- Open Access
Effects of Different Lightweight Functional Fillers for Use in Cementitious Composites
© The Author(s) 2017
- Received: 13 August 2016
- Accepted: 21 November 2016
- Published: 8 February 2017
The effects of different lightweight functional fillers on the properties of cement-based composites are investigated in this study. The fillers include fly ash cenospheres (FACs) and glass micro-spheres (GMS15 and GMS38) in various proportions. The developed composites were tested for compressive, flexural and tensile strengths at 10 and 28-day ages. The results indicated that both FACs and GMS38 are excellent candidates for producing strong lightweight composites. However, incorporation of GMS15 resulted in much lower specific strength values (only up to 13.64 kPa/kg m3) due to its thinner shell thickness and lower isostatic crushing strength value (2.07 MPa). Microstructural analyses further revealed that GMS38 and GMS15 were better suited for thermal insulating applications. However, higher weight fraction of the fillers in composites leads to increased porosity which might be detrimental to their strength development.
- functional fillers
- glass microsphere
- mechanical properties
Lightweight concrete (LWC) has gained much more interest from the researchers in the last few decades although its use could trace back to 3000 BC (Chandra and Berntsson 2002). The reasons for such increased interest are its unique advantages over normal weight concrete such as reduction in dead loads leading to smaller structural member as well as foundation size, ease of the shipping and transportation in case of precast structural members, and reduced overall construction cost. In addition LWC offers excellent durability in chemical and frost attack with reduced permeability (Li 2011), greater fire resistance (ACI 216.1 1997) and better thermal insulation (ACI 213 2003). The unit weight of LWC lies in the range of 1200–1800 kg/m3 (Li 2011) while for structural LWC in general ACI Committee 213 defines the range as 1120–1920 kg/m3 (ACI 213 2003). In order to achieve the desired unit weight with adequate mechanical properties, careful selection and efficient utilization of lightweight filler (LWF) materials is imperative.
Traditionally, different types of LWFs have been studied for their use in the cement-based composites such as expanded perlite (Demirboǧa et al. 2001; Kramar and Bindiganavile 2010; Lanzón and García-Ruiz 2008; Lu et al. 2014), expanded glass beads (ASTM D790-10 2010; Bouvard et al. 2007), shale (de Gennaro et al. 2008; Ke et al. 2009; Lotfy et al. 2015), expanded polystyrene beads (Bouvard et al. 2007; Miled et al. 2007; Saradhi Babu et al. 2005), expanded clay (Chandra and Berntsson 2002; Gao et al. 2014) etc. and the unit weight has been successfully achieved within the stipulated guidelines. However such composites had lower mechanical strength and reduced overall performance. For example Yu et al. used recycled expanded glass and achieved 28-day density of the composites as low as 1280 kg/m3 with good durability, however, the corresponding compressive strength was limited to 23.3 MPa (Spiesz et al. 2013; Yu et al. 2013). Also, Chen and Liu developed expanded polystyrene foam composite with density of 400 and 800 kg/m3 with excellent thermal insulation properties (lower thermal conductivity coefficient) but the extremely low corresponding strength values of 3 and 13 MPa hampered the use in structural applications (Chen and Liu 2013). Similarly, Topku and Isikdag used perlite aggregate as LWF and produced the composites within the density range of 1800–2040 kg/m3 and deduced that any amount of perlite aggregate greater than 30% has negative effects on the parameters relating to mechanical strength. Even at 1800 kg/m3 density, the strength was limited to 37.3 MPa (Topçu and Işıkdağ 2008). Kramar and Bindiganavile further confirmed that compressive strength declines cubically while flexural strength and fracture toughness decreases linearly with perlite addition (Kramar and Bindiganavile 2013). Mladenovic studied the expanded vermiculite, clay, glass and perlite and similar observations for alkali-silica reactivity and found that these LWFs are suitable in terms of this particular durability related property (Ducman and Mladenovic 2004). Likewise, Hassanpur et al. comprehensively reviewed the problems associated with the use of LWFs (expanded perlite, expanded clay, pumice, etc.) in cementitious composites and concluded that inclusion of different kinds of fibers may significantly improve the toughness, ductility and energy absorption of the resulting composites, however the lower mechanical strength (compressive strength) remains an unresolved issue (Hassanpour et al. 2012).
