EFFECTS OF ORDINARY PORTLAND CEMENT BENTONITE BLEND ON COMPRESSIVE STRENGTH OF CONCRETE MIXES USING

1

30% bentonite blend 1:3:6
Days
Compressive strength (N/mm2)
30% bentonite blend 1:1.5:3
Days
ompressive strength (N/mm2)
Days
EFFECTS OF ORDINARY PORTLAND CEMENT-BENTONITE BLEND ON COMPRESSIVE STRENGTH OF CONCRETE MIXES USING
19mm SIZE COARSE AGGREGATE
BY
SERIKI, OLUWASEGUN OLUWASEYI
(CVE/07/0368)

SUBMITTED TO
THE CIVIL ENGINEERING DEPARTMENT,
FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE, ONDO STATE

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF BACHELOR OF ENGINEERING (B. Eng.) IN CIVIL ENGINEERING

AUGUST 2011.
CHAPTER ONE
1.0 INTRODUCTION
Revolutionary developments relating to novel materials for concrete production and to modifications and improvements in the behaviour of traditional materials have been taking place in the past two decades. These developments have been facilitated by increased knowledge of the atomic and molecular structure of materials, studies of long-term failures, the development of more powerful instrumentation and monitoring techniques, decreases in the cost-effectiveness of traditional materials, and the need for stronger and better performing materials suitable for larger structures and longer spans, as well as for increased ductility. (Nawy, E. G., 1996)

The construction industry has taken considerable strides forward over the last two or three decades with regard to trials in the use of one or another cementitious materials generally identified as pozzolans, for the compounding of various cement based products. This have not only resulted in improving the compressive strength value attained thereby but also in qualities like ability to set and harden under water. Among these coal fly-ash, blast furnace slag, rice hull ash, silica fume, or metakaolin are the most common ones. Other like gypsum, gypsum fines, Portland cement, cement kiln dust, lime dust, stone dust, and calcined clay are also in use.

Due to economic and environmental concerns, different methods of making cement products are being considered. One method to achieve the goal of reducing carbon dioxide emissions and greenhouse gases is to formulate cements using a lower portion of calcinated material, thereby reducing carbon dioxide emissions per unit of product. Another approach is that of including a lower percentage of cement and /or gypsum than it is common with standard cement or gypsum and to ensure an increased compressive strength and/or flexural strength is yet attained thereby. This as one which is durable, and suitable for all types of applications, also benefits the environment. Additionally, a need exists for improved cement and gypsum products that permit the use of less expensive aggregates to reduce the cost of the cement product.
Bentonite is a form of calcined clay (i.e. clay that has gone through heat process to be in its powder form) that consists of a primary mineral called montmorillonite which gives it its properties. Calcined clay seems to have the greatest overall potential as alternative pozzolanic material for concrete due to its availability in large quanta and the relatively cheap price. Though the mineralogy of clays varies a lot, which may influence the reactivity, its interaction with CSH gel formed during ordinary Portland cement has been found beneficial to the final form of the hardened concrete. The benefit of it being used as partial replacement of a portion of the ordinary Portland cement has been found not only on strength improvement, but more on durability enhancement.(Detwiler et al., 2001)
1.1 JUSTIFICATION FOR THE STUDY
Bentonite as blend with ordinary Portland cement is a review of the use of supplementary cementitious materials in concrete. Environmental concerns both in terms of damages caused by the extraction of raw material and CO2 emissions during cement manufacture, have brought about pressures to reduce cement consumption by use of supplementary materials. In addressing environmental problems and economic advantages, mixtures of Portland cement (PC) and pozzolan are very commonly used in concrete production [Sabir et al., 2001]. This study is therefore to investigate the range of ordinary Portland cement – bentonite mix proportion that can be found useful for particular use in the construction industry.
1.2 AIM AND OBJECTIVES
The research study was aimed at the following;
1. Study the effects of bentonite on the rheology of fresh concrete.
2. Evaluation of the effects of ordinary Portland cement - bentonite blend in various mix proportions on the compressive strength of concrete.

1.3 SCOPE OF THE STUDY
This research work will be limited to the effect of bentonite when it is used to replace a portion of ordinary Portland cement in a prescribed mix as related to hydration properties, workability and compressive strength properties of resulting concrete only. Comparisons of the blended mixes would be made with ordinary Portland cement mixes [as a control at the chosen water-cement ratios].

2.0 LITERATURE REVIEW
2.1 POZZOLANS
2.1.1 DEFINITIONS, TERMS AND CLASSIFICATION OF NATURAL POZZOLANS
The definition of natural pozzolans according to ASTM Standard C618 is siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. Within the building industry, the term "pozzolan" covers all the materials which react with lime and water giving calcium silicate and aluminate hydrates. All pozzolans have to be rich in reactive silica or alumina plus silica [Bensted and Barnes, 2002]. According to ASTM standard C618 the requirements as to chemical composition of natural pozzolan are approximately 70 % in contents of silicon dioxide (SiO2), aluminium oxide (Al2O) and iron oxide (Fe2O3) of which is present in bentonite structure. The loss on ignition is required to be a maximum of 10 %, according to the same standard. The acidic and amphoteric oxides (silica, alumina and ferric oxide) contents vary widely from one pozzolan to another. Silica is a major component in natural and processed pozzolans.
Pozzolans are classified into natural materials and artificial. Artificial ones are mostly industrial by-products. Bentonite needs to be calcined from its crystalline form for better performance. According to Souza and Molin (2005), calcination (also referred to as calcining) is the thermal treatment process applied to ores and other solid materials in order to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials. Calcination is to be distinguished from roasting, in which more complex gas-solid reactions take place between the furnace atmosphere and the solids.
2.2 DELETERIOUS SUBSTANCES IN AGGREGATES
According to Goldbeck, A. T (1933), there are three categories of deleterious substances that may be found in aggregates:
Impurities which interfere with the processes of hydration of cement.
Coatings preventing the development of good bond between aggregate and the cement paste.
Certain individual particles which are weak or unsound in themselves.

