- Email: [email protected]
Porosity of green concrete based on a gap-graded blend
Proc. Int. Symp. ,,Brittle Matrix Composites JO" A.MBrandt, J.Olek, MA.Glinicki, C.K.Y.Leung, eds. Warsaw, October 15-17, 2012 IFTR and Woodhead Puhl., Warsaw 2012
POROSITY OF GREEN CONCRETE BASED ON A GAP-GRADED BLEND
Nghi L.B. LE and Piet STROEVEN Faculty of Civil Engineering and Geosciences, Delft University of Technology Stevinweg 1, 2628 CN Delft, the Netherlands, e-mail: [email protected]
ABSTRACT Portland cement (PC) production contributes by about 6% to global emissions of C02. Reduction of the PC content in the binder exerts therefore a direct positive effect on such emissions. Partial replacement of the Portland cement by pozzolanic mineral admixtures has been proven a possible option. The green character is even reinforced by making use of incinerated vegetable waste, such as rice husk ash (RHA). Moreover, as a result of the gap-grading effect on particle packing density with RHA as the fine component, high strength concrete can be achieved with RHA-blended cement. This has been published earlier, so the paper only briefly covers these aspects. Characteristics of the capillary pores developed in the hydrating binder have direct impact on the transport-based durability properties. However, assessment of such pore characteristics of the RHA-blended cement constitutes a complicated problem, especially in experimental approaches. This paper, therefore, presents a new economic approach to conduct such investigation on the realistically produced cementitious materials in virtual reality. The fresh packed state of the paste's particles is simulated by a discrete element modeling (DEM) system, HADES. The hydration of a (blended) cement is simulated by a so-called 'extended integrated particle kinetics model' (XIPKM). Pore characteristics, such as topology, location distribution, degree of percolation and fraction of main channels in the pore network, are explored by a new modern approach named 'double random multi-tree structuring' (DraMuTS). The size distribution of throats that significantly limit the transport in the pore network system is derived from star volume measurements. Four examples of plain PC and of RHAblended PC with two wlb ratios are presented. The expected positive effects on transport-based durability issues due to RHA-blending are discussed.
Keywords
(blended) cement, rice husk ash, gap-grading, virtual cementitious materials, multi-phase hydration simulation, porosimetry, pore size distribution, main pore channels.
INTRODUCTION
One of the obvious contributions to reducing detrimental effects of Portland cement production on global warming as a result of C0 2 emissions is to reduce PC content significantly. Use of an admixture of vegetable origin such as RHA will additionally contribute to waste management and by its incineration energy to energy conservation. Experiments have demonstrated the profitable effects of blending Portland cement with a pozzolanic admixture that is significantly finer (Bui [1]). Indeed, similar to aggregate packing, the blended binder can therefore make profitable use of the gap-grading principle. Major evidence is also coming from tests where blending material was inert (Goldman and Bentur [2]). Basically, optimization of such mixtures has been considered in terms of compressive strength (Bui [1] and Vu [3]).
316
Nghi L.B. LE and Piet STROEVEN
Assessment of transport-based durability properties, which are intimately connected to pore network characteristics in materials, would be another issue of engineering significance. Investigating pore systems (porosimetry) is not a simple task, however. Experimental methods are unfortunately laborious, time-consuming and thus expensive. Additionally, they become more complicated in case of blending the binder. Fortunately, with the fast development of computer facilities, the use of virtual materials becomes a more economic way to conduct research on materials. Such new numerical porosimetry methods should therefore be developed. A robotics-inspired line of thinking is elaborated in Stroeven et al. [4]. In this paper, the hydration ofa cement paste in which the PC is blended by a pozzolanic admixture is discussed. Its microstructure is simulated by a new modem hydration model denoted 'extended integrated particle kinetics model' (XIPKM), in which two main phases of the Portland cement, i.e., tricalcium silicate (C 3S) and dicalcium silicate (C 2 S) as well as two phases of the RHA, i.e., silica (Si0 2) and inert are taken into consideration. Fresh binder particles and their packing structure are simulated by a DEM system, HADES, which is able incorporating arbitrary grain shapes and taking into account mechanical contacts among particles as well as the influence of external forces. Thereupon, the pore network is explored and its properties investigated by a new modem approach, 'double random multi tree structuring' (DRaMuTS) inspired from the rapidly-exploring random tree (RRT) approach in robotics.