In the recent years, researchers have also been trying to investigate some other materials like cenosphere (Chávez-Valdez et al. 2011; Kwan and Chen 2013; Pichór 2009; Wang et al. 2012, 2013; Wu et al. 2015; Xu et al. 2015) and aerogel particles (Gao et al. 2014; Hanif et al. 2016; Kim et al. 2013; Ng et al. 2015) for their use as LWF. Xu et al. found that even though incorporating cenosphere in magnesium oxy-chloride cement composite reduces the compressive strength, still high levels of strength (60 MPa) are achieved (Xu et al. 2015). Similar findings for cenosphere particles in OPC based composites were presented by Wang et al. (2012, 2014) and Wu et al. (2015). This indicates their potential for producing strong lightweight composites. Further, the studies carried out on the influence of aerogels by Hanif et al. (2016) and Ng et al. (2015) showed that aerogel is ideally suited for thermally insulated composites while the mechanical strength is not high enough because of the mechanical properties of aerogel particles (Woignier and Phalippou 1988).
Nevertheless, the need to broaden the knowledge on cenosphere behavior in the cement-based composites is still increasing. Moreover, alternate LWFs need to be sought for improved properties of the resulting composites. Although various fillers have been previously used, the objective of getting the reduced unit weight at adequate strength levels is still challenging (Sharifi et al. 2016). The aim of current study is to evaluate the properties of the composites with incorporation of an alternative lightweight material, the hollow glass microspheres, and comparing the resulting properties with cenosphere incorporated composites.
Physical characteristics of lightweight fillers.
Bulk density (kg/m3)
BET surface area (m2/g)
Iso-static crush strengtha (MPa)
Glass micro-sphere GMS15
Glass micro-sphere GMS38
Fly ash cenosphere
Elemental analysis of the raw materials.
Fly ash cenospheres
2.2 Mix Proportions and Specimen Casting
Mixture proportions (by weight).
Superplasticizer (% by binder weight)b
PVA fiber (%)
The mixed fibrous pastes were cast into the pre-lubricated steel molds and compacted for removing the entrapped air. Specimen prisms (Area 40 mm × 40 mm and length 160 mm) and cubes (40 mm side) were cast for testing strength in flexure and compression, respectively. The tensile strength test specimen size was 50 mm × 15 mm (area) and 350 mm (length). After casting, the specimens were wrapped using a plastic sheet and kept under room temperature. After one day, the specimen were demolded and retained in the curing room where the relative humidity and temperature were maintained at 95% and 25 °C, respectively. The curing of the specimen was continued till their testing age. Mechanical strength parameters were tested after 10 and 28 days while the microstructural studies were carried out after 28-day age.
2.3 Experimental Methods and Procedures
2.3.1 Mechanical Testing
The mechanical tests included compression, flexure, and tension. Compressive strength testing was conducted in an automatic compression testing machine, at the loading rate of 1.0 kN/s. Three-point bending test was done on the with the span length of 100 mm and the loading rate of 0.15 mm/min. Load at mid-point was directly recorded whereas the mid-span deflection was measured with two LVDTs (linear variable differential transformers) attached parallelly on the specimen. The load and deflection values were used to develop stress–strain plots. Moreover, the elastic modulus was also determined. For determining the tensile strength, direct tension tests on the relevant specimen were done with a length of 150 mm in testing portion and the loading of 0.05 mm/min. Extension in the specimens was measured with two LVDTs mounted on the specimen in the direction parallel to the loading direction. For all the mixes, three specimen were tested for each property under investigation and the average of these were reported for corresponding test.
