Pore structure evolution and strength development of hardened cement paste with super low water-to-cement ratios

Construction and Building Materials 227 (2019) 117108

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Pore structure evolution and strength development of hardened cement paste with super low water-to-cement ratios Laibo Li a, Haiming Zhang b, Xiangyang Guo c, Xiangming Zhou d, Lingchao Lu a,⇑, Mingxu Chen a, Xin Cheng a,* a

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Qilu University of Technology, Jinan 250353, China Shandong Provincial Academy of Building Research, Jinan 250000, China d Department of Mechanical, Aerospace and Civil Engineering, Brunel University London, Uxbridge, Middlesex UB8 3PH, United Kingdom b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Ice particles were introduced as

mixing water to prepare cement paste with super low w/c.
Porosity of hardened cement paste decreased with the increase of w/c.
Compressive strength of hardened cement paste increased with the increase of w/c.

a r t i c l e

i n f o

Article history: Received 8 May 2019 Received in revised form 8 September 2019 Accepted 27 September 2019

Keywords: Pore structure evolution Strength development Super low water-to-cement ratios Cement paste

a b s t r a c t In the current study, the ice particles instead of liquid water was firstly utilized as mixing water to prepared cement pastes with super low water-to-cement (w/c) ratios and homogenous structures. The effect of w/c ratio on the properties, especially the pore structure evolution and strength development, of hardened cement pastes were investigated. Mercury intrusion porosimetry (MIP) was introduced for the evolution of pore structure of hardened cement paste. Hydration heat-evolution was employed for characterising the hydration process of cement paste with super low w/c ratios. Experimental results suggested that the water absorption rate decreased by 49.3% while its open porosity decreased by 43.5% with the increase of w/c ratios from 0.08 to 0.16. The initial porosity of cement paste compact reached up to 36.6%, and the pores were mainly capillary pores of diameter in range of 0.1–8 lm. When hydrated for 3 days, the capillary pores with diameter more than 1 lm were vanished totally. Furthermore, 28-days total porosity of samples with w/c ratio of 0.16 decreased by up to 46.1% compared to that of samples with w/c ratio of 0.08. With the w/c ratio increased from 0.08 to 0.16, the 28-days compressive strength increased by 68.9% to a value of 127.5 MPa. Additionally, the total porosity was the mainly factor that determined the compressive strength of hardened cement paste with super low w/c ratios. Ó 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. E-mail addresses: [email protected] (L. Lu), [email protected] (X. Cheng). https://doi.org/10.1016/j.conbuildmat.2019.117108 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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L. Li et al. / Construction and Building Materials 227 (2019) 117108

1. Introduction Concrete is the most widely used construction materials today. It was roughly estimated that the output of concrete globally reached up to 25 billion tons each year. In other words, the output of concrete averaged over 3.8 tons per person per year in global. Twice as much concrete was used in the construction industry around the world than all the other construction materials (e.g. glass, ceramics and wood et al.) combined [1,2]. Among all the components of concrete, such as cement, aggregate, water and admixtures, water largely determined the engineering performance, mechanical properties and porosity of concrete. Previous studies [3–7] demonstrated that the strength, shrinkage, water absorption and other engineering properties, and the quality of pore structure of concrete were enhanced when the w/c ratio value was decreased. For instance, Rao [4] in a role of w/c ratio on the strength development in mortars study found out that the strength of mortar increased as the w/c ratios increased at any age of mortar. Schulze [5] reported that shrinkage and water absorption were a function of w/c ratio, and the shrinkage and water absorption of concrete were decreased by the low w/c ratios. Similar results were obtained by Piasta and Zarzycki [6] and Singh [7]. Therefore, reducing the water content or w/c ratio was beneficial to enhance the engineering properties of concrete. However, there was a fact worthy noting here. The workability of the fresh concrete was decreased by the decrease of the w/c ratio, which limited the use of the positive effect of low w/c ratio [8–10]. Hence, superplasticizers were adopted to decrease the w/c ratio and increase the workability of the fresh concrete. In general, superplasticizers were organic polymers, which enable workability of fresh concrete with low w/c ratio by shielding attractive forces between the powder particles, like Van-der-Waals- and electrostatic forces [11,12]. Because of the particles could be dispersed sufficiently in the mixing water and homogeneous and workable mixture could be obtained by superplasticizers, concrete with low w/c ratios (e.g. ultra-high performance concrete) were applied widely [12–15]. From the structure and compositions point of view, the application of superplasticizers had two main disadvantages. The first presented that the application of superplasticizers was a limited solution [3,16,17]. For the concrete with super low w/c ratios, under w/c ratio of 0.15 and deeper, particles were not dispersed uniformly in the mixing water and no homogeneous structure mixtures could be prepared. The second consists in that the complexity of concrete compositions was increased by the application of the mentioned materials [18–20]. Therefore, in order to prepare concrete with super low w/c ratios and homogenous structure, and simplify compositions of mentioned concrete, pressure was introduced to compact the concrete. Previous literatures [21–24] reported that compacts of fresh concrete or bottle-hydrated cement pastes were widely employed for investigating the microstructure, porosity, and mechanical properties et al, and the structural model of C-S-H gel et al. For instance, Panzera et al. [21,22] designed and prepared a cement paste for combining aerostatic bearings with low w/c ratios by compacting mixtures of particles and water. Sereda and Feldman [23] studied the relationship between the sorption and dimensional changes of micro-pore in detail by compacting fine powders and water. Ekincioglu et al. [25] prepared a macro-defect-free cement material with w/c ratio of 0.11 using the shear mixing method, and Zˇivica [3] prepared a cement paste with a w/c ratio of 0.075 using the high-pressure dehydration technology. However, Shen et al. and Lecomte et al. [26,27] thought that it was difficult to prepare a cement paste with low w/c ratios and homogenous structure due to the difference of physical form