2.2.1 CLAY AND OTHER FINE MATERIALS
Clay is a fine grained material consisting mainly of aluminium silicates that occur naturally in soils and sedimentary rocks (Microsoft® Encarta® 2009. © 1993-2008). Clays are rarely found separately and are usually mixed with other clays and with microscopic crystals of carbonates, feldspars, micas and quarts. Clay minerals are divided into four major groups: kaolinite, montmorillonite/smectite, illite and chlorite groups. The compositions of clay minerals depend on geographic area and the bedrock, and vary a lot all over the world.
Clay may be present in aggregates in the form of surface coatings which interfere with the bond between aggregate and the cement paste. Good bond is essential to ensure a satisfactory strength and durability of concrete. Other fine materials which can be present in aggregate, silt and crusher dust can be removed by washing the aggregate.

2.2.2 ORGANIC IMPURITIES
Natural aggregate may be sufficiently strong and resistance to wear and yet they may not be satisfactory for concrete-making if they contain organic impurities which interfere with the chemical reactions of hydration. Materials such as decay of vegetable matter, organic loam are more likely to be present in sand than in coarse aggregate, which is easily washed.
2.3 CONCRETE PROPERTIES
Concrete, as a structural member has to pass through two stages, plastic stage and the hardened stage. The properties of both plastic and the hardened concrete are to utmost importance to the concrete user.
2.3.1 PROPERTIES IN THE PLASTIC STATE
The properties of concrete in the plastic state refer to the properties the concrete exhibits when it is freshly prepared. These properties are evident from the time of mixing to the time when the concrete has hardened due to the hydration and setting of the cement.
SLUMP: is an indication of the relative consistency of the plastic concrete. Concrete of plastic consistency does not crumble but flows sluggishly without segregation.
WORKABILITY: Is a measure of the ease of placing, consolidating and finishing of the freshly mixed concrete. Concrete should be workable but must not segregate or bleed excessively.
SETTING TIMES: Knowledge of the rate of reaction between cementing materials and water is important to determine a concrete mixes` setting time and hardening. The setting times of concrete mixtures do not correlate directly with the setting times of the cement paste because of water loss and temperature differences.
SEGREGATION: This can be defined as the isolation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform.
BLEEDING: According to Neville and Brooks (1996), bleeding is also known as water gain. It is a forms of segregation in which some of the water in the mix tends to rise to the surface of the freshly placed concrete.
2.3.2 PROPERTIES OF CONCRETE IN ITS HARDENED STATE
Strength
Compressive strength
Tensile strength
Flexural strength
Bond strength
Durability
Resistance to freezing and thawing
Cracking
Rebar corrosion
Internal problems
Creep
Shrinkage

2.4 BENTONITE
Bentonite is a geological extract found freely in its natural state. Bentonite is formed out of volcanic ash. It is a form of clay that consists of a primary mineral called montmorillonite that gives it its properties. Montmorillonite is a dual-layered, dual-dimensional mineral that has aluminium and silicate. These minerals give bentonite a layer of cards that look like crystalline packets and are called platelets (MSDS, Properties, Technology, Ullagaram, India.)
According to Wikipedia, for industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite.

2.4.1 SODIUM BENTONITE
Sodium bentonite expands when wet, absorbing as much as several times its dry mass in water. Because of its excellent colloidal properties, it is often used in drilling mud for oil and gas wells and for geotechnical and environmental investigations.
The property of swelling also makes sodium bentonite useful as a sealant, especially for the sealing of subsurface disposal systems for spent nuclear fuel and for quarantining metal pollutants of groundwater. Similar uses include making slurry walls, waterproofing of below-grade walls, and forming other impermeable barriers, e.g., to seal off the annulus of a water well, to plug old wells, or to line the base of landfills to prevent migration of leachate. Sodium bentonite can also be "sandwiched" between synthetic materials to create geo-synthetic clay liners (GCL) for the aforementioned purposes. This technique allows for more convenient transport and installation, and it greatly reduces the volume of sodium bentonite required.
Various surface modifications to sodium bentonite improve some rheological or sealing performance in geo-environmental applications, for example, the addition of polymers.
2.4.2 CALCIUM BENTONITE
Calcium bentonite in Adelaide, Australia is a useful adsorbent of ions in solution, as well as fats and oils, being a main active ingredient of fuller's earth, probably one of the earliest industrial cleaning agents. Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite's properties by a process known as "ion exchange" (patented in 1935 by Germans U Hofmann and K Endell). In common usage, this means adding 5–10% of a soluble sodium salt such as sodium carbonate to wet bentonite, mixing well, and allowing time for the ion exchange to take place and water to remove the exchanged calcium. Some properties, such as viscosity and fluid loss of suspensions, of sodium-beneficiated calcium bentonite (or sodium-activated bentonite) may not be fully equivalent to those of natural sodium bentonite. For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geo-synthetic liners.

