Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 172 (2017) 165 – 171 Modern Building Materials, Structures and Techniques, MBMST 2016 Production of high-performance concrete by addition of fly ash microsphere and condensed silica fume J.J Chena, P.L Ngb,c*, L.G Lid, A.K.H Kwanc a Foshan University, Foshan, China Vilnius Gediminas Technical University, Vilnius, Lithuania c Department of Civil Engineering, The University of Hong Kong, Hong Kong, China d Guangdong University of Technology, Guangzhou, China b Abstract With reference to the packing model of concrete materials, addition of fly ash microspheres (FAM) to fill the voids between cement grains, followed by addition of condensed silica fume (CSF) to further fill the voids between FAM would reduce the water content to achieve the desired flowability This could allow the adoption of lower water/cementitious materials (W/CM) ratio to produce High-Performance Concrete (HPC) This study was aimed to evaluate the effects of FAM and CSF on the packing density of cementitious materials and the flowability and strength of cement paste The results showed that the addition of FAM and CSF can significantly increase the packing density, thereby enhancing flowability and strength performance concurrently ©©2017 Authors Published by Elsevier Ltd This 2016The The Authors Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of MBMST 2016 Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: Condensed silica fume; fly ash microsphere; high-performance concrete Introduction Because of the conflict between demand for high strength and demand for high flowability, the development of HPC has encountered a bottleneck In this regard, increasing the packing density of the cementitious materials should provide room for further advancement of HPC This is because the filling water (the water filled into the * Corresponding author Tel.: +852-28598024 E-mail address: irdngpl@gmail.com 1877-7058 © 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of MBMST 2016 doi:10.1016/j.proeng.2017.02.045 166 J.J Chen et al / Procedia Engineering 172 (2017) 165 – 171 voids between the solid particles) is not effective in providing flowability and it is the excess water (the water in excess of that needed to fill the voids) that forms water films coating the solid particles to provide flowability [1] At the same W/CM ratio, the flowability may be improved by increasing the amount of excess water, or alternatively, at the same flowability requirement, the W/CM ratio could be lowered to improve strength [2] Literatures indicate that adding a supplementary cementitious material (SCM) finer than cement to fill into the voids is effective in increasing the packing density of the cementitious materials Long et al [3], Yahia et al [4] and Diederich et al [5] pointed out that the use of fillers in cement mortar could increase the packing density and liberate part of the mixing water otherwise entrapped in the voids to improve the flowability Olhero and Ferreira [6] and Kwan and Fung [7] demonstrated that the addition of CSF could improve the flowability of mortar through increasing the packing density Zhang et al [8-9] proposed a wide and gap-graded particle size distribution based on the close packing theory and showed that such particle size distribution would increase the packing density and reduce the water requirement Although many studies on the addition of one SCM (i.e., double blending cement with one SCM) were reported, there have been relatively few studies on the addition of two SCMs (i.e., triple blending cement with two SCMs) In theory, the addition of a SCM finer than cement to fill into the voids between the cement grains and the addition of another even finer SCM to fill into the voids between the larger particles should be able to reduce the voids to a greater extent than possible with the addition of just one SCM To validate the above postulation, a comprehensive research programme was carried out Fly ash microsphere (FAM), which is a superfine fly ash captured from the exhaust smoke of coal-fired power station and generally has a mean particle size of several micrometres, and CSF, which is even finer, were added to ordinary Portland cement (OPC) to form triple-blended cementitious materials so as to evaluate the benefits of such triple blending Materials A strength grade 52.5N OPC, FAM and CSF had been tested to verify their compliance with British Standard BS EN 197-1: 2000, Chinese Standard GB/T 1596-2005 and American Standard ASTM C 1240-15, respectively Their particle size distributions were measured by a laser diffraction particle size analyzer and the results obtained are plotted in Fig A polycarboxylate ether-based superplasticizer (SP) in aqueous solution state was used to disperse the cementitious materials As the SP is a surface reactant and it is the SP dosage per solid surface area that governs its effectiveness, the SP dosage was set as a saturation dosage of 26×10-6 kg/m2 for all the cement paste samples CSF FAM OPC Fig Particle size distributions of OPC, FAM and CSF 167 J.J Chen et al / Procedia Engineering 172 (2017) 165 – 171 Experimental Programme The flow spread and cube strength