between solid and liquid, and the micro bleeding, bulb, settlement and segregation et al. were unavoidable. Hence, in order to avoid the negative factors as described above, the cement compacts without any mixing water were prepared and saturated by water through capillary absorption by Shen et al [26,28]. Micromorphology and structure of cement pastes with low w/c ratios were researched by the mentioned method. For this method, the structure of hardened cement paste was still heterogeneous due to the water appears graded distribution in the cement compacts. In addition, the height of dry cement compacts could not more than 35 mm due to the limited capillary force, which also limited the application of the method in civil engineering [28]. The physical form of ice particles is the same as cement. There is only a weak attractive force between ice particles and cement due to the inexistence of ‘liquid bridges’. Micro-bleeding, bulb, settlement and segregation could be removed during the mixing of ice particles and cement. Therefore, ice particles were adopted to totally replace liquid water in order to prepare a cement pastes with super low w/c ratios and homogenous structure. This study aimed to determine the effects of w/c ratio on the pore structure evolution, strength development and other physico-chemical properties, such as water absorption rate, open porosity, bulk density, matrix density, and non-evaporable water content of hardened cement pastes with super low w/c ratio and homogenous structure. Another important intention of this paper is to supply useful data to develop the knowledge in the concrete industry, especially for concrete prepared with super low w/c ratios. 2. Experimental procedures 2.1. Materials Ordinary Portland cement (OPC, Grade 52.5R according to Chinese National Standard GB 175-1999, manufactured in Shandong Cement Co., Ltd, China) was adopted as the binder to prepare the cement paste. An X-ray fluorescence spectrometer (XRF, Tiger S8, Germany) was employed to determine the chemical compositions of the OPC, and the chemical compositions and physical properties of OPC were listed in Table 1. The main crystal phases of OPC were measured by X-ray diffraction (XRD) and quantified by quantitative X-ray diffraction (QXRD), the data of which are presented in Fig. 1 and Table 2. Particle size distribution of OPC was tested by using a laser particle size analyzer (LS13320, USA) and the result was shown in Fig. 2. Ice particles were prepared and used to totally replace mixing water to fabricate the cement paste. The prepared process of ice particles in detail was conducted as follows: 1) an electric sprayer filled with supercooled water was placed in an environment of 20 ± 2 °C, and the sprayer around 90 ± 5 cm from the ground; 2) turn on the switch to make sure the sprayer work normally; The atomized supercooled water became the ice particles in the process of falling; 3) a square-hole sieve was used to remove the ice

Table 1 Chemical compositions and physical properties of OPC. Chemical compositions (wt%)

Physical properties

CaO SiO2 Al2O3 SO3 MgO Fe2O3 K2O Los Surface area (m2/kg) Density (kg/m3)

61.80 19.13 4.80 3.61 3.07 3.14 0.70 2.68 320 3043

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L. Li et al. / Construction and Building Materials 227 (2019) 117108

Fig. 1. X-ray analysis result of OPC (C = CaO, $ = SiO2, A = Al2O3, F = Fe2O3).

Fig. 2. Particle size distribution of OPC.

with particle sizes of more than 300 lm. Fig. 3 showed the set-up for preparing the ice particles. In addition, the particle size distribution of ice was analyzed by square-hole sieves with different aperture sizes (i.e. 90 lm, 98 lm, 105 lm, 125 lm, 154 lm, 200 lm and 300 lm), as presented in Fig. 4.