GAP-GRADED BLENDING EFFECTS
The RHA is produced in the traditional way from Vietnamese rice husks and grinded until its internal porous structure collapsed, significantly reducing water demand Bui [l]. Mean . particle size was 5 µm (Bui et al. [5]). For additional details, see Bui et al. [5] and Stroeven et al. [6]. 70% crushed basalt and 30% of fine sand of fluvial origin constituted a gap-graded aggregate mixture. 500 to 550 kg/m3 Portland cement of two qualities were used. Three water/binder ratios (w/b) were investigated and replacement percentages of 10, 20 and 30 were envisaged. Naphthalene-based superplasticizer additions were employed to get cohesive mixtures with high slump values. lOOmm cubes were used for compressive strength testing at different stages of maturation. Detailed test results have been published in Bui [1] and Bui et al. [5]. However, Fig. l reveals the blending efficiency resulting from gap-grading that is only realized when RHA is combined with the coarser cement (PC30). Since strength is improved due to increase in PC quality, the results in Fig. 1 are presented in relative terms. Fig. 2 presents data from the computer simulation study. Shown are gradient structures of X 3 values, supposedly being proportional to physical (van der Waals) contributions to strength, where ,1, denotes mean free spacing, i.e., average of uninterrupted distance between all particle pairs (Hu [7]). Blending obviously significantly improves strength as a result of particle packing effects. Of course, PC40 mixtures are superior over PC30 mixtures, however at the expense of higher costs. Particularly, the normalized values for the coarsest cement demonstrate the even higher packing efficiency in the gap-graded case for 10% cement replacement. At higher dosage this effect of optimized packing is absent, however. Nevertheless, green concrete could be produced with high dosage of fine-grained RHA without loss of strength. Moreover, energy is produced during RHA incineration; this energy conservation aspect is another "green element" involved in the gap-graded blending concept whereby use is made of a vegetable waste component. The gap-grading effect is not depending on pozzolanity. In the research by Goldman and Bentur [2] it is demonstrated that even blending with carbon black (inert!) can give rise to proper results because of the physical (van der Waals) strength contribution compensating for
Porosity ofgreen concrete based on a gap-graded blend
317
the loss of chemical strength. Also in our research we found blending by incinerated diatomite earth to lead to higher compressive strength than by meta-kaolin-blended mixtures due to higher fineness and thus better packing with the cement grains (Stroeven et al. [8]).
~
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Fig. l. Relative compressive strength values of gap-graded aggregate RHA-blended concrete.
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Fig. 2. RHA blending reveals increased density in virtual concrete when gap-graded with the PC.
Nghi L.B. LE and Piet STROEVEN
318
PRODUCTION OF VIRTUAL CEMENTITOUS MATERIALS Simulation of fresh cement particles by HADES To obtain matured virtual cement paste, firstly, fresh cement particles need to be generated. The packing of the fresh cement particles is simulated in this study by HADES (HAbanera's Discrete Element Simulator). HADES is an advanced DEM system for making realistic particle packing simulations, also incorporating arbitrary grains shape. HADES is a dynamic force-based system, which allows simulating particle packing under the influence of external forces. Mechanical interaction in HADES is based on an algorithm that evaluates the interaction forces exerted between nearby objects. The forces are functions of distances and of areas of the segments. Several forces can be applied in this way on a particle such as spring force, cohesion force, damping force and friction force. HADES renders possible implementing particle packing in containers with periodic boundaries, simulating bulk conditions, with rigid boundaries, simulating aggregate's surfaces, or with mixed conditions. Gradual reduction of the container size while particles move makes it possible achieving high packing densities as met in practice. This is illustrated in Fig. 3.
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Fig. 3. Spherical particles dynamically compacted from loose (left) into dense random state (right) by the DEM system HADES. Simulation of hydration process In this study, a new numerical multi-phase model for simulating hydration of (pozzolanic blended) cement is utilized. The hydrating grains are simulated in this model by spherical integrated particles based on the so called 'integrated particle kinetics model' (IKPM) by Navi and Pignat [9] , in which remaining fresh (unhydrated) part of a particle modelled as a spherical core is coupled with its hydration product (i.e., CSH) as a shell coating the core into a single particle (integrated particle). This IKPM model is developed for a single phase material (C3S). In the present model, each fresh spherical core of a particle also incorporates information of its components, i.e. volume fractions of phases. Hence, the model is referred to as 'extended integrated particle kinetics model' (XIPKM). Although the present model takes into account only two phases, i.e., tri-calcium silicate (C3S) and di-calcium silicate (C 2S) for cement and Si0 2 and inert for RHA, its algorithms can readily be expanded to cover additional phase components. Fig 4 is an illustration of the hydration model with three types of particles. Beside the two integrated particles (left and
Porosity ofgreen concrete based on a gap-graded blend
319
middle ones in Fig. 4.) described above for modeling hydrating cement and pozzolanic grains, another hydration product (CH) is modelled as spherical particles (right one in Fig. 4). lt has been shown that the CH product diffuses and nucleates randomly either in pore space or precipitates on the surface of the existing CH grains. An evolution scheme for the number of CH grains during the hydration process was also proposed by Navi and Pignat [9].
Si0 2
•
•
inert
•
CSH-in
• csH• cH
Fig. 3. Particle models of cement, pozzolanic admixture and hydration product.