2.5 PROPERTIES OF BENTONITE
2.5.1 PHYSICAL PROPERTIES
As a form of sedimentary clay that is formed out of volcanic ash, Bentonite has a highly viscous, gel-like structure. This makes it ideal for use in many industries. It is primarily used in its physical state in the wine industry. Because of its physical properties, it is a useful binder in wine. When hydrated, it can change its molecular and electrical properties. This makes it porous, and useful as a binding agent.
2.5.2 CHEMICAL PROPERTIES
Bentonite is a source of many useful chemicals. As a mineral, it is primarily composed of silica, which makes up almost two thirds of its substance. It also has smaller proportions of other minerals in varying ratios: Silicon 29.8%; Aluminium 10.6%; Iron 2.6%; Magnesium 1.4%; Calcium 0.69%; Sodium 1.80%; Potassium 0.42%; Hydrogen 1.19%; Oxygen 51.5%.These minerals can be extracted from bentonite and used in various applications.
2.6 EFFECT OF PAKISTANI BENTONITE ON PROPERTIES OF CONCRETE
Ahmed et al (www.aggregate research.com, 2011) evaluated a Pakistani bentonite (from Jehangira, Swabi District) as a cement replacement material in concrete, the ordinary Portland cement concrete was replaced with the bentonite blend at 10%, 20%, 30%, 40% and 50% by cement mass although the material type was not specified. The strength activity index of bentonite was determined as received (20°C) and heated (treatment at 500°C and 900°C). The strength activity index of bentonite conformed to the ASTM Standard C618 specifications, except for the 900°C heated bentonite.
In wider studies, Ahmed et al (2011) also found the water adsorption, for mortars containing up to 30% bentonite blend, first decreased for mortar containing up to 30% bentonite and then steadily increased at higher bentonite loadings. When immersed in 5% Na2SO4and 2% MgSO4 solution, the greatest compressive strength was observed for mortar containing 30% bentonite. When finally the concrete mixes, as constituted in the preceding paragraph was investigated, the water demand was found to increase with increasing bentonite content. Although the compressive strength of concrete decreased progressively as the substitution level of bentonite was increased, the compressive strength of concrete containing 30% `as received' bentonite was found to be 70% of the control concrete, whereas for concrete containing 30% 500°C heated bentonite, the strength was found to be 79% of the control concrete. It can be concluded that this Pakistani bentonite can be used to replace up to 30%of cement to produce concrete with sufficient compressive strength for low-cost construction resistant to sulphate attack.

CHAPTER 3
3.0 MATERIALS AND METHODOLOGY
3.1 CONCRETE TECHNOLOGY
Concrete is the product of mixing, aggregate, cement and water. The setting of concrete is a chemical reaction between the cement and the water, not a drying process. This reaction is called hydration, it evolves heat as does any chemical reaction, and the process is irreversible. There is an initial set when the concrete will cease to be liquid but have little strength (e.g. 6 to 24hrs. old), thereafter the concrete will gradually gain strength over time until it achieves the strength required. Differing mix proportions and cement types will achieve required strengths in differing time spans.
3.1.1 CONSTITUENTS OF CONCRETE
3.1.1.1 AGGREGATES
Aggregates are usually distinguished between fine and coarse aggregate.
Aggregates used were natural sand (fine) and 19mm size of granite as coarse aggregate.
The aggregate used had minimum inherent strength requirement for structural concrete. (Neville A.M., 1973) The fine aggregate used was sieved and all debris was removed.
3.1.1.2 CEMENT
Cement used was the local ordinary Portland cement, purchased in Akure, Ondo State.
3.1.1.3 BENTONITE
The bentonite used was a sodium type, manufactured by Changsha May Shine Chemical Co., Ltd [Province: Hunan, China] but purchased from Lagos.

3.1.1.4 WATER
Clean water from an approved source free from impurities was used.
3.2 EXPERIMENTAL WORKS
3.2.1 WET SIEVING
This type of sieving is resorted to because clay or fine dust adhering to larger particles would not be separated in ordinary dry sieving.
Washing of aggregate could either be carried out manually (hand) or mechanically. Washing requires water, sieve (No 200) and container. The washing was carried out manually or simply by hand. The sand sample was initially weighed with the aid of the weighing balance. Quantifiable mass of sand was collected, all unwanted materials were hand-picked and oven dried. A 500g of oven dried sample was collected, soaked with water, thoroughly stirred and left for a day to allow for settlement. The sample was thoroughly washed until all traces of silt/ clay removed which was confirmed by visual inspection. The sample was oven dried and weighed to determine the weight of silt or clay initially present.
3.2.2 MOISTURE CONTENT
The moisture content was carried out for the fine aggregate. It is necessary so that allowance can be made for the same when adding water to a concrete mix.
Procedure: An empty can for the sample was weighed and recorded as W1 and a 50g of the sample was collected into the can with the aid of spatula, weighed and recorded as W2. The W2 sample was placed in an oven of 120oc for 30 minutes. It was collected for weighing after it had cooled and recorded as W3. It is mostly expressed in percentage and used as decimal in calculation.
M.C=W2-W3 (W3-W1) X 100%

Where W1 = Weight of empty can.
W2 = Weight of can + wet sample
W3 = Weight of can + dry sample
3.2.3 SLUMP TEST ON FRESH CONCRETE
Workability refers to the ease with which concrete can be placed and compacted (Neville A.M 1973). In this study, slump test is the method used to assess the above. Since workability of a given concrete increases with the amount of water added, the slump test provided a means of controlling the water content of successive batched of the same mix. The mould for the slump test was a frustum of a cone, 300mm high. It was placed on a smooth surface with the smaller opening at the top, and concrete was filled in three layers. Each layer was tamped 25 times with a standard 16mm diameter steel rod, rounded at the end, and the top surface was smoothened with a trowel. The mould was firmly held against its base during the entire operation. Immediately after filling, the cone was slowly lifted, and the unsupported concrete slump. The decrease in the height of the highest part of the slumped concrete is called slump and was measured by a ruler.
3.2.4 PREPARATION OF CONCRETE CUBE
Specimens were cast in wooden mould of 150x150x150mm cube. A thin layer of oil was applied to the inside surfaces of the mould in order to prevent the development of bond between the mould and the concrete. The mould was filled in three layers and each layer receiving 25 blows from a 16mm diameter size tamping rod to ensure full compaction. After compaction, excess concrete were removed and the surface was dressed by means of a trowel. Then, the cube was stored undisturbed for about 24 hours at room temperature. At the end of this period, the mould was stripped and the cube was further cured in water.