2.3.2 Testing for Morphological and Microstructural Characterization
Microstructural studies were carried out by using scanning electron microscopy (SEM) in which hydration products identification, observation of LWF particle distribution in the matrix, and evaluation of pore structures were done. SEM was done with the help of JSM-6390 and JSM-6700F (ultra-high resolution scanning electron microscope; 1 nm at 15 kV and 2.2 nm at 1 kV) (Jeol USA Inc.). Also, thermogravimetric and differential thermal analysis (using TGA Q5000 and TG/DTA 92 Setaram II) methods were employed for determining the heat flow and weight change (phase transformation) with temperature. For this purpose, samples were prepared by grinding the broken pieces at 28-day age, and subjected to heating from room temperature to 900 °C at the rate of 10 °C per minute, under nitrogen environment (nitrogen flow rate of 25.00 mL/min and inlet gas pressure 1 bar). The corresponding values of weight loss and heat flow were determined.
Porosity and pore volume characteristics in the composites were evaluated by mercury intrusion porosimetry (MIP) method (Ma and Li 2013). For this test, the samples were prepared by breaking down to smaller sizes and dried by solvent replacement method in which the solvent (ethanol) was changed every 6 h during the first few days and then every day until one week. Later the specimen were subjected to vacuum drying. The dried samples were subjected to mercury intrusion during which the pressure was increased up to 400 MPa (initially low pressure followed by high pressure). The mercury-concrete contact angle was taken as 140° (Ma 2014). Washburn equation (Washburn 1921) was used to convert the pressure into relevant pore diameter while Katz–Thompson model (Katz and Thompson 1986) was employed to determine the permeability.
3.1 Density and Compressive Strength
Mechanical properties of the hardened specimen.
Unit weight (kg/m3)
Compressive strength (MPa)
Flexural strength (MPa)
Flexural strain capacity (%)
Unit weight (kg/m3)
Compressive strength (MPa)
Flexural strength (MPa)
Tensile strength (MPa)
Flexural strain capacity (%)
Elastic modulus (GPa)
However, the decrease in the density directly influenced the mechanical properties of the composites. As comprehended from the Table 4, the compressive strength decreased drastically for GMS15 composites as compared other LWF composites (GMS38 and FAC). The compressive strength at 10-day age for GMS15 composites was too low to be determined due to the minimum stress limitation of the compression testing machine (5 MPa). For 28-day compressive strength values, it was found that the incorporation of 10% of GMS15 reduced the strength by more than 80% as compared to the control mix (CM) while with 30 wt% incorporation, the corresponding decline was about 94%. This very low strength values were due to both the total air content (in the matrix) and required mixing water (to achieve uniformity and consistency of fresh mortar mix) associated with the LWF incorporation which were exacerbated by the lower isostatic crush strength (2.07 MPa) of GMS15 particles. On the other hand, the composites with GMS38 and FAC showed much better mechanical performance. The strength decrease in GMS38 and FAC composites was found about 49 and 43% for LWF weight fraction of 10 and 25%, respectively. It was interesting to see that 10 and 25 wt% incorporation of GMS38 and FAC, respectively resulted in composites having similar densities but the corresponding compressive strength was 6% higher for FAC composites. Similar phenomenon had been observed for other levels of LWF weight fraction for these composites. Further, the total water content in the mixes varied which was also a factor in strength reduction as according to Abram’s law (Abrams 1927), it directly influences the strength. The higher the water to cementitious materials ratio, the lower is the strength. However, it was seen that FAC modified composites exhibited better mechanical characteristics regardless of higher water content which is due to the chemical composition of FAC particles and tougher shell. All the FAC modified composites either had similar or higher water to binder ratio as compared to their GMS15 and GMS38 containing counterparts, still higher strength levels were achieved for these composites. This showed that FAC was superior to GMS38 and GMS15 in producing lightweight composites with better mechanical properties. Also, GMS38 have proved to be better than GMS15 in a similar way. Both the GMS15 and GMS 38 incorporated composites contained same water amount at the same weight fraction level of LWFs, but their mechanical performance particularly the compressive strength varied greatly.