POROSIMETRY BY DRaMuTS
Pore characteristics in 30 virtual hydrated cementitious paste are investigated by the DRaMuTS method that has been introduced in Stroeven et al. [IO]. By this method, randomized data structures are built incrementally in two stages. In the first stage, a system of virtual trees, which consist of nodes and lines connecting pairs of such nodes (like branches in real trees), randomly and incrementally grow in the pore medium. This stage is to rapidly explore the pore space. The expansion of the trees by DraMuTS is illustrated by Fig. 5. Due to the tree structure of a main trunk and branches, the virtual tree system allows distinguishing not only between isolated pores and connected (percolated) pores that connect external surfaces of the considered specimen, but also between main channels (direct connection to external surfaces) and dead-end branches of such connected pores. In the second stage, a system of probing points is generated uniformly at random in pore space. As a result, such
(6)
(4)
\ ( 5): nearest vertex (I): tree vertex (6): intersections (2): tree edge (7): new vertex (3): obstacle (4 ): random point
Fig. 5. The expansion of the tree system in DRaMuTS approach.
Nghi L.B. LE and Piet STROEVEN
320
points can be used for statistical assessment of pore characteristics in which point classifications are realized by connections of such points to the tree systems. For example, the connected fraction of pores can be estimated by the fraction of the total number of points that can be associated with the percolated tree branches. Such probing points are also used to assess pore size characteristics. A relevant characteristic of the irregular pore network system would be the pore throat size distribution. At each point, the local pore throat is defined as the smallest pore section among the ensemble of intersection planes through the relevant point. The area of the local pore throat at a point i is then calculated by star method (Gundersen et al. [11]) as A; (i.e., diameter) is given by s;
= 2.Jlf, where lj
= Jr!J
and thus the throat size
is an intercept (i.e., the distance between the
point and the perimeter of the pore section) of an isotropic intercept system in the intersection plane through the point.
EXAMPLES
Four examples of plain PC and of RHA-blended PC with two wlb ratios are herein presented. The material grains are modelled as spheres and packed by HADES into parallelepiped pockets. Afterwards, the hydration simulations are implemented with the input data detailed in Table 1. The pocket is set up with two rigid surfaces to represent the material region between neighboring aggregate grains.
Table 1. Hydration simulation input Sample
W40Pc
W40Po20
W25Pc
W25Po20
wlb
0.40
0.40
0.25
0.25
% replacement
0
20
0
20
Number of cement particles
2091
1601
2644
2002
Number ofRHA particles
0
3457
0
4323
- Portland cement (PC), Blaine: 286 (m2/k:g) - Model cement, PSD: Rosin Rammler with n = 1.052, b = 0.040; size range
3~40
(µm)
- Model RHA, PSD function: Bui [1 ]; size range 3~ 13 (µm) - Cement composition: 80.5 % C3 S and 19.5% C2S - RHA composition: 87.5% Si02 and 12.5% inert - Pocket, size: lOOxlOOxlOO (µm); boundaries: 2 opposite rigid surfaces and 4 periodic ones. (PSD stands for particle size distribution) The microstructure of the hydrated samples at ultimate degree of hydration are depicted in Fig. 6. With the help of the digital-based voxel (volume pixel) system whereby each voxel represents a single phase, the hydration system can simulate the phases at the complicated interference situation of hydrating nearby particles. Fig. 7 presents the random tree structures at the sensitivity level of 30,000 tree branches, whereby the whole trees are shown on the left and the main pore channels on the right.
Porosity ofgreen concrete based on a gap-graded blend
321
W40Po20
•
W25Pc Cement RHA
-=SH-in -=SH-out
W25Po20 H
Fig. 6. 20 sections of the samples at ultimate degree of hydration.
(a) W40Pc
(b) W40Po20 Fig. 7. Visualization of entire trees (left) and main trunks (right) of pore network.
Nghi L.B. LE and Piet STROEVEN
322
~~·W25Pc
70 80 90 100 wnll (Jlm)
lO 20 30 40 50 60 70 80 90 lOO
Distance from left rigid wall (µm)
Fig. 8. Gradient structures of pore density.
01_......_........ _ .........