PLATE 3.2.4 CONCRETE MOULD BEFORE & AFTER CASTING
3.2.5 CURING
The curing carried out for the various ages viz 7, 14, 21and 28 days, by soaking cubes in metallic tanks into which water has been poured. This however was not done for the mix with bentonite only which curing was done by sprinkling with water regularly. It was then covered by cement bag to shield same from direct sunlight avoid excessive evaporation; which would affect its strength development.

3.2.6 COMPRESSIVE STRENGTH
The compressive strength is usually obtained experimentally by the means of compression test equipment. The standard cubes were tested at the prescribed ages used for the study at 7, 14, 21 and 28 days.
In compression test, the cube was placed with the cast faces in contact with the platens of the testing machine, i.e. the position of cube when tested is at right angles to the as-cast. According to B.S. 1970, the load on the cube was applied at the rate of 15MN/m2/min, because of the non-linearity of the stress/strain relationship for concrete at high stresses; the rate of increase in strain increased progressively as failure approach.
The compressive strength can thus be expressed as,
COMPRESSIVE STRENGTH= CRUSHING LOAD (N)AREA OF CUBE (mm2)

CHAPTER FOUR
4.0 RESULTS AND DISCUSSIONS
4.1 RESULTS OF LABORATORY TESTS
4.1.1 Aggregates: Gradation of Aggregates used
Fine aggregate
The gradation of the particles in the fine aggregate was determined using the wet sieving method and the result is shown below.
Table 4.1.1 The results for wet sieving for the 500g collected oven dried sample
Sieve no
Diameter (mm)
Mass Retained
(g)
% retained
% passing
7
2.36
3.4
0.75
99.25
10
1.70
1.4
0.34
98.91
14
1.18
7.5
1.79
97.12
25
0.60
40.9
9.78
87.34
31
0.50
76.4
10.26
77.08
36
0.425
5.3
1.27
75.81
72
0.212
161.8
38.67
37.14
100
0.150
77.2
18.45
18.69
200
0.075
40.5
9.68
9.01

Pan
36.5
8.72
0
TOTAL
450.9

Weight of oven dried Sample =500g
Weight of mass retained =450.9g
% OFCLAYSILT=500-450.9 500 X 100%

This shows the sample contains 9.82% of silt /clay fractions. This has been washed off due to the effect it could have on the concrete mix.

Coarse aggregate
The particle size distribution test was carried out on coarse aggregates and the results are as shown below.
Mass of aggregate tested = 2000g
Diameter(mm)
Mass retained(g)
% Retained
% Passing
25
100
5
95
20
1250
62.5
32.5
14
450
22.5
10
12
150
7.5
2.5
PAN
50
2.5
0

The particle size distribution curve of the fine aggregate is as shown in the Figure 4.1.1

Fig 4.1.1: Combined Particle size distribution graph for fine and coarse aggregates
Comments
Fine aggregate parameters
Effective size: This is the diameter in a particle size distribution curve corresponding to 10% finer and is denoted as D10.
D10 = 0.08mm (from Particle size distribution curve)
Uniformity co-efficient: This is denoted by Cu and is expressed as
Cu=D60D10
Where D60 = Diameter corresponding to 60% finer =0.22(from PSDC)
D10 = Effective size = 0.08, Therefore, Cu = 2.75
Co-efficient of Gradation: This is denoted by Cc and is given by
Cc= D302 / (D60 X D10)
Where D30= Diameter corresponding to 30% finer
From the Particle size distribution curve, D30 = 0.19 and from values of D60 and D10 above, Cc= 2.051 < 3.
Since the co-efficient of gradation is less than 3, it conforms to the limit given in the BS 1377: PART 2; 1990 for well- graded soils. Hence, the fine aggregate can be said to be well graded.
Coarse aggregate parameters: Also, from Figure 4.1.1, we obtain the following:
Effective size: This is the diameter in a particle size distribution curve corresponding to 10% finer and is denoted as D10.
D10 = 14.0mm (from Particle size distribution curve)
Uniformity co-efficient: This is denoted by Cu and is expressed as
Cu=D60D10
Where D60 = Diameter corresponding to 60% finer =20.5mm (from PSDC)
D10 = Effective size = 14.0mm, Therefore, Cu = 1.46 < 4
Co-efficient of Gradation: This is denoted by Cc and is given by
Cc= D302 / (D60 X D10)
Where D30= Diameter corresponding to 30% finer
From the Particle size distribution curve, D30 = 20.0mm and from values of D60 and D10 above, Cc= 1.4 < 3.
From the result obtained, according to BS 1377: PART 2; 1990, the uniformity coefficient shows the aggregate used is gap graded.
4.1.2 MOISTURE CONTENT TEST RESULTS
The moisture content of the fine aggregate (natural sand) gives the percentage of the water content in the soil sample.
Table 4.1.2 Results of moisture content test carried out
Weight of Wet Sample = 50g
Description
Notation
Weight (g)
Weight of container
W1
39.6
Weight of container + wet sample
W2
89.6
Weight of container + oven dried sample
W3
85.9
The moisture content was calculated from
M.C=W2-W3 (W3-W1) X 100%
=(89.6-85.9)(85.9-39.6) X 100%
= 7.99%
As by this result, there was a deliberate reduction of 8% of the volume of water during the proportioning of each mix.
4.1.3 SLUMP TEST RESULTS
Table 4.1.3 shows the slump value for all the concrete mixes
MIX DESIGN
WATER-CEMENT RATIO
VARIATION
SLUMP VALUE(mm)

