Cementitious Composites
2. Materials and Methods 1. Raw Materials
The cement used for this research is PO (ordinary portland cement) 42.5 cement, made by Huaxin Cement Co., Ltd. (Huangshi, China), with a relative density of 3141 kg/m3(refer to Tables1and2for chemical analysis and technical specifications). The fly ash used was first-grade ash made by China Railway Major Bridge Engineering Group Co., Ltd. Ninth Company (Nanjing, China) where d(0.5) is 11.6, d(0.9) is 43.5 (refer to Table1for detailed chemical elements, d is diameter). The fine aggregate used was quartz sand, with a fineness of 100 mesh, which means that the diameter per particle was 0.15 mm. A highly effective water reducer called polycarboxylate superplasticizer (made by Wuhan Geruilin Building Materials Technology Co., Ltd., Wuhan, China) was used. The mother liquor is white transparent thick liquid, and the solid content is 40%, which is diluted to 20% for practical use.
Table 1.Chemical analysis of cement and fly ash.
Oxide SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 Loss
Cement 21.6 5.85 2.84 59.80 2.24 0.67 0.21 2.06 3.70
Fly ash 46.43 38.02 3.11 7.51 0.23 0.89 0.34 0.68 2.78
Table 2.Technical specifications of cement.
Stability Setting Time (min) Flexural Strength (MPa) Compressive Strength (MPa) Specific Surface Area (m2/kg)
Initial Final 3d 28d 3d 28d
Qualified 193 273 4.3 6.7 22.6 43.4 342
The polyvinyl alcohol fibers (PVA fibers) used for the study encapsulated two types, and respectively, they are high-strength high elastic modulus PVA fibers (fabricated by Sinopec Chongqing SVW (Sichuan Vinylon Works) Chemical Co., Ltd., Chongqing, China), and REC 15 model PVA fibers (fabricated by Kuraray Co., Ltd., Tokyo, Japan). They were chopped to 12 mm on average (refer to Table3for concrete performance specifications).
Table 3.Performance indexes of PVA fiber.
Type Diameter
(μm) Aspect
Ratio
Tensile Strength (MPa)
Tensile Elastic Modulus (GPa)
Ultimate Elongation (%)
Domestic PVA 31 387 1400 35 6.5
REC15 40 300 1560 42 7.0
The vinyl acetate emulsion (hereinafter named ‘VAE emulsion’) that was used for the study is the new model VAE CW40-905 (fabricated by Sinopec Chongqing SVW Chemical Co., Ltd., refer to Table4 for detailed performance specifications).
Table 4.Performance indexes of VAE.
Solid Content (%)
Viscosity
(mPa·s) PH Ethylene
Content (%)
Dilution
Stability MFFT (◦C)
54.4 1500–2500 4.0–6.5 ≤5.0 ≤3.5 ≤-3
(MFFT: Minimum film forming temperature).
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The hydroxyl propyl methyl cellulose (HPMC) used for the study is HK-type, including 50,000 and 150,000 viscosity (refer to Table5for technical specifications). Note that, due to the viscosity of the HPMC being high, and its quantity of use being low, the dried powder does not easily scatter evenly during the process of stirring. Therefore, the HPMC was made into a 1% solution in advance, and the impact of water was omitted in its volume of water content.
Table 5.Performance indexes of HMPC.
Nominal Viscosity (mPa·s×104)
Methoxyl Content (%)
HydroxyPropyl Content (%)
Water Content
(wt%)
Ash Content (wt%)
Density (g/cm3)
Fineness (Mesh)
5/15 17.0–24.0 4.0–12.0 ≤5.0 ≤1.0 1.26–1.31 >80
The boric anhydride used for the study is made by Sinopharm Chemical Reagent Co., Ltd.
(Beijing, China). Therein, the purities are as follows: sulfate (SO4)≤0.02%, heavy metal (calculated by Pb)≤0.005%, silicon and alkali metal content (calculated by SO4)≤0.1%, and purity≥98.0%.
The butylbenzene emulsion was supplied by the Sinopharm Chemical Reagent Co., Ltd. (Beijing, China), model no. JNYS-BS-8825GH.
2.2. Specimen Preparation 2.2.1. Ratio of the Mixture
The fundamental proportions of the engineered cementitious composites included cement, fly ash, and quartz sand. Per 500 g, the water–binder ratio that was adopted was either 0.3 or 0.35, and a 2% (2% fiber mixing amount can ensure ECC has a tensile strain of 3% to 7%. When the fiber mixing amount exceeds 2%, although the tensile properties of the material increase with the increase of the mixing amount, it cannot guarantee sufficient workability. In addition, it may result in fiber agglomeration and uneven dispersion [27].) The volume fraction of PVA fibers was added as well as a small quantity of HPMC density-increasing stabilizer and a highly effective polycarboxylate superplasticizer. Standard maintenance of the sample was adopted. The HPMC and the fraction of the water reducer were adjusted, accordingly, as in the results and discussion, so that cementitious composites with good fluidity could be obtained.