2.2. Sample preparation In this paper, the liquid water was totally replaced by the frozen water (i.e. ice particles) for preparing the cement paste. Hence, the subjects of the paper were the hardened cement pastes from OPC and frozen water (i.e. ice particles) with w/c ratios (i.e. frozen water-to-cement, as follows) ratios of 0.08, 0.12 and 0.16. The mixing process of cement paste in detail was conducted as follows: 1) OPC was kept in the environment of 20 ± 2 °C until the temperature of OPC was the same as the ambient temperature; 2) A mixer (Fig. 5(a)) similar to the egg beater was used to mix the OPC and ice particles in the low temperature environment as described above for 20 min; 3) The mixer containing the mixture of OPC and ice particles was moved from the environment of 20 ± 2 °C to the environment of 25 ± 2 °C, and the mixture was mixed until the ice particles melted completely. After which the flow ability of the cement paste was tested complying with GB/T 8077-2000 [29]. The flow ability of cement paste was measured in terms of flow spread using the mini slump cone test. The test method was conducted as follows: 1) Pour the cement paste slowly into the slump cone until (Fig. 6(b)) the slump cone is completely filled up (Fig. 6(c)); 2) Lift the slump cone gently and allow the cement paste to spread; 3) At the time of 30 s since lifting the cone, measure the diameters of the cement paste patty in two orthogonal directions, calculate the average diameter as the flow spread of the cement paste. The test result (Fig. 6(d)) suggested that the flow ability of the cement paste with super low w/c ratio was too low to compact by vibration. Then, the mixture was added in a special steel mould (Fig. 5(a)) with cavity size of 20  20  40 mm3 for the preparation of 20 mm-edge cube specimens. After which, the steel mould with the mixture was moved to a precise press machine controlled by an automatic pressure controller (Fig. 5(a)). The maximum pressure

was 25 MPa, and the pressure was kept for 3 min. The maximum pressure of 25 MPa was determined by the relationship between the initial porosity, w/c ratio and compaction pressure. The initial porosity (P0) could be calculated according to Eqs. (1) and (2), which was 32.9% at the w/c ratio of 0.16 in this paper. After which, the initial porosities of cement pastes at the different compact pressure were measured by ‘the ethanol absorption method’ [30]. The relationship between the initial porosity and compact pressure was established by the nonlinear fitting technology (Fig. 7). Then, the maximum pressure could be calculated by substituting the value into the expression.

P0 ¼

VW  100% VW þ VC

w=c ¼

ð1Þ

V W  qW V C  qC

ð2Þ

where P0 is the initial porosity, %; VW is the volume of water at the total volume of 1 cm3, cm3; VC is the volume of cement at the total volume of 1 cm3, cm3; w/c is water-to-cement ratio; qW is the density of water, g/cm3; qC is the density of cement, g/cm3. The specimens were then squeezed out from the steel mould. The demoulding specimens (Fig. 5(a)) were kept in a curing environment of 20 ± 2 °C and 95+% RH for 24 h. After that, the specimens were then soaked in water at 20 ± 2 °C for 2- and 27-days. Subsequently, the hydration of specimens was stopped by leaving samples immersed in absolute ethyl alcohol for 24 h. Fig. 5 shows the process for the specimen preparation. In addition, cement powder without any mixing water was compacted at the maximum pressure of 25 MPa and the pressure holding time of 3 min, representing the cement paste hydration for 0 day. 2.3. Test methods 2.3.1. Basic physical characteristics The water vacuum saturation method [31] was employed to determine the water absorption rate, open porosity, bulk density and matrix density of hardened cement pastes. The specimens

Table 2 QXRD data of OPC (wt%). Mineral

C3$

C2$

C3A

C4AF

CaSO412H2O

Amorphous

Content

51.95

12.21

4.66

6.00

3.45

21.71

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containing the paste was put into a conical flask. Then the conical flask was subsequently filled with paraffin oil and closed with a rubber stopper, through which a graduated pipette with a total volume of 1 ml and an accuracy of 0.01 ml was inserted; the conical flask was immersed in a controlled water bath at 20 °C. Measurements were performed by reading the oil level of the graduated pipettes at regular intervals during 28 days on three replicate specimens. 2.3.4. Thermal analysis The non-evaporable water content [32,33] of hardened cement pastes was determined by burning method at approximately 950 °C. The burning method was conducted as follows: 1) specimens were dried at 105 ± 2 °C no less than 24 h; 2) the specimens were then ground to a particular fineness of below 74 lm; 3) some 5–10 g of specimens powders (exact mass M1) were burned at 950 °C to constant weight (mass M2). The non-evaporable water was calculated according to the following equation:

W NC ¼

Fig. 3. Set-up of ice particles preparation.