0
l 2 3 4 5 6 Number of tree branehes ( ~un) x 104
Fig. 9. Sensitivity analysis of connected pore fraction vs. number of tree branches. Fig. 8 shows the gradient structures of pore density normalized in the direction perpendicular to the rigid walls. The gap-graded blending by RHA has reduced the extension of the interfacial transition zone (ITZ) - the zone of highyst porosity, as is well-know (Scrivener and Nemati [12]). It can be observed also that this reduction by the gap-graded blending is far more apparent for wlb=0.4 than for w/b=0.25. Note that the simulated material is scaled down (maximum grain size by a factor of about 3) with respect to the real cement. So, simulated ITZ extents are somewhat proportionately reduced, too. The sensitivity analysis of connected pore fraction vs. the number of generated tree branches is presented in Fig. 9. It demonstrates that a larger number of tree branches is required for obtaining a stable level of pore connectivity in case of gap-graded blending. The effect of increasing sensitivity of the approach by taking lager numbers of tree branches is obvious; the smaller pores and hidden pores due lo pore tortuosity contributing to pore connectivity would be detected. Therefore, it can be inferred that pore paths connecting two external surfaces of a sample would become more tortuous and narrower due to gap-graded blending. Thus not only strength is enhanced but certainly also durability of the matured "green" concrete. Fig. 10 and Fig. 11 present the throat size distribution in which throat sizes are obtained by application of the star method at random points (in pores) generated in the second stage of DRaMuTS. Fig. 10 demonstrates that RHA gap-graded blending leads to refinement in pore
Porosity ofgreen concrete based on a gap-graded blend
323
-W40Pc -W40Po20 1-W25Pc 1-W25Po20
2
4
6
8
IO
12
14
16
Pore size (µm) Fig. 10. Throat size distribution for different wlb ratios.
14
16
Fig. 11. Throat size distribution in different zones.
structure. Obviously, this effect will also contribute significantly to transport-based durability. Fig. 11 shows that in plain PC, the pore throats in the ITZs are coarser than that in the middle zone (MZ) - the zone between two rigid walls not including the ITZs. However, in case of RHA blending, the pore throats in the ITZs are finer than in the MZ.
CONCLUSIONS The hydration system based on MP-IPKM and the DRaMuTS method for porosimetry have been demonstrated economic and effective tools for concrete research. To be unbiased, the material particles in the fresh state are simulated by the DEM system HADES, based on a concurrent algorithm (inherent to DEM), which realistically describes the mechanism of particle contacts. The 'multi-phase integrated particle kinetics model' is proposed to simulate hydration of cement grains as well as of RHA admixtures. The pore network is then explored and properties are thereupon statistically assessed by the modem porosimetry method DRaMuTS. The method renders possible separating between continuous pores, dead-end pores branching off such trunks, and isolated pores. Gap-graded blending by a fine vegetable waste-based mineral admixture like RHA can produce a greener high performance concrete with a relative high strength because of better grain packing. Moreover, gap-graded blending also leads to reduced transport capacity of the hardened material due to increased tortuosity and refinement of the continuous fraction of the pore system. These features can be expected improving transport-based durability properties.
REFERENCES 1. Bui D.D., Rice husk ash as a mineral admixture for high performance concrete. PhD Thesis, Delft University of Technology. DUP Science, Delft 2001 2. Goldman A., Bentur A., The influence of microfillers on enhancement of concrete strength. Cement and Concrete Research, 23, 1993, 962-72 3. Vu D.D., Strength properties of metakaolin-blended paste, mortar and concrete. PhD Thesis, Delft University of Technology. DUP Science, Delft 2002 4. Stroeven P., Sluys L.J., Le N.L.B., Assessment of pore characteristics in virtual concrete bypath planning. In: Proc. "Brittle Matrix Conference 10 ".Warsaw 15-17 Oct. 2012
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5. Bui D.D., Hu J., Stroeven P., Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete. Cement and Concrete Composites, 27, 2005, 35766 6. Stroeven P., Bui D.D., Sabuni E., Ash of vegetable waste used for economic production of low to high strength hydraulic binders. Fuel, 78, 1999, 153-59 7. Hu J., Porosity in concrete - morphological study of model concrete. PhD Thesis, Delft University of Technology. OPTIMA Grafische Communicatie, Delft 2004 8. Stroeven P., Bui D.D., Vu D.D., Promoting sustainable development by Portland cement blending. In: Proc. "I.A.B.S.E. Symposium". Melbourne 2002 9. Navi P., Pignat C., Simulation of cement hydration and the connectivity of the capillary pore space. Advanced Cement Based Materials, 4, 1996, 58-67 10. Stroeven P., Le N.L.B., Sluys L.J., He H., Porosimetry by double random multiple tree structuring. Image Analysis & Stereology, 31, 2012, 55-63 11. Gundersen H.J.G., Bagger P., Bendtsen T.F., Evans S.M., Korba L., Marcussen N., M0ller A., Nielsen K., Nyengaard J.R., Pakkenberg B., S0Rensen F.B., Vesterby A., West M.J., The new stereological tools: Disector, fractionator, nucleator and point sampled intercepts and their use in pathological research and diagnosis. Acta Pathologica, Microbiologica et lmmunologica Scandinavica, 96, 1988, 857-81 12. Scrivener K.L., Nemati K.M., The percolation of pore space in the cement paste/aggregate interfacial zone of concrete. Cement and Concrete Research, 26, 1996, 35-40