of cement paste samples mixed with different FAM, CSF and water contents were measured In order to study the effects of FAM and CSF on packing density, the packing densities of cementitious materials having different FAM and CSF contents were also measured It should be noted that the FAM and CSF contents are expressed in volumetric ratio of total cementitious materials From the packing density results obtained, the water film thickness (WFT) of the cement paste samples were evaluated as the excess water to solid surface area ratio, and then the roles of WFT in the flowability and strength of cement paste were studied In this study, totally combinations of cementitious materials were adopted, as shown in Table A total of 88 cement paste samples were produced for testing The W/CM ratio by mass was varied from 0.14 or 0.16 to 0.30 in increments of 0.02 (the lower limit of W/CM ratio was set such that the water content was still enough for homogenous mixing of the cement paste) Each cement paste sample was of 1.550 litres size of batch It was produced by mixing the designed mix proportions of OPC, FAM, CSF, SP and water together using a standard mixer complying with BS EN 196: Parts 1-3 Table Cementitious materials combination and packing density results Mix no OPC content (%) 100 80 60 90 70 50 80 60 40 FAM content (%) 20 40 20 40 20 40 CSF content (%) 0 10 10 10 20 20 20 Packing density 0.641 0.703 0.755 0.696 0.743 0.778 0.727 0.768 0.767 Test Methods 4.1 Determination of packing density and WFT The wet packing method [1,10] was used to measure the packing density of the cementitious materials In this method, solid particles are mixed with water at different water/solid ratios to determine the voids ratio and solid concentration of each resulting mixture by measuring its apparent density The maximum solid concentration is taken as the packing density of the solid particles The WFT is calculated according to previous study [2] It has the physical meaning of being the average thickness of the water films coating the solid particles and is the single most important parameter governing the rheology of cement paste [11,12] 4.2 Measurement of flow spread The mini slump cone test [13] was used to measure the flow spread of the cement paste samples for evaluation of workability The mini slump cone has a base diameter of 100 mm, a top diameter of 70 mm and a height of 60 mm After filling the slump cone with the paste sample, the slump cone was lifted vertically upwards to allow the paste to slump downwards The flow spread was measured as the average of two perpendicular diameters of the patty 4.3 Measurement of cube strength Three 70.7 mm cubes were made from each cement paste sample for strength measurement All the cubes were stored in the laboratory at a temperature of 24 ± 2°C until demoulding at 24 hours, and then put into a lime-saturated water curing tank controlled at a temperature of 27 ± 2°C until the age of 28 days for undergoing the cube compression test The cube strength was reported as the average of compression test results of the three cubes 168 J.J Chen et al / Procedia Engineering 172 (2017) 165 – 171 Test Results 5.1 Packing density The packing density result is tabulated in the last column of Table Results demonstrate that CSF is more effective than FAM in increasing the packing density because it is finer and can fill into the voids between cement grains more readily without loosening the packing of the cement grains Triple blending of OPC with FAM and CSF further improved the packing density of the cementitious materials This is because the FAM particles can fill into the voids between cement grains while the CSF particles can fill into the voids between FAM particles to successively fill up the voids for maximizing the packing density It should however be noted that the packing density does not always increase with the FAM and CSF contents A probable reason is that when FAM or CSF is added to beyond its respective optimum content, the FAM or CSF would be more than enough to fill the voids and its further addition would loosen the packing of the larger solid particles 5.2 Flow spread The flow spread results are plotted against the W/CM ratio in Fig Generally speaking, mixes with higher W/CM ratios would give rise to larger flow spread values Comparing the flow spread-W/CM ratio curves, it can be seen that the effect of adding FAM and/or CSF is dependent on the W/CM ratio Basically, the addition of FAM would greatly increase the flow spread at low W/CM but only slightly increase the flow spread at high W/CM In contrast, the addition of CSF would increase the flow spread at low W/CM but decrease the flow spread at high W/CM The flow spread results are plotted against the WFT in Fig In all cases, the paste did not flow (flow spread = 0) when the WFT was negative or very small, and started to flow at a WFT of around to 0.03 Pm, which agrees closely with the value of 0.025 Pm reported in the literature [14,15] Generally, at the same WFT, the flow spread was higher at a higher FAM content and marginally lower at a higher CSF content This may be attributed to the fact that the FAM particles, being finer than the cement