M1 M2 M1

 WL

1  WL

 100%

ð3Þ

where WNC is non-evaporable water content, %; M1 is the weight before burning, g; M2 is the weight after burning, g; WL is the loss on ignition of cement, % (Table 1). Thermogravimetric (TG) analysis was performed by using a simultaneous thermal analysis instrument ((TGA/DSC1/1600HT, Sweden) at 10 °C/min up to 950 °C. The experimental atmosphere was argon with flowing rate of 50 ml/min. Furthermore, the weight of each sample was 40 ± 2 mg, and the particle size of that was no more than 74 lm. 2.3.5. X-ray diffraction (XRD) analysis Crystallized phases of hydrated cement paste were measured by an X-ray diffractometer (D8 ADVANCE, Germany) with copper anode kKa1 = 0.154 nm generated at 40 mA and 40 kV. The particle size of samples was kept less than 74 lm using a square-hole screen. The acquisition range for each sample was 7–65° (2h) with a step size of 0.02° (2h) and a rate of 5 s per step. In addition, Quantitative XRD (QXRD) technique was utilized to quantitatively evaluate the content of phases in the hardened cement paste. In this work, the internal standard method was employed to acquire the QXRD data. After which the data were processed by using the internal standard method with Topas 4.2 software package from Bruker AXS GmbH.

Fig. 4. Particle size distribution of ice particles.

were cured for 28 days followed by drying at 105 ± 2 °C in order to remove majority of the physically bound water. After which each specimens were removed into a desiccator with de-aired water. A vacuum pump was used to drawing-off the air from the desiccator in three hours. Specimens were then soaked in water not less than 24 h. 2.3.2. Pore structure The pore structures of specimens hydrated for 0- (i.e. cement powder compact), 1-, 3-, 28-days were characterized by mercury intrusion porosimetry (MIP). The experiments were carried out using an automatic mercury porosimetry (Pore Master-60, USA), whose testing range of pore diameters was from 5 nm to 200 lm and intrusion accuracy was ±0.11%. 2.3.3. Chemical shrinkage The chemical shrinkage of cement paste was tested according to ASTM standard C 1608-07 [40]. The fresh cement paste (~10 g) was weighed and inserted in a rubber bag. After which the rubber bag

2.3.6. Hydration heat evolution An accurate conduction calorimeter (TAM Air, USA) operating at 20 ± 0.2 °C was employed to determine the hydration heat flow, whose measurement accuracy was ±20 lW. The heat flow was recorded every 24 s until 72 h had passed. 2.3.7. Mechanical property The compressive strengths of hardened cement pastes were tested by a universal compression machine (MTS CMT 5504, USA). The measure of compressive strength was carried out after 1-, 3- and 28-days hydration of specimens, and the speed of loading was 2 mm/min. Each resultant value of compressive strength was an average calculated from six tests. 3. Results and discussion 3.1. Basic physical characteristics Fig. 8 shows the water absorption rate and open porosity of hardened cement paste curing for 28 days. With the increase of

L. Li et al. / Construction and Building Materials 227 (2019) 117108

Fig. 5. The flowchart of specimens preparation and characterizations.

Fig. 7. The relationship between the initial porosity and compact pressure.

Fig. 6. The test of flow ability of cement paste; (a) the cement paste with w/c of 0.16; (b) flow ability set-up for cement paste; (c) the slump cone containing cement paste; (d) the cement paste after removed the slump cone.

w/c ratios, the water absorption rate of samples decreased. The 28days water absorption rate of samples decreased by 49.3% to a value of 6.8 wt% with the increase of w/c ratios from 0.08 to 0.16. A same trend could be found for the test data of open porosity, which was for the samples with w/c ratio of 0.16 43.5% lower than that for the samples with w/c ratio of 0.08. This result was contrary to the study of Zˇivica [3], in which the porosity decreased with the decrease of w/c ratios. The possible reason was that the initial porosity of cement paste was determined by the compact pressure [42]. The compact pressure increased with the decrease of w/c ratios in [3], meaning the initial porosity decreased with the decrease of w/c ratios. In this paper, the initial porosity of cement pastes with different w/c ratios were uniform due to the

Fig. 8. Water absorption rate and open porosity.