1:2:4

0.7
Cement control
10% bentonite blend
20% bentonite blend
30% bentonite blend
50
100
30
20

0.8
Cement control
10% bentonite blend
20% bentonite blend
30% bentonite blend
60
90
110
100

1:3:6

0.7
Cement control
10% bentonite blend
20% bentonite blend
30% bentonite blend
80
10
15
5

0.8
Cement control
10% bentonite blend
20% bentonite blend
30% bentonite blend
90
60
20
10

1:1.5:3

0.7
Cement control
10% bentonite blend
20% bentonite blend
30% bentonite blend
150
140
140
20

0.8
Cement control
10% bentonite blend
20% bentonite blend
30% bentonite blend
150
150
150
30

COMPARISON OF VARIANT MIXES ON BASIS OF SLUMP
Slump of concrete at same w/c ratio for individual mixes

Fig. 4.1.3A Slump comparison for mix 1:2:4, water- cement=0.7
The slump value for this mix increases from the cement control mix to the 10% bentonite blend, and thereafter drops sharply for other mixes. Although generally it is believed that the slump value should progressively decrease from the cement control mix to the 30% bentonite due to the introduction of bentonite which increases hydration rate but factors such as weather conditions and mixing could have affected the result obtained.

Fig. 4.1.3B Slump comparison for mix 1:2:4, water-cement =0.8
The slump value for this mix increase from the cement control mix to the 20% bentonite and drop a bit at 30% bentonite. In correlation with the previous mix, increase in water cement ratio increased the slump value up to 20% bentonite blend, but at 30% its workability is reduced.
We can readily see that in the mix 1:2:4, the 10% bentonite blend is the most desirable due to its increased workability in both water-cement ratio variants (i.e. 0.7 and 0.8). Thus, when considering workability in the mix 1:2:4, the 10% bentonite blend is the recommended variant for use and it is also economical as lower cement will be used in the mix, when compared with the control.

Fig. 4.1.3C Slump comparison for mix 1:3:6, water- cement=0.7
Slump value dropped sharply for 10% bentonite blend and fluctuates between the 10% and 30% bentonite blend. This could be as a result of high ratio of mono-size aggregate with large pore sizes which allows free movement of water through the pore spaces, thereby reducing the water available for proper mixing and encourages the swelling of bentonite to form gel in the voids.

Fig. 4.1.3D Slump comparison for mix 1:3:6, water- cement=0.8
In Fig. 4.1.3.1.1D, the slump value decreased from the cement control mix to the 30% bentonite blend. This may be as a result of reason given for the previous same mix with different water cement ratio. It could also be as a result of the aggregate-cement ratio.
It can be inferred from the values shown in Fig. 4.1.3C and Fig. 4.1.3D, that the presence of more aggregates which are predominantly single-sized has affected the slump values hence we have reduced workability in the mixes.

Fig. 4.1.3E Slump comparison for mix 1:1.5:3, water- cement=0.7
The slump value decreased at 10% bentonite blend and maintained a constant value at 20% bentonite blend after which it decreased sharply at 30% bentonite blend. The high slump value shows workability increase for high cement ratio in a mix. The workability exhibited by this mix is basically due to the cement-aggregate ratio which is relatively higher in this case.

Fig. 4.1.3F Slump comparison for mix 1:1.5:3, water- cement=0.8
Constant slump value was maintained for the cement control mix to the 20% bentonite blend after which there was a sharp drop at 30% bentonite. Slump value was high due to high cement content in the mix and at 30% bentonite there was large deviation which may reveal it may not be suitable for construction. The fluid nature of this mix is highly attributed to increased fines and water-cement ratios.
Trend from findings
From the above figures, a likely conclusion can be drawn that when workability is being considered, bentonite-cement blend can be used up to about 20% in the 1:1.5:3 mix. Beyond this, it is not advisable for use.
Slump of concrete at the same w/c ratio but varying mixes

FIG. 4.1.3G MIX DESIGN with WATER-CEMENT = 0.7
Fig. 4.1.3G reveals that for the three mixes at 0.7 water cement ratio, the 1:1.5:3 mix has the highest workability followed by the 1:2:4 mix and finally 1:3:6 mix. This as earlier stated is due to the increased fines and cementitious material in the 1:1.5:3 mix which in turn increases its workability, and hence it has higher slump values.

FIG. 4.1.3.1.2H MIX DESIGN with WATER- CEMENT = 0.8
For the three mixes at 0.8 water cement ratio, 1:1.5:3 mix has the highest workability followed by the 1:2:4 mix and finally 1:3:6 mix from the cement control mix to the 20% bentonite blend but deviation occurred at 30% bentonite blend.
Trend from findings
The low values of slump for the 1:3:6 mix was expected due to its high aggregate content, and inability to achieve dense interlocked structure in packing. Furthermore, bentonite characteristics of high water retention capacity which not only causes more water to be trapped in the pores of the concrete mix, but to quickly push apart the aggregates.
At both water-cement ratios, the behaviour of the 1:2;4 mix is not easily understandable. Predominantly single sized aggregates used [with two or three particle sizes], have made impossible for the placing.
For up to 10% for all mixes, there seems to be a consistency in rhythm of slump values for both water-cement variants.
SUMMARY
As bentonite was introduced to the mixes, there was reduction in slump value with an exception in the 1:2:4 mix, whose workability increased instead [as opposed to expected results]. This behaviour may be as a result of environmental conditions such as humidity that occurred during casting, or other aggregate properties.
Bentonite, a clay material swells and absorbs water thereby increases the rate of hydration and reduces workability as seen in majority of the mixes except the 1:2:4 mix at 10%, which recorded a deviation in both water/cement ratios. No slump test was carried out for the bentonite control mix due to its sticky or plastic nature; therefore, bentonite control mix had a zero slump value. Conclusively, since bentonite used was swelling clay, it increased the hydration rate of the mixes and reduced the setting time.
Slump values for all mixes shows 1:3:6 mix has the lowest workability, followed by 1:2:4 mix and then 1:1.5:3 mix was the most workable except for 30% bentonite blend which deviated.
Hence, it could be inferred that when workability is been considered, bentonite may be used comfortably in the 1:1.5:3 and 1:2:4 mixes up to about 20% using a water/cement ratio=0.8.
Conclusively, slump shows workability (ease of concrete placement and to attain a highly dense structure). Predominantly, single size with two or three close particle sizes have made any denseness impossible for the placing. Slump value may not be used as a guide for strength development prospects, hence, reliance will be on the CSH gel or inherent formation status and the curing conditions exposed to.