POROSITY OF GREEN CONCRETE BASED ON A GAP-GRADED BLEND
Nghi L.B. LE and Piet STROEVEN Faculty of Civil Engineering and Geosciences, Delft University of Technology Stevinweg 1, 2628 CN Delft, the Netherlands, e-mail: [email protected]
ABSTRACT Portland cement (PC) production contributes by about 6% to global emissions of C02. Reduction of the PC content in the binder exerts therefore a direct positive effect on such emissions. Partial replacement of the Portland cement by pozzolanic mineral admixtures has been proven a possible option. The green character is even reinforced by making use of incinerated vegetable waste, such as rice husk ash (RHA). Moreover, as a result of the gap-grading effect on particle packing density with RHA as the fine component, high strength concrete can be achieved with RHA-blended cement. This has been published earlier, so the paper only briefly covers these aspects. Characteristics of the capillary pores developed in the hydrating binder have direct impact on the transport-based durability properties. However, assessment of such pore characteristics of the RHA-blended cement constitutes a complicated problem, especially in experimental approaches. This paper, therefore, presents a new economic approach to conduct such investigation on the realistically produced cementitious materials in virtual reality. The fresh packed state of the paste's particles is simulated by a discrete element modeling (DEM) system, HADES. The hydration of a (blended) cement is simulated by a so-called 'extended integrated particle kinetics model' (XIPKM). Pore characteristics, such as topology, location distribution, degree of percolation and fraction of main channels in the pore network, are explored by a new modern approach named 'double random multi-tree structuring' (DraMuTS). The size distribution of throats that significantly limit the transport in the pore network system is derived from star volume measurements. Four examples of plain PC and of RHAblended PC with two wlb ratios are presented. The expected positive effects on transport-based durability issues due to RHA-blending are discussed.
Keywords
(blended) cement, rice husk ash, gap-grading, virtual cementitious materials, multi-phase hydration simulation, porosimetry, pore size distribution, main pore channels.
INTRODUCTION
One of the obvious contributions to reducing detrimental effects of Portland cement production on global warming as a result of C0 2 emissions is to reduce PC content significantly. Use of an admixture of vegetable origin such as RHA will additionally contribute to waste management and by its incineration energy to energy conservation. Experiments have demonstrated the profitable effects of blending Portland cement with a pozzolanic admixture that is significantly finer (Bui [1]). Indeed, similar to aggregate packing, the blended binder can therefore make profitable use of the gap-grading principle. Major evidence is also coming from tests where blending material was inert (Goldman and Bentur [2]). Basically, optimization of such mixtures has been considered in terms of compressive strength (Bui [1] and Vu [3]).
316
Nghi L.B. LE and Piet STROEVEN
Assessment of transport-based durability properties, which are intimately connected to pore network characteristics in materials, would be another issue of engineering significance. Investigating pore systems (porosimetry) is not a simple task, however. Experimental methods are unfortunately laborious, time-consuming and thus expensive. Additionally, they become more complicated in case of blending the binder. Fortunately, with the fast development of computer facilities, the use of virtual materials becomes a more economic way to conduct research on materials. Such new numerical porosimetry methods should therefore be developed. A robotics-inspired line of thinking is elaborated in Stroeven et al. [4]. In this paper, the hydration ofa cement paste in which the PC is blended by a pozzolanic admixture is discussed. Its microstructure is simulated by a new modem hydration model denoted 'extended integrated particle kinetics model' (XIPKM), in which two main phases of the Portland cement, i.e., tricalcium silicate (C 3S) and dicalcium silicate (C 2 S) as well as two phases of the RHA, i.e., silica (Si0 2) and inert are taken into consideration. Fresh binder particles and their packing structure are simulated by a DEM system, HADES, which is able incorporating arbitrary grain shapes and taking into account mechanical contacts among particles as well as the influence of external forces. Thereupon, the pore network is explored and its properties investigated by a new modem approach, 'double random multi tree structuring' (DRaMuTS) inspired from the rapidly-exploring random tree (RRT) approach in robotics.