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values of compact pressure were identical and the amount of hydration products could increase with the w/c ratios. Therefore, the open porosity decreased with the increase of w/c ratios in the current study. In addition, previous researches [34,35] suggested that the relationship between water absorption and porosity of ordinary hardened cement paste was an extremely significant linear correlation. Hence, the relationship between the two properties as described above was established in this particular case, as presented in Fig. 10. The index of correlation between water absorption rate and open porosity up to 0.998, representing that the correlation between two parameters was similar to that of the ordinary hardened cement paste. It also confirmed that decreasing open porosity was the dominating factor of the decrease of water absorption rate in this paper. The results of bulk density and matrix density of 28-days hardened cement pastes are presented in Fig. 9. There is not any substantial effect of w/c ratio on the matrix density of hardened cement paste compacts. The difference of matrix density of all samples was less than 3.1%. On the contrary, the data of bulk density increased widely. With the increase of w/c ratios, the bulk density increased by up to 11.5%. In addition, Fig. 10 also reveals a strong correlation between open porosity and bulk density, and the correlation coefficient between two parameters reached up to

Fig. 9. Bulk density and Matrix density.

0.992. Therefore, the increase of bulk density could be attributed to the decrease of the open porosity. 3.2. Pore structure evolution Fig. 11 shows the pore size distribution of cement powder compact (i.e. Hydration for 0 day) and 1-, 3- and 28-days hardened cement pastes. The most capillary pores of cement powder compact focused in 0.1–8 lm, and the threshold pore diameter was 1.62 lm. In a comparison with the compact pressure of 18.3 MPa in [26], the diameter of capillary pores was below 6 lm and the threshold pore diameter was about 1.35 lm. This difference could be attributed to that the specific surface area of cement in [26] was higher than that in this paper. After 1 day hydration, the capillary pores with diameter of 0.3–8 lm reduced dramatically, and, on the contrary, the capillary pores in the 0.01–0.3 lm range increased largely. Therefore, the pore size distribution curves became bimodal curves (one peak was located at ~ 1.35 lm and another peak was located at ~0.08 lm) from unimodal curves. This result was consistent with the study of Shen et al. [26], in which the bimodal curves was also presented in the test results of pore structure after 1 day hydration of cement. In addition, with the increase of w/c ratios from 0.08 to 0.16, the values of the bimodal curves decreased. At a curing time of 3 days, the capillary pores of diameter more than 1 lm were vanished totally for all samples. The threshold and maximum pore diameter of 3-days samples decreased with the increase of w/c ratios. For instance, the threshold pore diameter of 3-days samples decreased from 0.15 lm to 0.08 lm with the increase of w/c ratios from 0.08 to 0.16. A reverse trend can be found for the threshold and maximum pore diameter of samples curing for 28 days. For ordinary hardened cement paste, reducing the w/c ratio was beneficial for decreasing the capillary pore diameter [36,37]. On the contrary, the diameter of capillary pores decreased with the increase of w/c ratios from 0.08 to 0.16 in this particular case. This was an important discovery of the effect of w/c ratio on the capillary pores diameter in this study. Evolution of cumulative porosity is presented in Fig. 12. The total porosity of hardened cement pastes decreased with the increase of hydration time at the same w/c ratio. This result was contrary to that of ordinary hardened cement paste. For instance, the work of Chen et al. [43] on the pore structure evolution of cement pastes with low w/c ratios suggested that the total porosity and the capillary pore diameter decreased with the decrease of w/c ratios at the same curing time. In addition, the decrease of porosity was mainly due to macropores of diameter more than 1 lm in this paper (Figs. 11 and 13). This can be attributed to the hydration products, especially for C-S-H gels with diameter below 0.1 lm, increased with the increase of curing time. In addition, the total porosities determined by MIP decreased with the increase of w/c ratios from 0.08 to 0.16, which agreed with the results of basic physical properties test by water vacuum saturation method (Fig. 8). For instance, compared to the samples with w/c ratio of 0.08, the 3- and 28-days total porosity of samples with w/c ratio of 0.16 decreased by up to 39.6% and 46.1%. The probably reason was that the amount of hydration products increased with the increase of w/c ratios from 0.08 to 0.16. 3.3. Chemical shrinkage

Fig. 10. Relationship between open porosity and water absorption rate, bulk density.

Chemical shrinkage was the absolute volume reduction in a cement paste, due to the fact that the volume occupied by the hydration products was lower than that of the reactants [46]. In the current study, the chemical shrinkage of cement paste mainly occurred during the early-age hydration (Fig. 14). For instance, the 1-day chemical shrinkage of cement paste with w/c ratio of 0.16 was 0.1178 ml/g cement while its 28-days chemical shrinkage

L. Li et al. / Construction and Building Materials 227 (2019) 117108

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Fig. 11. Pore size distribution.

