EXPERIMENTAL STUDY ON THE MECHANICAL BEHAVIOUR OF FIRED SAND-CLAY AND GLASS POWDER-CLAY BRICKS

Mechanical behaviour of fired bricks containing varied amount of fine sand (FS) and waste glass powder (GP) was investigated. FS and GP were added to bricks at varied amount of 0, 5, 10, 15, 20, 25, 30, 35 and 40 wt. %. Firing was done at 1200 oC and samples produced were evaluated for compressive and flexural strengths while microstructural analyses of 25 wt. % FS and GP-clay bricks were examined. Results showed that compressive strength was highest at 30 wt. % GP for GP-bricks while for FS-clay bricks, compressive strength rose to 11.4 and 12.8, at 35 and 40 wt. % FS addition. Flexural strength for GP-clay and FS-clay bricks peaked at 30 wt. % GP (3.63 MPa) and 40 wt. % FS (2.45) respectively. Flexural modulus increased progressively and exponentially as FS and GP proportion increased. Work done in resisting deformation and deflection during bending reduced with increased amount in both additives. Flexural strain was inversely related to load and stiffness. In conclusion, addition of GP and FS in increasing amount resulted in improved mechanical properties in the bricks. Also, increased proportion of GP and FS was found to improve response to loading in fired bricks.


INTRODUCTION
Ceramics are inorganic materials made of metal and non-metal compounds and are composed of silica, alumina, magnesia, zirconia, and other compounds. Properties of these materials include resistance to corrosion and chemical attack due to their inert nature, poor conduction of heat and electricity and high compressive strength. They are also hard, brittle and heavy with poor tensile properties [1][2][3]. Fired clay are ceramics which are made hard by firing, while unfired clay ceramics are made hard by sun drying or oven drying. Concrete or cement bricks are a form of ceramic products which are made strong by the addition of cement followed by further curing [4]. Oven drying is done on green ceramic body at 110 o C for water removal and decomposition of some organic elements present. Firing process involves exposure of ceramic body to high temperature for a period of time to enhance hardness, strength and other properties [5][6][7]. The process of sintering enhances bond within particles leading to improved properties [8]. Firing of clay is undergone in the production of potteries, wares, roof tiles and bricks. Properties of fired bricks include porosity, shrinkage, density, and strength. Clay in its raw form is porous which affects strength and density in the sense that higher porosity leads to reduced strength [7,9]. For structural application, reduced porosity in bricks is necessary in order to ensure structural integrity of buildings. Reduced porosity in bricks results in reduced inter particle distance leading to enhanced bond between particles [10]. The process of reducing porosity in fired clay body involves the incorporation of additives like waste glass powder/shavings, eggshell powder, silica nano particles [11] and other additives. Adding of eggshell as biofillers to fired clay bricks [12] was noted to produce fired bricks of high compressive strength and hardness, good thermal expansion coefficient and lower water absorption at 25 wt. % eggshell powder addition. Addition of waste glass, was recorded to reduce porosity and water absorption while increased compressive strength was noted, when waste glass powder was added in increasing proportion of 0, 10, 20, 30, and 40 wt.% [13,14]. In this study, waste glass powder and fine sand were added to fired bricks and comparison was made on mechanical behaviour of such bricks at varied proportion addition; for application in masonry.

Materials Preparation
Materials used include sand, glass bottles and clay. The sand was obtained from a stream; washed and sun dried for 3 days. Clay used was excavated from a depth of 1.5 m in a borrow pit in Aule, Ondo State, Nigeria. Water was added to the clay, stirred and left undisturbed for two days. The water was poured off leaving behind the clay. Fresh water was added, stirred and left undisturbed for another two days and the water was poured off while the left over clay was spread in a cotton material and allowed to sun dry for 7 days. Dried clay lumps were broken into smaller pieces, followed by crushing and milling. Waste glass bottles (bottles of soft drinks and alcoholic drinks) were bought from a shop where waste glass products were sold for recycling. The bottles were washed and sun dried for one day followed by crushing and milling before sieving. The sand, waste glass and milled clay were sieved using an electric sieve shaker (Model RX 29) which top sieve has an aperture of 4750 µm. Clay (sieved to 300 µm), glass powder (GP) and fine sand (FS) which were sieved to -150 µm were collected and used for sample preparation.