4.1.4 COMPRESSIVE STRENGTH TEST RESULT
Table 4.1.4 showing compressive strength of all mixes
VARIANT
WATER- CEMENT RATIO
AGE
WEIGHT
(Kg)
PEAK LOAD
(N)
PEAK STRESS
(N/mm2)
1:2:4
Cement control mix
0.7
7
14
21
28
8.57
8.60
8.70
8.47
453.12
508.32
553.08
591.84
20.14
22.60
24.88
26.30
1:2:4
10% bentonite blend
0.7
7
14
21
28
8.23
8.90
8.80
8.77
192.84
223.44
305.88
361.92
8.58
9.92
13.96
16.09
1:2:4
20% bentonite blend
0.7
7
14
21
28
8.17
8.63
8.77
8.37
162.24
211.68
258.12
319.92
7.21
9.41
11.47
14.22
1:2:4
30% bentonite blend
0.7
7
14
21
28
8.90
8.67
8.20
8.33
152.4
177.0
231.72
257.64
6.78
7.87
10.30
11.45
1:2:4
Cement control mix
0.8
7
14
21
28
8.23
8.33
7.93
8.37
379.44
434.76
453.12
481.68
16.86
19.32
20.04
21.41
1:2:4
10% bentonite blend
0.8
7
14
21
28
8.33
8.57
8.37
8.37
272.52
284.04
297.72
312.6
12.11
12.62
13.24
13.90
1:2:4
20% bentonite blend
0.8
7
14
21
28
8.77
8.43
8.60
8.70
185.40
196.44
205.80
208.80
8.24
8.74
9.14
9.28
1:2:4
30% bentonite blend
0.8
7
14
21
28
8.67
8.77
8.67
8.93
124.20
145.92
163.08
193.08
5.52
6.49
7.25
8.58
1:2:4
bentonite control mix
1.5
7
14
21
28
6.67
7.27
7.33
7.17
30.60
42.31
46.58
51.53
1.36
1.88
2.07
2.29
1:3:6
Cement control mix
0.7
7
14
21
28
8.43
8.17
8.20
8.33
230.4
316.08
347.52
398.88
10.24
14.04
15.20
17.74
1:3:6
10% bentonite blend
0.7
7
14
21
28
8.67
8.40
8.43
8.53
196.20
208.08
214.92
294.24
8.72
9.25
9.55
13.10
1:3:6
20% bentonite blend
0.7
7
14
21
28
7.80
8.13
8.07
7.97
122.16
122.76
138.24
220.56
5.42
5.46
5.52
9.80
1:3:6
30% bentonite blend
0.7
7
14
21
28
8.17
7.87
8.13
8.47

82.08
90.48
114.36
122.16
3.65
4.02
5.09
5.42
1:3:6
Cement control mix
0.8
7
14
21
28
8.13
8.13
8.23
8.20
162.72
314.88
363.84
423.12
7.24
13.99
16.18
18.80
1:3:6
10% bentonite blend
0.8
7
14
21
28
8.20
8.47
8.63
8.73
251.28
309.12
334.92
340.32
11.17
13.74
14.89
15.12
1:3:6
20% bentonite blend
0.8
7
14
21
28
8.37
8.60
8.53
8.57
125.52
162.72
198.72
210.48
5.58
7.24
8.83
9.35
1:3:6
30% bentonite blend
0.8
7
14
21
28
8.53
8.73
8.83
8.50
119.64
130.08
149.16
167.52
5.32
5.78
6.62
7.44
1:3:6
bentonite control mix
1.5
7
14
21
28
7.53
7.63
7.43
7.43
16.43
18.00
22.95
24.30
0.73
0.80
1.02
1.08
1:1.5:3 Cement control mix
0.7
7
14
21
28
8.27
8.20
8.10
8.20
301.44
371.52
439.44
451.92
13.40
16.51
19.52
20.09
1:1.5:3 10% bentonite blend
0.7
7
14
21
28
8.03
8.03
8.00
8.00
223.44
247.20
276.84
339.60
9.94
10.99
12.30
15.10
0.7
7
14
21
28
8.37
8.37
8.53
8.57
229.68
245.52
262.32
283.20
10.21
10.91
11.65
12.52
0.7
7
14
21
28
8.33
8.73
9.00
9.57
144.00
162.48
171.96
201.24
6.41
7.22
7.64
8.94
1:1.5:3 Cement control mix
0.8
7
14
21
28
8.27
8.33
8.33
8.50
313.32
336.36
350.52
452.16
13.92
14.95
15.59
20.10
0.8
7
14
21
28
9.03
9.03
8.77
8.67
213.36
317.04
345.60
383.28
9.48
14.09
15.36
17.03
0.8
7
14
21
28
8.17
8.20
8.53
8.17
186.00
202.08
218.52
317.04
8.27
8.98
9.71
14.09
0.8
7
14
21
28
8.53
8.57
8.53
8.60
151.92
181.20
187.80
223.32
6.76
8.05
8.34
9.92
1:1.5:3 bentonite control mix
1.5
7
14
21
28
6.50
6.60
6.70
6.67
28.80
32.41
34.43
40.28
1.28
1.44
1.53
1.79