grains, can squeeze themselves into the voids between cement grains and thus push the angular cement grains apart to reduce their inter-particle friction Also, the FAM particles themselves, having highly spherical shape, can roll like ball bearings to improve the flowability of the cement paste Multi-variable regression analysis has been carried out to correlate the experimental data in Fig 3, where the symbols y denotes the flow spread (mm), x1 denotes the WFT (μm), x2 denotes the FAM content (%), and x3 denotes the CSF content (%) A very high R2 value of 0.932 has been achieved, indicating that the flow spread is closely related to the WFT, FAM content and CSF content 5.3 Cube strength The 28-day cube strength results are plotted against the W/CM ratio in Fig The optimum W/CM ratio for maximum strength was lower at higher FAM and CSF contents Hence, the addition of FAM and/or CSF would allow a lower W/CM ratio to be adopted to achieve a higher strength The highest strength attained was 159 MPa for the mix with FAM content of 0% and CSF content of 20% and at a W/CM ratio of 0.16 169 J.J Chen et al / Procedia Engineering 172 (2017) 165 – 171 400 Flow spread (mm) 300 200 FAM= 0%, CSF= 0% FAM=20%, CSF= 0% FAM=40%, CSF= 0% FAM= 0%, CSF=10% FAM=20%, CSF=10% FAM=40%, CSF=10% FAM= 0%, CSF=20% FAM=20%, CSF=20% FAM=40%, CSF=20% 100 0,10 0,15 0,20 0,25 0,30 0,35 W/CM ratio Fig Flow spread versus W/CM ratio Flow spread (mm) 400 300 200 y = A + BeCx A = 402-0.627x2-0.234x3 B = -399+0.225x2-1.149x3 C = -5.412-0.212x2-0.009x3 R2 = 0.932 100 FAM= 0%, CSF= 0% FAM=20%, CSF= 0% FAM=40%, CSF= 0% FAM= 0%, CSF=10% FAM=20%, CSF=10% FAM=40%, CSF=10% FAM= 0%, CSF=20% FAM=20%, CSF=20% FAM=40%, CSF=20% 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 WFT (Pm) Fig Flow spread versus and WFT Concurrent Strength-Flowability Performance The effects of FAM and CSF contents on the concurrent strength-flowability performance are illustrated by plotting the 28-day cube strength against the flow spread in Fig Each performance curve in the figure represents the strength and flowability that could be concurrently achieved by the cement paste at given FAM and CSF contents Comparing the performance curves for different FAM and CSF contents, it is evident that the performance curve would be shifted to the right when FAM is added and shifted upwards when CSF is added Such shifting of 170 J.J Chen et al / Procedia Engineering 172 (2017) 165 – 171 the curves indicates that the addition of FAM would increase the flowability at the same strength whereas the addition of CSF would increase the strength at the same flowability 28-day cube strength (MPa) 180 150 120 90 60 FAM= 0%, CSF= 0% FAM=40%, CSF= 0% FAM=20%, CSF=10% FAM= 0%, CSF=20% FAM=40%, CSF=20% 30 FAM=20%, CSF= 0% FAM= 0%, CSF=10% FAM=40%, CSF=10% FAM=20%, CSF=20% 0,05 0,10 0,15 0,20 W/CM ratio 0,25 0,30 0,35 Fig 28-day cube strength versus W/CM 180 28-day cube strength (MPa) 150 120 90 60 FAC= 0%, CSF= 0% FAC=40%, CSF= 0% FAC=20%, CSF=10% FAC= 0%, CSF=20% FAC=40%, CSF=20% 30 FAC=20%, CSF= 0% FAC= 0%, CSF=10% FAC=40%, CSF=10% FAC=20%, CSF=20% 0 100 200 Flow spread (mm) 300 400 Fig Concurrent strength and flowability performance According to the results shown in Fig 5, different strategies of adding FAM and/or CSF should be adopted for the production of different types of concrete To produce high-flowability concrete, double blending with FAM should be adopted To produce high-strength and high-flowability concrete, double blending with either FAM or CSF should be adopted To produce ultra-high-strength concrete, double blending with CSF or triple blending with both FAM and CSF should be adopted At this juncture, it is noteworthy that in contrast to double blending, part of J.J Chen et al / Procedia Engineering 172 (2017) 165 – 171 171 the CSF may be replaced by FAM in triple blending to reduce the cost of production while maintaining a similar strength-flowability performance (note that the performance curve at 20% FAM and 10% CSF and the performance curve at 0% FAM and 20% CSF are very close to each other) Conclusions From the experimental investigations presented in this paper, the following conclusions can be drawn: (1) Double blending of OPC with either FAM or CSF, and triple blending of OPC with both FAM and CSF can significantly increase the packing density Relatively, the finer CSF is more effective in increasing the packing density (2) The addition of FAM and/or CSF can more substantially increase the flow spread at low W/CM ratio than at high W/CM ratio Correlations of the flow spread to the WFT yielded very high R2 values of well above 0.9, indicating that the WFT principally governs the flowability of cement paste On the other hand, at the same WFT, the flow spread is higher at higher FAM content and marginally lower at higher CSF content (3) The optimum W/CM ratio for maximum strength is generally lower at higher FAM and CSF contents Hence the addition of FAM and/or CSF would allow the W/CM ratio to be lowered in order to increase the strength (4) From plots of concurrently achieved strength and flowability, it can be seen that the addition of FAM would 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