Fig. 12. Cumulative porosity.

was 0.2617 ml/g cement. The work of Ye [47] on the percolation of capillary pores in hardening cement pastes suggested that continuous pore paths of cement paste with low w/c ratios may become depercolated at early-age hydration due to that hydration products are deposited in the originally water filled space. Therefore, the early-age chemical shrinkage of cement paste played a leading role in the total chemical shrinkage. In addition, the chemical shrinkage of cement paste increased with the increase of w/c ratios at the same hydration time. The 28-days chemical shrinkage of cement paste with w/c ratio of 0.08 was 0.2028 ml/g cement. With a w/c ratio of 0.16, the 28-days chemical shrinkage reached up to 0.2617 ml/g cement, an increase of 22.5% compared with that of

the paste with w/c ratio of 0.08. Hence, decreasing the w/c ratio was beneficial to reduce the chemical shrinkage of cement paste with super low w/c ratios.

3.4. Thermal analysis The non-evaporable water contents of hardened cement pastes are shown in Fig. 15. Thermogravimetric (TG) analysis data of the hydrated OPC and the content of CH are depicted in Fig. 16. The content of non-evaporable water (Fig. 15) and CH (Fig. 16) of specimens increased with the increase of w/c ratios at the same hydra-

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L. Li et al. / Construction and Building Materials 227 (2019) 117108

Fig. 13. Pore volume fraction.

Fig. 14. Chemical shrinkage of cement paste. Each curve is a smoothed average from three samples.

tion time. This indicated the amount of hydration products of hydrated cement paste with the increase of w/c ratios. Due to the initial porosity (i.e. the porosity of cement powder compact) of 36.6% (Fig. 12) was determined by a compact pressure of 25 MPa and the capillary pores can be filled by the hydration products of cement, the diameter of pores (Fig. 11), total porosity

Fig. 16. TG analysis of hydrated OPC.

(Fig. 12), water absorption rate (Fig. 8) decreased with the increase of the hydration products content. However, the volume fraction of capillary pores (Fig. 13) with diameter below 0.1 lm increased with the increase of the hydration products content due to a main hydration product (i.e. C-S-H gel) was porous materials with pore diameter smaller than 0.1 lm. In addition, the change of the 28days matrix density (Fig. 9) could be explained by the experiment data of non-evaporable water content because of the matrix density of products of cement hydration was lower than that of unhydrated cement. 3.5. XRD analysis

Fig. 15. Non-evaporable water content of hydrated OPC.

The mineral compositions of hydrated cement pastes cured for 28 days were presented in Fig. 17 and QXRD analysis results for

L. Li et al. / Construction and Building Materials 227 (2019) 117108

9

Fig. 17. XRD patterns of the cement pastes hydrated for 28 days.

relative phase content of hydrated OPC were listed in Table 3. It could be seen that there was not any substantial effect of w/c ratio on the crystalline mineral compositions of hydrated cement pastes (Fig. 17). The amount of hydration products (e.g. CH, Ettringite, amorphous) increased with the increase of w/c ratios, and the amount of unhydrated mineral (i.e. C3S) decreased with the increase of w/c ratios (Table 3). From the relative content of C3S, CH, Ettringite and amorphous, it could be concluded that the hydration of cement was accelerated by w/c ratios from 0.08 to 0.16, especially the hydration of C3S. This result was consistent with Figs. 11 and 12 in which the diameter of pores and the total porosity decreased with the increase of w/c ratios from 0.08 to 0.16. In addition, this result was also consistent with Figs. 15 and 16 in which the non-evaporable water content and the amount of hydration products increased with the increase of w/c ratios from 0.08 to 0.16.

3.6. Hydration heat evolution Fig. 18 showed the hydration heat evolutions of OPC with super low w/c ratios. Compared with the cement paste with a conventional w/c ratio, there was not an obvious shoulder peak in the hydration process of PC with w/c ratios from 0.08 to 0.16. The work by Bullard et al. [41] on the mechanisms of cement hydration indicated that the 1st shoulder peak was caused by the renewed dissolution of tricalcium aluminate (C3A; C = CaO, A = Al2O3) with formation of ettringite (AFt; C3A3C$H32, $ = SO3, H = H2O) and the 2nd shoulder peak was caused by the formation of calcium aluminate sulfate hydrate (AFm; C3AC$H12), as shown in Fig. 19. The probable reasons were that the decreasing of w/c ratios limited the hydration of C3A and the formation of AFt, and the critical super saturation of AFt occurred more easily in the cement paste with lower w/c ratios resulting in the formation of AFt in advance. In addition, the cumulative heat of hydration of PC with a w/c ratio of 0.08 for 3 days was only 60.01 J/g. With a w/c ratio of 0.16, the