I. COMPARISON OF COMPRESSIVE STRENGTH FOR MIXES
A) Strength of concrete with same w/c ratio but varying mixes
Water-cement ratio of 0.7

FIG. 4.1.4A Three cement control mixes with water- cement = 0.7
The graph shows 1:2:4 mix has the highest compressive strength followed by 1:1.5:3 and finally 1:3:6 mix. Conventionally, 1:1.5:3 mix should have the highest compressive strength but high workability of the mix may have resulted in internal cracks caused by shrinkage on drying, and this reduction in strength may also be due to stresses induced when detaching the concrete cube from mould. The lower strength values of the 1:3:6 mix is due to the higher aggregate content of the mix and the lower aggregate-cement ratio of the mix.

FIG. 4.1.4B 10% bentonite blend with water- cement= 0.7
On a close observation of the Fig. 4.1.4B, it is noticeable that as at the fourteen days curing age, 1:1.5:3 mix had the highest compressive strength and 1:3:6 mix had the lowest, but within the next fourteen days, up to 28-days curing age 1:2:4 mix had the highest compressive strength gains and then maintained the lead till 28days.

FIG. 4.1.4C 20% bentonite blend with water- cement= 0.7
In the period up to 21-days curing age, 1:1.5:3 mix had the highest compressive strength in comparison with the other two mixes but as clearly shown above, the 1:2:4 mix showed a higher rate of strength gain to eventually attain the highest compressive strength at 28-days curing age. This behaviour may be due to the water-cement ratio used which is relatively low and also the presence of bentonite in the mix, and it is obvious that 1:1.5:3 mix has a higher cement-aggregate ratio than the others, but the contrasting slower rate of strength gain between 21 and 28-days curing age does seems however to be explainable on the above basis alone.

FIG. 4.1.4.1.1D 30% bentonite blend with water- cement= 0.7
From Fig. 4.1.4D, it is evident that the 1:2:4 mix still maintains the highest strength. The possible reasons for it being higher than that of the 1:1.5:3 mix is that, the increased fines and cement in the 1:1.5:3 mix may lead to shrinkage and internal cracks in the concrete cube, thereby reducing its strength. It may also be due to environmental factors such as abrasion, humidity and evaporation of water in pores. There is no distinction between strength gained rate from 21 to 28-days curing age in all mixes.

FIG. 4.1.4.1.1E Bentonite control mix with water- cement= 1.5
As shown in FIG. 4.1.4E, the water –cement ratio used for casting the bentonite control mix was 1.5. This is due to the fact that at water-cement ratio of 0.7, the mix was not rather workable for all the mix ratios. The water- cement ratio had to be increased to 1.5 in order to achieve minimal workability needed for its placement. This phenomenon that occurred in the course of this experiment was earlier anticipated, due to the high absorption rate of bentonite as a material. It can thus be inferred from the above results that full replacement of cement with bentonite cannot be on the prospects of any strength gain thereby. In fact, such strength values shown in the plot above is so low that it cannot be used for any structural concrete construction.
Trend from findings
It is worthy to note however, that an evolving trend occurs in the first fourteen (14) days of curing with mix 1:1.5:3 developing considerate compressive strength values and to a large extent, conforming to standard i.e. it is relatively greater than the mix 1:2:4and 1:3:6 up till about 14days but beyond that it reduces. The higher rate of strength gained between the 21 and 28 –days, however cannot be explained in the basis earlier discussed. There is no distinction between strength gained rate from 21 to 28-days curing age in all mixes.

B) Water-cement ratio of 0.8

FIG. 4.1.4F Cement control mix with water- cement= 0.8
In Fig. 4.1.4F, it is clearly shown that the 1:2:4 mix has the highest strength, leading all others at all ages. It is however surprising that these other mixes had much closer values of compressive strength values to that of the 1:2:4 mix at the 28-days curing age. Although, according to standard, the 1:1.5:3 mix should have had the highest strength but the deviation could be due to the single sized nature of the coarse aggregate.
Also (it's being angular) is critical too. All these affect void sizes occupied mainly by mortar created in the concrete to which additional effect of factors such as humidity and temperature become important.

Fig. 4.1.4.1.1G 10% bentonite blend with water- cement= 0.8
In Fig. 4.1.4G, the 1:1.5:3 mix started out with a low early strength at 7days but then gains more strength at 14 days and then attain the highest strength at the end of 28days. This conforms to standard and hence this mix is a desirable mix that can be recommended for use in relatively minor construction works and it is also economical, as lower quantity cement will be used.

Fig. 4.1.4.1.1H 20% bentonite blend with water- cement= 0.8
Again in Fig. 4.1.4H, we see the 1:1.5:3 mix gaining prominence in strength and maintain it until the 28days curing. At this point, the mixes have begun conforming to standard. At 0.8 water- cement ratio and at variant with 20% bentonite blend, although having strength values reduced in comparison to the cement control mix, the mixes are conforming to standard in terms of usual ranking order of strength values.

Fig. 4.1.4J 30% bentonite blend with water- cement= 0.8
As shown above, it is evident that beyond the 20% bentonite blend, the strength characteristics is no longer desirable in all the mixes as the strength values are well below acceptable standards. It is however worthy of note, that the 1:1.5:3 mix still maintains the highest strength.
Trend from findings
It is evident from that the 1:1.5:3 has the highest compressive strength gain at 28-days curing age, except in the control mix. It also shows considerable rapid rate of strength gain between the 21 and 28-days curing age in all the mixes.