GAP-GRADED BLENDING EFFECTS
The RHA is produced in the traditional way from Vietnamese rice husks and grinded until its internal porous structure collapsed, significantly reducing water demand Bui [l]. Mean . particle size was 5 µm (Bui et al. [5]). For additional details, see Bui et al. [5] and Stroeven et al. [6]. 70% crushed basalt and 30% of fine sand of fluvial origin constituted a gap-graded aggregate mixture. 500 to 550 kg/m3 Portland cement of two qualities were used. Three water/binder ratios (w/b) were investigated and replacement percentages of 10, 20 and 30 were envisaged. Naphthalene-based superplasticizer additions were employed to get cohesive mixtures with high slump values. lOOmm cubes were used for compressive strength testing at different stages of maturation. Detailed test results have been published in Bui [1] and Bui et al. [5]. However, Fig. l reveals the blending efficiency resulting from gap-grading that is only realized when RHA is combined with the coarser cement (PC30). Since strength is improved due to increase in PC quality, the results in Fig. 1 are presented in relative terms. Fig. 2 presents data from the computer simulation study. Shown are gradient structures of X 3 values, supposedly being proportional to physical (van der Waals) contributions to strength, where ,1, denotes mean free spacing, i.e., average of uninterrupted distance between all particle pairs (Hu [7]). Blending obviously significantly improves strength as a result of particle packing effects. Of course, PC40 mixtures are superior over PC30 mixtures, however at the expense of higher costs. Particularly, the normalized values for the coarsest cement demonstrate the even higher packing efficiency in the gap-graded case for 10% cement replacement. At higher dosage this effect of optimized packing is absent, however. Nevertheless, green concrete could be produced with high dosage of fine-grained RHA without loss of strength. Moreover, energy is produced during RHA incineration; this energy conservation aspect is another "green element" involved in the gap-graded blending concept whereby use is made of a vegetable waste component. The gap-grading effect is not depending on pozzolanity. In the research by Goldman and Bentur [2] it is demonstrated that even blending with carbon black (inert!) can give rise to proper results because of the physical (van der Waals) strength contribution compensating for
Porosity ofgreen concrete based on a gap-graded blend
317
the loss of chemical strength. Also in our research we found blending by incinerated diatomite earth to lead to higher compressive strength than by meta-kaolin-blended mixtures due to higher fineness and thus better packing with the cement grains (Stroeven et al. [8]).
~
·~...------------. t20
~1rn
-+-IO%RRAJ
f
c......
Im
=~~=~1
(; 110
!"'
HS
W!hll.3 1PC30
IM (} 7
;;mi
r
--t0%RRA --t5\l'RHA
100
II ~5
i:
l421lS:l.SAl:
-.o-20%RHA
0 1144.l
~(day•)
ZS3:54249~6J10:71.84!J191l
Age (da)aj
m...------------. ~no
--u11HlliA
--1!'%11HAI
.....,..20'%1lHA
0 7 I
JmtL;c;;;::==~-11:
95
~o
8$ 411~6l
WT/84:-9! 98
~1<1a,,;1
. , . . - - - - - - - - - - - - , WIB=i.34
;.< w
!..
.Jl!j
(} 7 i4 2,1$J$42
Ag•(dliy>(I )4~
i: r i
c..,....u•CJO
'w lu:l
(j
,.
'l4 :lt 2SJS .U.ot9-!i6 6..l '1ll.71 ~ 91 98
Ago{dllt•)
Fig. l. Relative compressive strength values of gap-graded aggregate RHA-blended concrete.
ll~---~-----i...-----"
ill}
11·
lll 20 !l!l Df!AAlltitlro:~~~)
Fig. 2. RHA blending reveals increased density in virtual concrete when gap-graded with the PC.
Nghi L.B. LE and Piet STROEVEN
318
PRODUCTION OF VIRTUAL CEMENTITOUS MATERIALS Simulation of fresh cement particles by HADES To obtain matured virtual cement paste, firstly, fresh cement particles need to be generated. The packing of the fresh cement particles is simulated in this study by HADES (HAbanera's Discrete Element Simulator). HADES is an advanced DEM system for making realistic particle packing simulations, also incorporating arbitrary grains shape. HADES is a dynamic force-based system, which allows simulating particle packing under the influence of external forces. Mechanical interaction in HADES is based on an algorithm that evaluates the interaction forces exerted between nearby objects. The forces are functions of distances and of areas of the segments. Several forces can be applied in this way on a particle such as spring force, cohesion force, damping force and friction force. HADES renders possible implementing particle packing in containers with periodic boundaries, simulating bulk conditions, with rigid boundaries, simulating aggregate's surfaces, or with mixed conditions. Gradual reduction of the container size while particles move makes it possible achieving high packing densities as met in practice. This is illustrated in Fig. 3.
. ·-. ...... ... ....• .
...•. ·•· • ..•.,,_-•...• ... .. ... ...•,. ·. t..
.
. • •! :......- •
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Fig. 3. Spherical particles dynamically compacted from loose (left) into dense random state (right) by the DEM system HADES. Simulation of hydration process In this study, a new numerical multi-phase model for simulating hydration of (pozzolanic blended) cement is utilized. The hydrating grains are simulated in this model by spherical integrated particles based on the so called 'integrated particle kinetics model' (IKPM) by Navi and Pignat [9] , in which remaining fresh (unhydrated) part of a particle modelled as a spherical core is coupled with its hydration product (i.e., CSH) as a shell coating the core into a single particle (integrated particle). This IKPM model is developed for a single phase material (C3S). In the present model, each fresh spherical core of a particle also incorporates information of its components, i.e. volume fractions of phases. Hence, the model is referred to as 'extended integrated particle kinetics model' (XIPKM). Although the present model takes into account only two phases, i.e., tri-calcium silicate (C3S) and di-calcium silicate (C 2S) for cement and Si0 2 and inert for RHA, its algorithms can readily be expanded to cover additional phase components. Fig 4 is an illustration of the hydration model with three types of particles. Beside the two integrated particles (left and
Porosity ofgreen concrete based on a gap-graded blend
319
middle ones in Fig. 4.) described above for modeling hydrating cement and pozzolanic grains, another hydration product (CH) is modelled as spherical particles (right one in Fig. 4). lt has been shown that the CH product diffuses and nucleates randomly either in pore space or precipitates on the surface of the existing CH grains. An evolution scheme for the number of CH grains during the hydration process was also proposed by Navi and Pignat [9].
