The continuous research in the area of concrete material and innovations has been made to cope with challenges of many construction aspects. One of the concrete that is becoming famous usage nowadays among the contractors is lightweight concrete due to its physical properties/characteristics make it an obvious choice as a medium of construction. Foam concrete
(FC) is a type of porous concrete (Fig 1), produced by mechanical mixing of foam (bubbles of size 0.1–1.0mm) prepared in advance with the concrete mixture composed of cement-sand-flyash matrix. Foam is prepared in a special device called foam generator and later mixed by using special mixer. By controlling the dosage of foam, density range of 400 –1600 kg/m3 and compressive strengths varying from one 1 MPa to 15 MPa, can be produced thus providing flexibility for various applications such as structural, partition and insulation material (Fig 2). With the integration of air-pores into the base matrix, it gives low self-weight, high workability, excellent insulating values, but lower strength in contrast to normal strength concrete. Foam concrete is a free flowing, self-levelling; material that does not require
compaction. It is particularly suitable for high-technology special structures where, apart from the response to reduce the cost and environmental impact are considered. Although FC possesses a great number of benefits, its compressive strength is low. Attempts have been made to improve the compressive strength of FC by adding nano materials.
Performance of Foam Concrete with Nanoparticles
In recent years, nanotechnology is gaining popularity in the field of civil engineering and construction. Nanotechnology deals with particle at nano-scale, i.e., 10-9 m. One of the methods recently employed to improve the technical characteristics of concrete is the use of additives consisting of nano dispersive particles. Concrete can be tailored by the incorporation of nanoparticles and nanotubes to control material behaviour and add novel properties by the grafting of molecules into cement particles, cement phases, aggregates, and additives to provide surface functionality. Nanosized particles have a high surface area to volume ratio providing the potential for tremendous chemical reactivity when it is admixed. Several challenges will need to be solved to realize its full potential, including the proper dispersion of the nanoscale additives, scale-up of laboratory results and implementation on larger scale, and a lowering of the cost benefit ratio. Various nanomaterials used in construction materials and their benefits are:
– Nano Silica (SiO2) – Nano Silica mixed in concrete can improve mechanical properties, control the degradation of fundamental C-S-H (Calcium-Silicate-Hydrate) reaction of concrete, can block water penetration and therefore lead to improvements in durability.
– Titanium Dioxide (TiO2) Titanium Dioxide nano powder added to concrete can used for its ability to break down dirt or pollution and then allow it to be washed off by rain water on everything from concrete to glass.
– Carbon Nanotube (CNT) addition to concrete can strengthen and monitor concrete. The addition of small amounts (1% wt) of CNT’s can improve the mechanical properties of samples. The oxidized multi-walled nanotubes (MWNT’s) show the best improvements both in compressive strength (+25 N/mm2) and flexural strength (+8 N/mm2).
Grigorij et al.  investigated the use of carbon nanotubes (0.05% by mass, dia 100nm and length 20 m), which is synthesized from aromatic hydrocarbons and used as reinforcement for the production of foam non-autoclave concrete, allows to decrease its heat conductivity up to (12–20) % and to increase its compressive strength up to 70%. The microstructure investigation results have shown a better pore size uniformity in foam concretes containing an admixture of carbon nanotubes. In concretes without carbon nanotubes, due to intensive percolation of wall pores causes them to combine into larger ones, which in turn increases the heat conductivity of the foam concrete and deteriorates its maintenance characteristics. Alireza Fiouz and Sina Saadat  investigated the properties of foamed concrete containing nano-silica (12nm and 1 to 6% of the cement weight), and micro-silica (230nm and 1 to 6% of the cement weight), as well as comparing the properties of these two concretes with the witness concrete. The increase of 1 to 6% of nanosilica particles in foamed concrete contributes to an increase in the amount of concrete compressive strength, as compared with the witness sample. By using the XRD and FT-IR techniques the micro structural characters of the foamed concrete can be studied. Jadvyga Keriene et al.  studied the influence of multi-walled carbon nanotubes (MWCNTs) additive on characteristics of non-autoclaved aerated concrete (NAC) and autoclaved aerated concrete (AAC). It was established that MWCNTs used as additive in NAC and in AAC production process acts as nucleators of crystallization influencing the hydration process and structure formation leading to increase in crystallinity of hardened binding material, as well as to increase in flexural and compressive strength of concretes and decrease in shrinkage during heating.