3.2 Flexural and Tensile Behavior
All the FAC composites showed better post cracking behavior as compared to GMS15 and GMS38 modified composites. Although, at lower weight fraction of LWF (10%), the composites containing GMS15 and GMS38 also behaved well with flexural strain capacity values of 0.75 and 1% respectively, but still it can be seen that after the first crack, the matrix wasn’t able to sustain longer even with the help of fibers. The peak flexural strength values for composites with similar unit weight (GMS15-0.1, GMS38-0.2, and FAC-0.55) show that LWFs GMS15 and GMS38 behave in a similar fashion (mainly due to their similar chemical composition) while FAC incorporated composites expressed almost double the value as compared to others. However, it was found that GMS15 composites were more brittle in nature as compared to their counterparts (GMS38). The primary reason for such behavior was the thinner shell thickness (1–2 microns) and lower isostatic crushing strength value (Table 1). Moreover, it was pertinent to identify the loss of ductility in the composites with the increasing LWF content. This showed that using such LWFs in higher amounts, greater fiber weight fraction was needed if the desired characteristics of the composites required strain hardening. The strain capacity values for FAC modified composites were higher than the GMS15 and GMS38 incorporated composites due to the better bonding of FACs and fibers with the binder matrix. It was further confirmed and explained in microstructural investigations in Sect. 3.3.
The ratio of compressive strength to tensile strength is low in comparison with the conventional mortar mixes. This is true because of the fillers used in the study are hollow spherical shells with different isostatic crushing strength values (Table 1) which depend primarily on the hollow spherical shell thickness. Thus, the matrix strength is reduced with the inclusion of these particles which leads to the lower first cracking strength. The first crack strength as well as the ultimate strength of the composites found to be directly correlated to the isostatic crushing strength value which is dictated by the shell thickness (1–2 microns for GMS particles whereas several microns for FACs). Even the fibers were used to improve the tensile behavior, the lower matrix strength especially in case of composites modified with GMS particles hindered the development of pronounced strain hardening.
3.3 Microstructural and Morphological Characterization
Porosity and permeability values for the composites.
An interesting observation is pointed out regarding the permeability. Even though the porosity increases with increase of LWF amount in the composites, the permeability varies differently. For all the composites containing LWFs except GMS15-0.3, the permeability was found lower than that of the control mix (46.67 milliDarcy). This might be due to the filling effect of the very small sized particles of LWFs. For GMS15-0.3, higher water content was used which is the primary reason of porosity related properties but the low shell strength of GMS15 particles is also another factor. The thinner shells which broke under compressive stresses led to ultimately higher porosity and permeability. It was, generally, seen that the permeability increased with increasing LWF amount in the composite. Although, the permeability values obtained from Katz–Thompson model were not accurate rather over-estimated for the composites under evaluation (Ma 2014; Ma et al. 2014) still the values could be used for comparative assessment.
The porosity doubled with the incorporation of 10 and 25 wt% of glass microspheres (GMS15 and GMS38) and FACs, correspondingly. However, with every 10 wt% increase of LWF content, the porosity increment observed was 14.66, 6.76, and 5.04% for GMS15, GMS38 and FACs. The apparent results on porosity could be attributed to two reasons; first, the greater number of pores associated with the higher water content and air voids within the composite, and second, the breakage of weak LWF particles (due to lower iso-static crush strength representing thinner shell thickness) in the composites which led to greater mercury intrusion as some of the particles were of significantly larger size. It was evident from the MIP results that FACs and GMS38 were superior to GMS15. The correlation of porosity with mechanical strength (compressive strength) was linear for the composites. The critical diameter (d c ) was determined (from the log differential intrusion volume plot) as 0.07, 0.02, and 0.05 µm for GMS15, GMS38, and FAC composites, respectively.