Si0 2
•
•
inert
•
CSH-in
• csH• cH
Fig. 3. Particle models of cement, pozzolanic admixture and hydration product.
POROSIMETRY BY DRaMuTS
Pore characteristics in 30 virtual hydrated cementitious paste are investigated by the DRaMuTS method that has been introduced in Stroeven et al. [IO]. By this method, randomized data structures are built incrementally in two stages. In the first stage, a system of virtual trees, which consist of nodes and lines connecting pairs of such nodes (like branches in real trees), randomly and incrementally grow in the pore medium. This stage is to rapidly explore the pore space. The expansion of the trees by DraMuTS is illustrated by Fig. 5. Due to the tree structure of a main trunk and branches, the virtual tree system allows distinguishing not only between isolated pores and connected (percolated) pores that connect external surfaces of the considered specimen, but also between main channels (direct connection to external surfaces) and dead-end branches of such connected pores. In the second stage, a system of probing points is generated uniformly at random in pore space. As a result, such
(6)
(4)
\ ( 5): nearest vertex (I): tree vertex (6): intersections (2): tree edge (7): new vertex (3): obstacle (4 ): random point
Fig. 5. The expansion of the tree system in DRaMuTS approach.
Nghi L.B. LE and Piet STROEVEN
320
points can be used for statistical assessment of pore characteristics in which point classifications are realized by connections of such points to the tree systems. For example, the connected fraction of pores can be estimated by the fraction of the total number of points that can be associated with the percolated tree branches. Such probing points are also used to assess pore size characteristics. A relevant characteristic of the irregular pore network system would be the pore throat size distribution. At each point, the local pore throat is defined as the smallest pore section among the ensemble of intersection planes through the relevant point. The area of the local pore throat at a point i is then calculated by star method (Gundersen et al. [11]) as A; (i.e., diameter) is given by s;
= 2.Jlf, where lj
= Jr!J
and thus the throat size
is an intercept (i.e., the distance between the
point and the perimeter of the pore section) of an isotropic intercept system in the intersection plane through the point.
EXAMPLES
Four examples of plain PC and of RHA-blended PC with two wlb ratios are herein presented. The material grains are modelled as spheres and packed by HADES into parallelepiped pockets. Afterwards, the hydration simulations are implemented with the input data detailed in Table 1. The pocket is set up with two rigid surfaces to represent the material region between neighboring aggregate grains.
Table 1. Hydration simulation input Sample
W40Pc
W40Po20
W25Pc
W25Po20
wlb
0.40
0.40
0.25
0.25
% replacement
0
20
0
20
Number of cement particles
2091
1601
2644
2002
Number ofRHA particles
0
3457
0
4323
- Portland cement (PC), Blaine: 286 (m2/k:g) - Model cement, PSD: Rosin Rammler with n = 1.052, b = 0.040; size range
3~40
(µm)
- Model RHA, PSD function: Bui [1 ]; size range 3~ 13 (µm) - Cement composition: 80.5 % C3 S and 19.5% C2S - RHA composition: 87.5% Si02 and 12.5% inert - Pocket, size: lOOxlOOxlOO (µm); boundaries: 2 opposite rigid surfaces and 4 periodic ones. (PSD stands for particle size distribution) The microstructure of the hydrated samples at ultimate degree of hydration are depicted in Fig. 6. With the help of the digital-based voxel (volume pixel) system whereby each voxel represents a single phase, the hydration system can simulate the phases at the complicated interference situation of hydrating nearby particles. Fig. 7 presents the random tree structures at the sensitivity level of 30,000 tree branches, whereby the whole trees are shown on the left and the main pore channels on the right.
Porosity ofgreen concrete based on a gap-graded blend
321
W40Po20
•
W25Pc Cement RHA
-=SH-in -=SH-out
W25Po20 H
Fig. 6. 20 sections of the samples at ultimate degree of hydration.
(a) W40Pc
(b) W40Po20 Fig. 7. Visualization of entire trees (left) and main trunks (right) of pore network.
Nghi L.B. LE and Piet STROEVEN
322
~~·W25Pc
70 80 90 100 wnll (Jlm)
lO 20 30 40 50 60 70 80 90 lOO
Distance from left rigid wall (µm)
Fig. 8. Gradient structures of pore density.