Nano-Clay Modified Foam Concrete
Attemps have been made to explore the functions of various nano paricles such as nano silica (SiO2), nano alumina (Al2O3), Carbon nanotube (CNT) and nano clay, in the foam concrete. Transmission electron microscopic (TEM) images of the nano particles used for modifying or engineering foam concrete is shown in Fig 3. Statistical analysis have been done to find out the appropiae ratio’s of the mixes based on the setting time, consistency, flowability and bulk properties of the nano moieties incorporated foam concrete. It is found that among all the nano particles used, nanoclay showed better performance in all the aspects. Hence, in this article only nanoclay incorporated foam concrete is discussed. The density of the mix is kept constant as 1350 kg/m3. Only the percentage of nano-clay added to the concrete is varied as 1%, 3%, 5% and 7% by weight of cement. The production of FC with a density of 1350kg/m3 is carried out by using raw materials that consisted of ordinary Portland Cement (OPC) of 53 grade conforming to IS:12269, fine river sand passing through 1.18mm sieve conforming to IS:383, Class F flyash conforming to ASTM C618, potable water, protein based foaming agent and nano-clay as additive.
The mineral class of clay used in this study is montmorollinite. The crytalographic structure and cell parameters of the montmorollinite clay (80-150nm) is shown in Fig 4. The quantity of materials for making 1m3 of concrete of density 1350 kg/m3 is given in Table 1. The water-binder ratio is kept constant as 0.31 and the cement-sand ratio is maintained as 1:0.87 for all the mixes. To the protein foaming agent, prescribed percentage of weight of nano-clay material is dispersed and are mixed thoroughly by ultrasonification. The stable foam with a density of 75 kg/m3 is produced by using the nano-clay modified foam. The LFC cubes and cylinders are casted and are subjected to water curing in room temperature. The specimens are given the names as Mix 1, Mix 2 Mix 3 Mix 4 and Mix 5. The details of the specimen are given in Table 2. The Mix 1 specimen is the control sample (foam concrete sample without nano additive). The Mix 2, Mix 3, Mix 4, and Mix 5 specimens contains nano-clay as additive equal to the weight of 1% 3% 5% and 7% of cement respectively.
Functionalisation of Nano Clay with Foaming Agent
Proper functionalisation methods are required to achieve absolute efficiency in the nano clay admixed foamed cementitious composites. Ultrasonication technique (Fig 5) is used to functionalise the nanoclay with foaming agent since it has many openable-hydroxyl groups in its molecule. These molecules are acting as an complexing agent for the clay moiety. Ultrasonication is the act of applying ultrasound energy to agitate particles in a solution for various purposes. In the laboratory, it is usually achieved through an ultrasonic bath or an ultrasonic probe/horn, known as a sonicator. It is the most frequently used method for nanoparticle dispersion. Required amount of diluted foaming agents (to reduce the viscosity) and nanoclay are taken in beaker, dispersed via hand stirring and kept under ultrasonication for 2 hr. The cut-off ratio to control the overtemperature is kept as 5 min at the break of each 15 min agitation. Ice-bath is used for controlling the agglomeration of individual clay layers due to heat dissipation during operation, which also maintains the solvent viscosity. The well dispersed and functionalsed foaming agent is kept ready for casting.
Hydration plays a very important role in developing the strength of the cement composites, since it is the core reaction for the system. Understanding and interpreting the chemical and micro structural phenomena of different steps of the intrinsic cement hydration process through micro- analytical characterization are quite complex and interdependent. Resolving the individual mechanisms or the parameters which determine the key factor of hydration rate is pretty difficult. Therefore, fundamental studies of hydration offer significant scientific challenges in the experimental techniques. Hence hydration study is very much required to link the strength parameters with fundamental liquid/solid state reaction of cement phases. The mixes as prepared for the strength studies are used for conducting hydration studies. For this study, the samples at 28th day for each mix are collected. Hydration is stopped by pouring acetone on to the samples and they are dried and kept in vaccum dessicator in order to avoid carbonation and moisture attack until the testing day. The above mentioned characterisation are carried out to study the effect of functionalisation of the nanoclay with foaming agent towards foam concrete production.
FT-IR – Functional Group Studies
In order to characterize the functional groups effect in nano modified foam composites, FT-IR studies have been carried out on the corresponding mixes taken at 28th day of hydration and the same have been shown in Fig 6. It is observed from the FT-IR pattern that the appearance of humps in the frequency region of 3800 cm-1 to 3875 cm-1, indicated the effect of chemicaly adsorbed water in C-S-H for Mix 4 and 5. This may be due to the oozing out of extra water due to breaking of foams at early stage. These peaks are absence in other mixes, which shows the hindrence of free movement of -OH groups in these mixes. Compare to control mixes, in all the cases of nano clay modified mixes have shown the appearance of sharp peaks, except for the Mix 5, in the region of 2750cm-1 to 2860cm-1, which details about the functionalised -OH groups (through protein moiety or from clay) of interlayer stacking of alumina moiety of clay CO32- of cement paste (from CaCO3). It directly correlates the adsorption (nucleation/seed formation) of alpha- phase Al2O3 – with the hydration products of cement, which confirms the CASH formation. In Mix 3, the formation rate of calsium silicate hydrates are high at 28th day as it is proven by the appearance of sharp peak at 855 cm-1, the stretching frequency of the silicate moiety, which is a characteristic of C-S-H formation. However, in the other mixes though the appearance of this peaks are visible, it is due to the mixed effect of CH, as the conversion of CH is yet to complete, hence sharp peaks are not obtained.
