPC mortars with magnesia aggregate (MQ0) (a) and quartz sand (MQ100) (b) at 28-day curing.

interaction through Van der Waals force, mechanical interlocking and bonding capacity of struvite-K. Relatively weak ITZs existing in the former case may lead to crack propagation along the ITZs under mechanical loading, as usually reported for conventional Portland cement mortar (Fig. 9a). However, given a stronger ITZ, the aggregate could undertake more responsibility together with the paste for the stress transfer and thus led to the penetration of cracks through the aggregates (Fig. 9b).

3.3. Chemical interaction in ITZs As discussed above, the application of magnesia sands reduced the strength weakening effect of ITZ and enhanced the bonding capacity of the paste to aggregate interface, so as to achieve a higher performance mortar. However, in engineering applications a major concern with the use of reactive aggregates is the volume stability (e.g., drying shrinkage and expansion) of so formed MKPC

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2 mm

2 mm

2 mm

MKPC Paste

Magnesia Sand

(a)

(b)

(c)

Fig. 7. CT slice images and corresponding 3D reconstructed images. (a) 1-day curing before loading; (b) 28-day curing before loading; (c) after loading a uniaxial compression at 28-day.

20

Volume fraction of aggregate (%)

64

Porosity (%)

15

10

5

1-day, average porosity = 13.40% 28-day, average porosity = 12.35%

0 0

2

4

6

62 60 58 56 54 52 50

1-day, average volume fraction = 58.09% 28-day, average volume fraction = 55.62%

48 8

10

0

2

4

6

Z-axis (mm)

Z-axis (mm)

(a)

(b)

8

10

Fig. 8. Porosity (a) and volume fraction of magnesia aggregate (b) distributions along the Z-axis at 1-day and 28-day curing.

mortar. According to previous studies, apart from the external stress-induced ITZ failure, the alkali-aggregate reaction (AAR) (mainly alkali-silica reaction (ASR) [35])-induced ITZ failure in Portland cement concrete is detrimental due to the 2–3 times volume expansion of the reaction products [36]. It usually occurs in the ITZ due to the interaction between the alkalis in the pore solution of Portland cement paste and certain forms of siliceous constituents in the surface of aggregates, generating a waterabsorbing gel whose expansion can exert internal forces and lead to uneven cracking of the paste matrix [37]. Previous studies have shown that in order to achieve an excellent mechanical property

the MKPC paste needs to be formulated as the molar ratio of MgO-to-KDP between 3 and 6, where the reaction degree of KDP at 28 days is about 55–5% [38]. Thus, there is a high possibility that unreacted KDP will continuously react with reactive magnesia sands. Fortunately, it was theoretically and experimentally proven that the reaction causing the volume expansion hardly occurred in the MKPC mortar containing reactive magnesia sands. The primary reason is the dimensional stability of the paste-to-aggregate reaction process. According to the stoichiometric compositions shown in Eq. (1), 1 mol of magnesium oxide reacts with 1 mol of KDP and

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(a) Crack initiation around weak ITZs MKPC paste Acting force exerted by aggregate on paste

(b) Crack penetrating aggregate Aggregate

Crack

Acting force exerted by paste on aggregate

Stress transfer direction

Fig. 9. The role of ITZs in the failure process of MKPC with different types of aggregates (a) weaker ITZ (b) stronger ITZ.

Table 2 Attributes of reactants and resultant involved in the MKPC reaction.

Molar mass (g/mol) Density (g/cm3) Molar volume (cm3/mol)

Reactant

Resultant

MgO

KDP

Water

MKP

40.30 3.58 11.26

136.09 2.34 58.16

18.02 1.00 18.02

266.47 1.86 143.26

5 mol of water, producing 1 mol of MKP. As shown in Table 2, the total molar volume of reactants is 159.52 (11.26 + 58.16 + 18.02 * 5) cm3/mol, which is almost equal to that of the resultant (143.26 cm3/mol). Such a difference is negligible compared to that caused by AAR, and in the meantime, it can be offset by pores etc. The other reason is that the generation and growth of struvite-K in the interface between the aggregate and the cement paste is intimately associated with the environmental pH value. The reaction shown in Eq. (1) may be interrupted by the inappropriate pH condition, resulting in the isolation of the magnesia aggregate and dihydric phosphate. It is therefore concluded that the volume of the ITZ almost remains unchanged before and after the chemical interactions. In the experiment, the length change of the MKPC mortar prisms was monitored by a dial gage extensometer to evaluate the volume stability. The measurement results of length changes (the average of three identical specimens) of the MKPC mortars are shown in Fig. 10. The test data of the OPC counterpart (with the similar water-to-cement ratio and cement-to-sand ratio to the present MKPC mortar with magnesia sands) extracted from ref. [7] was also presented for reference. It can be clearly seen that regardless of the aggregate reactivity the MKPC mortar had much better volume stability than OPC mortar over the whole curing period. The MKPC mortar appeared to exhibit sharp shrinkage during the first week after casting and reached a relatively steady level after that. In fact, from 28-day to 45-day, only a marginal deformation increase (i.e., around 30 lm and 12 lm for the MQ100 and MQ0, respectively) was observed in the MKPC mortar. In the early hydration period, the volume change was mainly due to the chemical reaction as discussed in previous sections, while in the latter stage when the hydration rate became gentle, the evaporation or migration of free water was the primary reason. MQ0 was slightly inferior to MQ100 in terms of the volume stability (the correspond-

Curing time (day) 1

7

28

45

0

-200

Length change (µm)

Items

-400

-600

-800

OPC mortar MQ100 MQ0

-1000 Fig. 10. Length change of MKPC mortar.

ing length changes at 28 days were 425 mm and 392 mm, respectively), which was because that the incorporation of reactive magnesia sands promoted more reaction of KDP and increased the hydration degree thus resulted in a slightly larger shrinkage. 4. Conclusions Application of reactive magnesia sands in the preparation of MKPC mortar has led to a better performance as compared to MKPC mortar with quartz sands. The ITZ development in the MKPC mortar with reactive magnesia sands was studied from three

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aspects: interaction between the magnesia sand and the MKPC paste during hydration, ITZ failure mode under uniaxial compression, and ITZ volume stability. Based on the experimental results, the follow conclusions can be drawn: (1) Use of reactive magnesia sands to replace inert quartz sands improved the M/P molar ratio of the paste, and strengthened the bonding capacity of the paste to aggregate interface. The compressive strength of MKPC mortar with reactive magnesia sands could reach a mean value compressive strength of 36.65 MPa at 12 h, and outperformed about 27.30% than that with inert quartz sands at 28-day curing. (2) The ITZ development in MKPC mortar with reactive magnesia sands was on-site traced by X-ray CT at 1 day curing and 28-day curing before and after uniaxial compression. The porosity of such mortar and the volume of magnesia sands were found to decrease by 1.05% and 2.47%, respectively, after 28-day curing according to the image processingbased results. (3) The failure mode of the specimen subjected to uniaxial compression was analyzed and visualized to pinpoint the reasons of strength upscaling of the MKPC mortar with reactive magnesia sands. The magnesia sands in the center of the specimens were crushed rather than bypassed, which was mainly attributed to the strong chemical and mechanical interlock between the cement paste and the sand. (4) An excellent volume stability of the MKPC mortar with reactive magnesia sands was theoretically and experimentally confirmed, showing that the reaction at the ITZs led to no harmful expansion.

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