Sample Preparation
Two groups of samples were prepared: fine sand-clay (FS-clay bricks) samples and glass powder-clay (GP-clay bricks) samples. FS-clay brick samples contained sand at varying proportion of 0, 5, 10, 15, 20, 25, 30, 35 and 40 wt. % of fine sand while GP-clay bricks samples contained the same varying proportion of GP (Table 1). Clay was mixed with water and the additives in a mechanical mixer and the slurry moulded into shape using compression moulding machine at 10 MPa. Water was added during mixture at water to clay ratio of 7:20. The green bricks produced were left undisturbed for 24 hours after which they were oven dried for 12 hours at a temperature of 110 o C in order to remove moisture and other volatile content. This was followed by firing in an electric furnace at 5 o C/min until 1200 o C was attained. The temperature was maintained for 4 hours before allowing samples to cool to room temperature in the furnace. Bricks 150 mm × 150 mm × 150 mm and 400 mm x 100 mm x 100 mm were produced for this study.

Preliminary test on materials used
Tests were carried out to examine the specific gravity, bulk density and moisture content of sand and clay (in as received condition) as well as glass powder (after sieving) in line with existing procedure stated in Table 2. Sieving was done out on the materials used as per [15,16]. Also, chemical composition of the materials were analysed and the results presented in Table 4.

Tests on brick samples
Compressive strength Compressive strength test was carried out on each sample to determine its load bearing capacity in line with [17] procedure. The brick samples (150 mm × 150 mm × 150 mm) were initially oven dried at 110 o C until a constant mass was attained and tested using a universal testing machine (TBTUTM-600). The samples were placed flat horizontally between the plates of the machine and a load of 10 kg/min was applied. The maximum load at failure was recorded and the compressive strength calculated using the expression in Equation 1.

Compressive strength (MPa) =
Maximum load at fracture Cross sectional area

Flexural strength
This strength evaluates the ability of bricks to resist deformation by bending and was carried out on samples (400 mm x 100 mm x 100 mm) immediately after cooling to room temperature. The test was done in line with [18] with a loading rate of 15 kg/min and the result evaluated using Equation (2).
Flexural strength (MPa) = 3Fh/2bd 2 (2.) Where F is the maximum load at fracture, h is the length of the support plan/length between supports; b is the width of the sample, d is the thickness/depth of the sample. Table 2 showed the physical properties of materials used which was in consistent with works reported by [19][20][21].   Table 3 highlights the chemical composition of materials used. Silica content is higher in the materials and from evaluation made 70% of the mix materials has silica content.  [13,15] and [22] in terms of proportion of waste glass where maximum compressive strength was attained. According to [15] where 900 o C was employed in firing, maximum compressive strength was attained at 50 wt. % waste glass powder addition, though in present study, maximum strength was recorded at 30 wt. % GP. Authors [23] reported on fired clay samples at 1100 o C and a maximum compressive strength was recorded at 40 wt. % added content of milled glass (sieved to -100 µm) and further addition resulted in reduced strength. However, report from [13] attained maximum compressive strength at 1000 o C for 40 wt. % of waste glass (sieved to -150 µm) while at 1100 o C, maximum compressive strength was attained at 30 wt. % addition of waste glass. Further increase in waste glass content resulted in reduction in compressive strength. As waste glass proportion increased in the samples, there was increased glassy phase. However, as this glass phases expands, brittleness increased [13]. This explains the reduction in compressive strength beyond some certain proportion of waste glass addition. It can be deduced that when attaining ≥30 wt. % of waste glass powder, compressive strength reduces, though depends on firing temperature and sieve fraction of glass powder.