These studies are conducted for the hydration phases at each intervals for all the mixes and the same are shown in Fig.7 and the corresponding peaks for the hydrated products are highlighted. From the XRD pattern, it is evident that early stage hydration is activated in Mix 3 as confirmed by the peak broadening effect shown at 2 theta of 28o. This peak indicates the formation of portlandite and mixed phases of aluminate bearing groups. The appearance of characteristic semi-crytstalline peaks of C-S-H at 2 theta of 29.5o & 48.6o confirms the early activation of C-S-H in Mix 3 compare to other mixes. The aluminate reaction rate is also high in mix 3 as it is proven by the fast conversion of aluminate and ferrite conatining groups as appeared at 2 theta of 35.6, 38, 40 respectively. However, in Mix 2 the dissolution of gypsum rate is high as the shift in peak at 18.5 is noticed.
Mechanical Strength Characterization
The strength study refers to the mechanical strength of the concrete with and without the presence of nano additive. The compressive and split tensile strengths at the age of 7, 14 and 28 days are determined for all the mixes and are presented in Figs 8 & 9. The modification of foam concrete with 1%, 3%, 5% and 7% by nano clay particles contributes to only a slight increase in the amount of concrete compressive strength and the split tensile strength as compared with the witness sample. It is observed that among all, the Mix 7 with 3% of nano clay has higher compressive and split tensile strength than the other samples. The mixes 6,8,9 has attained low strength as compared to the witness sample. Further, characterisation studies also helps in understanding the reason behind enhanced/reduced mechanical strength for the functionalised foamed concrete.
Scanning Electron Microscope (Sem) Studies
SEM studies are conducted on the nanoclay with foamed cement paste specimens cured under water for 28 days to study the stabilisation of foam by the incorporation of nano clay. The morphological appearances of various percentages of functinalised nanoclay incorporated foam composites are shown in Fig 10 (a to d). It is to be noted in these figures that, more damage for the foam is occured except for Mix 2 and Mix 3 (as indicated). In Mix 2 and 3, many small size bubbles with uniform distribution have been seen and in Mix 2, very few foams are broken (as indicated). In the case of Mix 4 and 5, the damage is occured partly for the former and for the later case a cage like cluster formation is seen, it may be due to the overloading of clay particles (agglomeration), which may induce negative effect if it breaks.
From the experimental findings, the optimum percentage of nanoclay is found to be 3%. The present study can build platform to carry out research on importance of nanoclay in the enhanced performance of modified foam concrete for various applications due to its mineralogical functions.
The authors thank the Director and Advisor (Management), CSIR-Structural Engineering Research Centre, Chennai, for his constant support and encouragement.
1. Ramamurthy, K., Nambiar., E.K.K, and Indu Sivaranjani, G. “A Classification of Studies on Properties of Foam Concrete” Cement and concrete composites, 31(6), 2009, 388-396.
2. Grigorij Yakovlev, Jadvyga Keriene, Albinas Gailius and Ingrida Girniene, Cement based foam concrete reinforced by carbon nanotubes, ISSN 1392-1320 Materials Science, 12(2), 2006, 147-151.
3. Alireza Fiouz and Sina Saadat, Investigations of the foamed concrete properties containing nanosilica as compared with the foamed concrete containing microsilica, SP-289.30, 399-411.
4. Jadvyga Kerience, Modestas Kligys, Antanas LAukaitis, Grigory Yakovlev, Algimantas Spokauskas, Marius Aleknevicius, The influence of multi-walled carbon nanotubes additive on properties of non-autoclaved and autoclaved aerated concretes, Construction and Building Materials, 49, 2013, 527-535.
5. Indian standard:12269, Specification for 53 grade ordinary portland cement, Bureau of Indian Standards, New Delhi, 1987.
6. Indian standard:383, Specification for coarse and fine aggregates from natural sources for concrete, Bureau of Indian Standards, New Delhi, 1970.
7. ASTM, Standard Specification for fly ash and raw or calcined natural pozzolana for use as a mineral admixture in Portland cemen concrete, ASTM C618, Philadelphia, 1989.