cumulative heat increased by 142.2% to 145.34 J/g at the same time. It is evident that the cumulative heat of hydration of PC increased with increasing w/c ratios at the same hydration time, representing the hydration degree of PC and the amount of hydration products increased with increasing w/c ratios. 3.7. Mechanical property The compressive strengths of hardened cement pastes are presented in Fig. 20. The compressive strength increased with the increase of w/c ratios at the same hydration time. The 1- and 28days compressive strength of samples with w/c ratio of 0.16 increased by 329% and 68.9% compared to that of samples with w/c ratio of 0.08. In general, the compressive strength decreased with the increase of w/c ratios for ordinary concrete [44]. This was mainly due to that the compressive strength of hardened cement paste was determined by the volume of pores (e.g. capillary pores and gel pores) in the hardened cement paste [38,39], and the total porosity decreased with the increase of w/c ratios in this paper (Fig. 12). For the relationship between compressive strength and total porosity of hardened cement paste, there were three widely accepted functions: Balshin function, Ryshkewitch function and Schiller function [38,39]. The functions as described above were as follows: Balshin function

SC ¼ S0  ð1  PÞn Ryshkewitch function

SC ¼ S0  expðbPÞ Schiller function

SC ¼ c  lnðP0 =PÞ where SC is compressive strength of hardened cement paste with total porosity P; S0 is compressive strength of hardened cement paste with total porosity of 0%; n is a coefficient; b and c are empir-

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Table 3 QXRD data of hydration products of OPC. w/c

C3S

C2S

CH

Ettringite

Amorphous

Others

0.08 0.12 0.16

35.92 33.05 29.43

11.79 11.84 11.89

1.58 1.72 2.01

2.00 2.08 2.19

40.71 46.13 49.55

8.00 5.18 4.93

Fig. 20. Compressive strength of hardened cement paste. Fig. 18. Hydration heat evolution of OPC with super low w/c.

Balshin function, Ryshkewitch function and Schiller function. According to expressions of the functions, the total porosity was an important factor that determined the compressive strength of hardened cement paste with low w/c ratios. This result was consistent with the study of Li et al. [45], in which the relationship between compressive strength and total porosity also fitted Balshin function and the compressive strength of hardened cement paste was determined by the total porosity. Therefore, it also explained that the compressive strength was consistent with the Fig. 12 in which the total porosity decreased with the increase of w/c ratios from 0.08 to 0.16. 4. Conclusions

Fig. 19. Hydration heat flow of OPC with w/c ratio of 0.40, showing typical shoulder peak where a secondary formation of ettringite occurs and subsequent broad peak corresponding to the formation of AFm phase.

ical constants; P0 is total porosity of hardened paste with compressive strength of 0 MPa. In this paper, the relationship between compressive strength and total porosity was established according to Balshin function, Ryshkewitch function and Schiller function, and the results were shown in Fig. 21. Furthermore, the accuracy of models was estimated by statistical analysis, and the data of prediction models were presented in Table 4. From Fig. 21 it can be seen that all the correlation coefficients between experiment data and models were no less than 0.96 (i.e. R2  0.92) (Fig. 21(a), (c) and (e)), and the normabilities corresponding to models were no less than 0.97 (i.e. R2  0.95) (Fig. 21(b), (d) and (f)). In addition, the internally studentized residuals of prediction models were in the range of from-2 to 2 (Table 4). It proved the relationship between compressive strength and total porosity could be established by

In the current study, ice particles was adopted as mixing water to prepare the homogenous structural cement pastes with super low water-to-cement (w/c) ratios. From the basis of multiple perspectives, the effects of w/c ratios on basic physical properties, pore structure, non-evaporable water content, chemical shrinkage, hydration heat evolution and strength development of hardened cement pastes were investigated. The main conclusions can be summarized as following: (1) The water absorption rate and open porosity of hardened cement paste compacts curing for 28 days decreased with the increase of w/c. The 28-days water absorption rate decreased by 49.3% while its 28-days open porosity decreased by 43.5% with the increase of w/c from 0.08 to 0.16. On the contrary, the bulk density increased with the increase of w/c. (2) The total porosity of unhydrated cement paste reached up to 36.6%, and the pores were mainly capillary pores of diameter in range of 0.1–8 lm. After 3 days hydration, the capillary pores with diameter more than 1 lm were vanished totally. Furthermore, the threshold and maximum pore diameter and the total porosity of hardened cement pastes decreased with the increase of w/c ratios at the same curing time.