The critical diameter didn’t change for one particular set of composites (containing the same LWF). The peak of log differential plot (corresponding to d c ) for all the LWF composites tends to shifts towards left (smaller pore diameter range) in relation to that for CM. The greater mercury intrusion volume with increase of LWF weight fraction indicates a greater percentage of pore volume in the corresponding composites. Thus, it could be deduced that such composites would behave well for thermal insulation applications. The thermal conductivity coefficient of air (1.008 W/m K) is far lower than that of cement mortar (2.3), thus higher pore volume would lead to a reduced thermal conductivity coefficient. However, it could be seen that incorporation of glass microspheres lead to a more porous microstructure which can be more meaningful for thermal insulating applications whereas the corresponding composites have reduced mechanical strength too. This suggests that in order to achieve a balance of different characteristics, a blend of these fillers may be helpful.
FAC bearing composites could be seen the most dense among all the composites. The FACs bonded exceptionally well with the cementitious matrix thus improving the properties. However, partially consumed (not broken) FAC shells were also seen in SEM imagery which is due to the partial reactivity of FAC particles owing to the presence of amorphous silica and some percentage of lime. The partially consumed FAC particles observed along with the unreacted ones are shown in the SEM images in Fig. 13. The reactivity could be responsible for higher compressive strength even at larger levels of FAC addition. This is so because the pozzolanic reaction leads to greater calcium silicate hydrate (CSH) gel in the system. The denser, more compacted, and uniform microstructure in the FAC modified composites influenced certain other properties of the composites including the post-first-crack behavior, flexural and tensile strain capacity, and total porosity. Primarily, the lime and amorphous silica present in the FACs is responsible for denser microstructure due to the increased pozzolanic activity. The pozzolanic reactivity can be further assessed from the dehydroxylation and decarbonation peaks obtained in TGA, already discussed earlier.
A comprehensive and thorough study on the cement-based composites incorporating various types of lightweight functional fillers (LWFs), including fly ash cenosphere (FAC) and glass microsphere (GMS15 and GMS38), was conducted to evaluate the influence of the LWFs on various properties of the corresponding lightweight composites and asses their feasibility in different building structural applications. Both mechanical and microstructural properties were studied. The results indicated that both FAC and GMS38 were suitable for producing structural lightweight composites (density less than 1920 kg/m3 and minimum compressive strength of 17 MPa (ACI 213 2003)).
FACs and GMS38 particles proved to be excellent materials for producing lightweight and ultra-lightweight cementitious composites. They can be employed for structural (load-carrying) purposes where higher mechanical strength is required, as long as they are incorporated in reasonable proportions in the composites. On the other hand GMS15 are inadequate due to weaker shell (or more specifically, the iso-static crush strength).
Given the decent strength to unit weight ratio, FAC and GMS38 are suitable for producing precast non load-bearing members like, wall panels, partition walls, ceiling, etc. However, GMS15 are not adequate to be used for such applications because of much lower mechanical strength associated with the resulting composites.
FACs and GMS38 are well suited for fiber-reinforced composites to efficiently utilize the tensile properties of fibers. Good bonding in the fibrous mortars leading to excellent ductility indicates their promising use for fiber-reinforced composites. Whereas GMS15 particles couldn’t be very helpful in this regard due to the weakness of their shell which may break earlier under stresses.
Glass microspheres, both GMS15 and GMS38, are good candidates for producing thermal insulating composites due to the greater pore volume associated with their incorporation, however they should be added with other materials/fillers (e.g. FAC) to achieve better mechanical properties.
The adequate weight fraction of these LWFs to be incorporated in the cement composites is determined as 20, 55%, and less than 10% for GMS38, FACs, and GMS15, respectively. Greater amounts may pose higher permeability and porosity leading to reduced mechanical properties thus restricting the possible use.
This work was financially supported by the China Ministry of Science and Technology under Grant 2015CB655100. The authors would also thank Advanced Engineering Manufacturing Facility (AEMF) at HKUST for providing technical support for surface area measurement (using nitrogen adsorption method) of the LWFs used in this study.
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