Flexural modulus and strain
Further analysis involves evaluation of flexural modulus. The deflection exhibited during the test for flexural strength was measured and recorded and the flexural strain evaluated using Equation (3).
Flexural strain (α) = 6dt/L 2 (3.) Where d was the recorded deflection (mm), t was thickness of sample and L is the distance between two supports. Flexural Modulus (GPa) was evaluated as flexural strength divided by flexural strain, while work done in resisting deformation during deflection (Nm) was evaluated by multiplying maximum load at failure (N) by deflection (mm). Table 4 shows the average deflection, work done and flexural Modulus for Fine Sand (FS). Table 5 shows the average deflection, work done and flexural Modulus for Glass Powder (GP).    As glass powder and sand content increased, flexural modulus increased (Fig. 4a), average deflection decreased leading to continuous decrease in strain. This resulted into progressive increase in flexural modulus, indicating increase in stiffness as glass powder and sand content amount increased. The strong bond formed between particles of clay and fine sand amounted to increased stiffness and rigidity. Increased compactment and strong adhesion between GP-clay bricks particles further enhanced resistance to bending in samples with GP. The strong glassy phase formed in samples containing GP further enhanced the resistance to deflection in GP-clay bricks samples, leading to high degree of flexural modulus compared with FSclay samples. Flexural moduli for both samples (GP-clay and FS-clay bricks) were almost the same from 0 wt. % of to 15 wt. % addition of the additives. However, at 20% addition, the difference was becoming clearer. Between 20 wt. % and 25 wt. % addition of GP, there was a large increment of almost 91% in flexural modulus of GP-clay bricks compared to 51% in FSclay bricks. As the content of the additives increased to 30, 35 and 40 wt. %, progressive increase in flexural modulus was 101%, 32% and 41% respectively for GP-Clay bricks, while in the case of FS-clay bricks, the progressive increase was 62%, 32% and 26% respectively. The flexural modulus-curve was exponential and progressive for GP-clay bricks and FS-clay bricks. Percentage increment of flexural modulus, in GP-clay was much higher than that of FS-sand, as a result of increased strong glass phase formed in the samples as GP content increased, which further complemented the bond formed between GP and clay particles. From the Fig. 4a, addition of ≥ 25% of both additives resulted in significant resistance to bending in bricks. Flexural stiffness (Fig. 4b) also followed the same trend for both FS and GP-clay bricks in that at 20 wt. % content of the additives, stiffness increased exponentially. Fig. 5 shows the downward movement of the work done in resisting deflection at maximum load application for both GPclay and FS-clay bricks. This is attributed to increased resistance to deflection in the samples as content of both GP and FS were increasing. The additives were effective in the reduction of work done in resisting deflection. Work done in GPclay bricks is lower than that of FS-clay bricks except at 10 wt. % of the additives where it's vice versa. This implies that as additives increased in the sample, the work done against load applied in causing deflection reduced, indicating high resistance to deflection. Comparing the two forms of bricks, work done against load in GP-clay bricks is lower than that of FS-clay bricks for each mix proportion (except for 10 wt. %), indicating, there is higher resistance to load in GP-clay bricks than in FS-clay bricks.

Glass powder
Fine sand in load applied was lower compared to compositions below 30 wt. % which also resulted into lower reduction in strain experienced at ≥ 25 wt. % FS addition owing to increased compactment in the samples. As load applied lied between 17 and 45 N, strain at failure was reducing ( Fig. 6(b)). There was a 20% reduction in strain between 15 wt. % of GP and 20 wt. % of GP with a corresponding 4.4% increase in load to failure. At ≥ 20 wt. % GP, there was shrinkage in strain which resulted into corresponding lower percentage increase in maximum load to fracture for samples. Generally, with increased load, strain at fracture reduced for both forms of bricks, due to reduced work done in resisting deflection as FS and GP contents increased in the samples. In Fig. 6b, highest load was recorded at 30 wt. % of GP with a resulting strain of 0.009. At 35 and 40 wt. % of GP, strain further reduced to 0.006 and 0.004 respectively, which was a further reduction of 33.3 and 55.6 % respectively, owing to higher value of stiffness.

Flexural strain against stiffness for brick samples
The representative plots showing the flexural strain against flexural stiffness for FS-clay bricks and GP bricks are as shown in Fig. 7 (a) and (b) respectively. From Fig. 7 (a) and (b), flexural strain reduced with increased stiffness for both type of bricks. This is due to increased strength induced as composition of FS and GP increased in samples. Flexural strain was higher in FS-clay bricks than GP-clay bricks, while flexural stiffness was higher in GP-bricks than FS bricks. This can be attributed to the fineness of GP, with fineness modulus of 1.50, which is lower than that of sand (fineness modulus of 2.26). A finer particle of additive contributes to strength improvement in bricks [13].

CONCLUSIONS
Fine sand and glass powder were added to fired bricks at varied proportion of 5 wt. % to 40 wt. %, and it was concluded that the incorporation of the two additives enhanced the compressive and flexural strengths, and mechanical response to loading. Addition of glass powder up to 30 wt. % gave maximum compressive and flexural strengths; further addition may result in reduction in the strengths. Addition of fine sand sieved to -150 µm at ≥ 15 wt. %, improved mechanical properties of fired bricks at increased proportion. Therefore, addition of glass powder and sand can improve properties of fired bricks for structural application, though glass powder proved to be more effective.