01_......_........ _ .........
0
l 2 3 4 5 6 Number of tree branehes ( ~un) x 104
Fig. 9. Sensitivity analysis of connected pore fraction vs. number of tree branches. Fig. 8 shows the gradient structures of pore density normalized in the direction perpendicular to the rigid walls. The gap-graded blending by RHA has reduced the extension of the interfacial transition zone (ITZ) - the zone of highyst porosity, as is well-know (Scrivener and Nemati [12]). It can be observed also that this reduction by the gap-graded blending is far more apparent for wlb=0.4 than for w/b=0.25. Note that the simulated material is scaled down (maximum grain size by a factor of about 3) with respect to the real cement. So, simulated ITZ extents are somewhat proportionately reduced, too. The sensitivity analysis of connected pore fraction vs. the number of generated tree branches is presented in Fig. 9. It demonstrates that a larger number of tree branches is required for obtaining a stable level of pore connectivity in case of gap-graded blending. The effect of increasing sensitivity of the approach by taking lager numbers of tree branches is obvious; the smaller pores and hidden pores due lo pore tortuosity contributing to pore connectivity would be detected. Therefore, it can be inferred that pore paths connecting two external surfaces of a sample would become more tortuous and narrower due to gap-graded blending. Thus not only strength is enhanced but certainly also durability of the matured "green" concrete. Fig. 10 and Fig. 11 present the throat size distribution in which throat sizes are obtained by application of the star method at random points (in pores) generated in the second stage of DRaMuTS. Fig. 10 demonstrates that RHA gap-graded blending leads to refinement in pore
Porosity ofgreen concrete based on a gap-graded blend
323
-W40Pc -W40Po20 1-W25Pc 1-W25Po20
2
4
6
8
IO
12
14
16
Pore size (µm) Fig. 10. Throat size distribution for different wlb ratios.
14
16
Fig. 11. Throat size distribution in different zones.
structure. Obviously, this effect will also contribute significantly to transport-based durability. Fig. 11 shows that in plain PC, the pore throats in the ITZs are coarser than that in the middle zone (MZ) - the zone between two rigid walls not including the ITZs. However, in case of RHA blending, the pore throats in the ITZs are finer than in the MZ.
CONCLUSIONS The hydration system based on MP-IPKM and the DRaMuTS method for porosimetry have been demonstrated economic and effective tools for concrete research. To be unbiased, the material particles in the fresh state are simulated by the DEM system HADES, based on a concurrent algorithm (inherent to DEM), which realistically describes the mechanism of particle contacts. The 'multi-phase integrated particle kinetics model' is proposed to simulate hydration of cement grains as well as of RHA admixtures. The pore network is then explored and properties are thereupon statistically assessed by the modem porosimetry method DRaMuTS. The method renders possible separating between continuous pores, dead-end pores branching off such trunks, and isolated pores. Gap-graded blending by a fine vegetable waste-based mineral admixture like RHA can produce a greener high performance concrete with a relative high strength because of better grain packing. Moreover, gap-graded blending also leads to reduced transport capacity of the hardened material due to increased tortuosity and refinement of the continuous fraction of the pore system. These features can be expected improving transport-based durability properties.
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5. Bui D.D., Hu J., Stroeven P., Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete. Cement and Concrete Composites, 27, 2005, 35766 6. Stroeven P., Bui D.D., Sabuni E., Ash of vegetable waste used for economic production of low to high strength hydraulic binders. Fuel, 78, 1999, 153-59 7. Hu J., Porosity in concrete - morphological study of model concrete. PhD Thesis, Delft University of Technology. OPTIMA Grafische Communicatie, Delft 2004 8. Stroeven P., Bui D.D., Vu D.D., Promoting sustainable development by Portland cement blending. In: Proc. "I.A.B.S.E. Symposium". Melbourne 2002 9. Navi P., Pignat C., Simulation of cement hydration and the connectivity of the capillary pore space. Advanced Cement Based Materials, 4, 1996, 58-67 10. Stroeven P., Le N.L.B., Sluys L.J., He H., Porosimetry by double random multiple tree structuring. Image Analysis & Stereology, 31, 2012, 55-63 11. Gundersen H.J.G., Bagger P., Bendtsen T.F., Evans S.M., Korba L., Marcussen N., M0ller A., Nielsen K., Nyengaard J.R., Pakkenberg B., S0Rensen F.B., Vesterby A., West M.J., The new stereological tools: Disector, fractionator, nucleator and point sampled intercepts and their use in pathological research and diagnosis. Acta Pathologica, Microbiologica et lmmunologica Scandinavica, 96, 1988, 857-81 12. Scrivener K.L., Nemati K.M., The percolation of pore space in the cement paste/aggregate interfacial zone of concrete. Cement and Concrete Research, 26, 1996, 35-40