L. Li et al. / Construction and Building Materials 227 (2019) 117108

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Fig. 21. The relationship between compressive strength and total porosity. (a) Fitting by Balshin function; (b) Normability correspond with Balshin function; (c) Fitting by Ryshkewitch function; (b) Normability correspond with Ryshkewitch function; (e) Fitting by Schiller function; (f) Normability correspond with Schiller function.

(3) The non-evaporable water content, the amount of hydration products and the cumulative heat of hydrated cement paste increased with the increase of w/c ratios at the same hydration time, representing the hydration degree of the paste increased with the increase of w/c ratios.

(4) The compressive strength of hardened cement paste increased with the increase of w/c at the same curing time. The 28-days compressive strength of samples with w/c of 0.16 increased by 68.9% to a value of 127.5 MPa compared to samples with w/c of 0.08. Furthermore, the relationship

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L. Li et al. / Construction and Building Materials 227 (2019) 117108

Table 4 Statistical analysis results of prediction functions of compressive strength. Function

Actual value

Predicted value

Residual

Internally studentized residuals

Balshin function

15.4 41.7 73.5 40.1 74.5 103.2 66.1 103.4 127.5

34.6 54.3 82.1 46.8 76.6 103.6 54.3 87.9 110.3

19.2 12.6 8.6 6.7 2.1 0.4 11.8 15.8 17.2

1.496 0.983 0.669 0.521 0.164 0.030 0.916 1.203 1.334

Ryshkewitch function

15.4 41.7 73.5 40.1 74.5 103.2 66.1 103.4 127.5

34.6 43.8 58.0 48.7 62.7 86.0 78.7 103.8 134.2

19.2 2.1 15.5 8.6 11.8 17.2 12.6 0.4 6.7

1.496 0.164 1.203 0.669 0.916 1.334 0.983 0.030 0.521

Schiller function

15.4 41.7 73.5 40.1 74.5 103.2 66.1 103.4 127.5

29.3 55.3 81.5 47.1 76.0 102.7 54.5 87.6 111.4

13.9 13.6 8.0 7.0 1.5 0.5 11.6 15.8 16.1

1.169 1.142 0.671 0.584 0.127 0.045 0.976 1.325 1.347

between compressive strength and total porosity of hardened cement paste proved that the total porosity was the important factor that determined the compressive strength of hardened cement paste with w/c from 0.08 to 0.16.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research work is financially supported from the National Key Point Research and Invention Program of the Thirteenth through the grants of NO.2016YFC0701000, and NSFC-Shandong Joint Fund Key Project through the grants of U1806222. References [1] S. Marinkovic´, J. Dragaš, I. Ignjatovic´, et al., Environmental assessment of green concretes for structural use, J. Clean. Prod. 154 (2017) 633–649, https://doi. org/10.1016/j.jclepro.2017.04.015. [2] M.A. DeRousseau, J.R. Kasprzyk, W.V. Srubar, Computational design optimization of concrete mixtures: a review, Cem. Concr. Res. 109 (2018) 42–53, https://doi.org/10.1016/j.cemconres.2018.04.007. [3] V. Zˇivica, Effects of the very low water/cement ratio, Constr. Build. Mater. 23 (2009) 3579–3582, https://doi.org/10.1016/j.conbuildmat.2009.03.014. [4] G.A. Rao, Role of water-binder ratio on the strength development in mortars incorporated with silica fume, Cem. Concr. Res. 31 (2001) 443–447, https://doi. org/10.1016/S0008-8846(00)00500-7. [5] C. Freidin, Cementless pressed blocks from waste products of coal-firing powerstation, Cem. Concr. Res. 21 (2007) 12–18, https://doi.org/10.1016/ j.conbuildmat.2005.08.002. [6] V. Zˇivica, M.T. Palou, M. Krizma, et al., Acidic attack of cement based materials under the common action of high, ambient temperature and pressure, Constr. Build. Mater. 36 (2012) 623–629, https://doi.org/10.1016/ j.conbuildmat.2012.04.025. [7] S.B. Singh, P. Munjal, N. Thammishetti, Role of water/cement ratio on strength development of cement mortar, J. Build. Mater. 4 (2015) 94–100, https://doi. org/10.1016/j.jobe.2015.09.003.

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