2.2.2. Pre-Processing of the Fibers
The method of pre-processing the fibers was as follows:
a. VAE emulsion: Water was added to 50 g VAE emulsion to form a 400 mL volume of diluted solution in a 1 L beaker. The solution was stirred evenly, and an ultra-sonicator (TOSO25-52, Nanjing Toso Ultrasonic Cleaning Machine Co., Ltd, Nanjing, China) was applied to scatter the particles. Pre-calculated and pre-weighed PVA fibers was added and soaked under constant stirring, with the simultaneous application of the ultra-sonicator for 10 min. Afterwards, the fibers were removed and placed on a mesh to be baked at low temperature until it was half-dry.
Water was used to wash away the extra emulsion and scattered particles, and the fibers were re-baked for use.
b. Cement and boric anhydride: A total of 5 g boric anhydride was made up to a 300 g solution in water, stirred evenly with 100 g cement, and formed into a very thin cement slurry.
A pre-calculated and pre-weighed amount of PVA fiber was added and evenly mixed. Then it was removed and placed on a mesh to be baked at a low temperature for use.
c. Butyl-benzene emulsion and boric anhydride: A total of 5 g boric anhydride was made up to a 350 g solution with water, and 50 g butyl-benzene emulsion was added with mixing. The subsequent processing method was similar to that of Step a. for processing the VAE emulsion. The derived pre-processed fibers were then ready for use.
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2.2.3. Mixing Process
The method of the mixing process was as follows:
A. The preparation of the mortar without fibers. (According to the ASTM C305–14 [28]).
B. Add the fibers or modification fibers.
C. Repeat Step A and cast it in two layers.
D. Strip the mold after 24 to 48 hours and then put them in the standard curing condition.
2.3. Experimental Methods
2.3.1. Experiment on Rheological Properties
The experiments on rheological properties used the slurry without the fibers added. The study adopted a rotor-type viscosity meter to test the rheological properties of the cementitious composite slurry. An R/S-SST model of a soft-solid rheological property testing instrument (manufactured by Brookfield Co., Ltd., New York, NY, USA) was used, and it was paired with a V 80-40 propeller rotor.
The scope of the shearing stress tested was 6–200 Pa, and the shearing velocity could be of an infinitely variable speed within 0–1000 rpm. The time of the shearing velocity was set as:
a) Under 200 s−1constant shearing velocity, with the test conducted on changes in the apparent viscosity at 0 min, 30 min, and 120 min after the slurry was made. The time for each test was 60 s.
b) Within 180 s, the rotor’s shearing velocity was increased constantly from 0 s−1to 200 s−1, and the test changes of the apparent viscosity, along with the shearing velocity of different slurries at 1 min, 31 min, and 121 min, and the yield viscosity of the slurry, were calculated according to the Rheo V2.8 program (version 2.8, Brookfield Co., Ltd., New York, NY, USA).
c) Within 180 s, the rotor’s shearing velocity was increased constantly from 0 s−1to 200 s−1. Afterwards, in the same period of time, the rotor’s shearing velocity was evenly decreased from 200 s−1to 0 s−1.
The relationship of the shearing stress, along with the shearing velocities of different slurries at 1 min, 31 min, and 121 min, and the thixotropies of different groups of slurries, were judged as per the area outlined by the curve [29].
2.3.2. Test on the Strength of Pressure Resistance and Fracture Resistance
According to the Testing Methods of Strength of Cement Mortar (ISO Methods) [30], an electrical hydraulic universal testing machine (model: WDW 10, supplied by Wuxi Jianyi experiment instrument Co., Ltd., Wuxi, China) was applied to make a sample of mortar and test the strengths of pressure resistance and fracture resistance.
2.3.3. Test on Tensile Performance
The equipment used for the tensile performance test was an Instron 5882 electrical universal material testing machine (Instron, Boston, MA, USA). The test was implemented in an environment of 25◦C at 75% humidity using self-made tensile clamps (Figure1) as per the design and construction recommendations for HPFRCC materials, set out by JSCE (Japan Society of Civil Engineers) [31].
The tensile loading device is shown in Figure2, with a pair of clamps at the top and the bottom.
After the sample for tensile testing was ready, aluminum LVDT (linear variable differential transformer) clamps were configured and fixed securely at the tensile area of the sample. The test loaded the tensile test sample through the shift control method, and adopted a 0.4 mm/min loading velocity. Shift sensors (model: SCAH series, Abek Sensors Co., Ltd., Beijing, China) were located at the two sides of the sample test data of tensile stress, and transmitted information to the Keithley 2700 data collection apparatus (Keithley 2700, Keithley Instruments, Beaverton, OR, USA). The data for tensile stress and
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stress were collected per second. When processing the data, the average value was used for the data, as derived from the shift sensors at two channels.
(a) (b)
Figure 1.The specimen and device of the tensile test. (a) Tensile specimen; (b) Diagram of the tensile loading device
2.3.4. Test on Micro-Performance
A field emission scanning electron microscope (Quanta 450 FEG, manufactured by FEI, Hillsboro, OR, USA) was used for the observation and analysis of the scanning electron microscopy (SEM) shape and its appearance in the study. Resolution capability:≤3 nm@1 kV, largest pixel: 6144×4096, range of magnification: 10–10,000,000 times. SEM was used for viewing the shape and appearance of the hydrated products of the cementitious composites, the shape and appearance of the transitional area at the interface of the cementitious composites and the fibers, and the shape and appearance of sections after the fibers were pulled [32].