Summary
It can therefore be inferred that on comparing the strength values obtained in the Figures 4.1.4F, 4.1.4G, 4.1.4H and 4.1.4J, 1:1.5:3 mix is the optimum at water- cement ratio of 0.8 up to about 30% bentonite blend.

Compressive strength comparisons at same mixes but varying water-cement ratios
4.1.4.1.2 Compressive strength of same mixes but different water-cement ratios
Cement control variant

Fig 4.1.4.1.2A Cement control mix 1:2:4

Fig 4.1.4.1.2B Cement control mix 1:3:6

Fig 4.1.4.1.2C Cement control mix 1:1.5:3
In Fig. 4.1.4.1.2A, the mix with a water-cement of 0.7 has the highest compressive strength at all the ages. This is in line with the usual result expected of ordinary Portland cement; that higher water-cement ratio usually leads to lower compressive strength value.
In Fig. 4.1.4.1.2B, the mix with water-cement ratio of 0.8 had the highest compressive strength except for the first seven days of curing. This may be due to abrasive forces that are applied to the concrete cubes when detaching them from the wooden mould [when detaching the concrete cubes from mould with an applied force it may cause internal cracks in the concrete cube which may affect the compressive strength].
In Fig. 4.1.4.1.2C, the mix with water-cement ratio of 0.7 had the highest compressive strength at all ages. This is in line with the usual result expected of ordinary Portland cement; that higher water-cement ratio usually leads to lower compressive strength value.
Trend from findings:
Critically examining the three plots for the cement control variant, water-cement ratio of 0.7 mixes predominantly developed the higher strength for the 21 to 28-days except for the 1:3:6 mixes, although water-cement ratio of 0.8 mixes had the highest developing strength.

Summary
The three cement control mixes with water-cement ratio of 0.7 had the highest compressive strength at twenty eight days except for the 1:3:6 mix, whose reduction in strength may be due to internal cracks that resulted when detaching concrete cubes from mould.

B) VARIANT WITH 10% BENTONITE BLEND
Fig. 4.1.4.1.2D 10% bentonite blend 1:2:4
Fig. 4.1.4.1.2E 10% bentonite blend 1:3:6

Fig. 4.1.4.1.2F 10% bentonite blend 1:1.5:3
From Fig. 4.1.4.1.2D, the mix with water-cement ratio as 0.8 has the highest compressive strength up till the 14-days curing age, but was thereafter overtaken by the mix with water-cement ratio as 0.7 which had the highest compressive strength at 28-days curing age. This behavior may be due to the latter mix having, at the 14-days to 28-days curing age formed CSH-gel to fill up more proportion of the contrastingly lower volume of water-filled pores therein to acheive the higher strength value observed.
Fig. 4.1.4.1.2E clearly shows the water-cement 0.8 variant having the higher compressive strength values. This is because the bentonite absorbs water of which 1:3:6 mix contains more aggregates and hence needs more water to enable the cementitious gel flow effectively into the pore spaces of the mix, enhancincing cohesion of aggregates in the concrete and thereby improving the strength. Also, the higher water content cushions the hydation effect of bentonite-cement blend, hence making the 0.8 water-cement variant the preffered option.
Fig. 4.1.4.1.2F also shows that the water-cement 0.8 variant has the highest compressive strenght values. Since the 1:1.5:3 mix contains more fines, more water in the voids will favour compressive strength as it will improve workability and also reduce rate of shrinkage caused by the bentonite-cement blend.
Trend from findings
Critically examining the three plots for the cement control variant, water-cement ratio of 0.8 mixes consistently developed the highest strength for the first 21days even to the 28days except for the 1:2:4 mixes.
Summary
In summary, we can see that on comparing the three mixes, under normal conditions, water- cement ratio 0.8 is the optimum variant that favours compressive strength, except for in the 1:2:4 mix which shows a deviation in ultimate strength at 14-days curing age, it may conservatively be considered to be the optimal water- cement variant in all three mixes. The reason for the behaviour in figs. 4.1.4N and 4.1.4O is however unclear.
The optimal mix is 1:1.5:3 at water-cement ratio 0.8 because it possesses the highest ultimate strength values at 28days.

VARIANT WITH 20% BENTONITE BLEND

Fig. 4.1.4.1.2G 20% bentonite blend 1:2:4

Fig. 4.1.4.1.2H 20% bentonite blend 1:3:6

Fig. 4.1.4.1.2J 20% bentonite blend 1:1.5:3
Studying Fig. 4.1.4G closely, water- cement 0.8 variant attained a higher early strength at 7days curing. We then see the 0.7 water- cement variant gaining strength at 14days and attaining the highest compressive strength at 28days. This behavior may be due to the water retention capacity of bentonite i.e. ability to store water in its molecules and with longer days of curing,and this does not favour compressive strength in the water-cement ratio 0.8 which has higher water content.
In fig. 4.1.4H, water- cement variant 0.8 has highest compressive strength up till 21-days curing age, but then water- cement ratio 0.7 gained the highest compressive strength at 28-days curing age. This may be attributed to the presence of more aggregates in the 1:3:6 mix, which has more pore spaces/voids in its structure, leading to the cementitious material (gel) filling up the pore spaces i.e. the presence of more water in the water- cement ratio 0.8 allows the free flow of the gel into the voids more than what is obtainable in the 0.7 water- cement variant.
In Figures 4.1.4J, it is worthy to note that the water- cement ratio 0.7 has the higher values of compressive strength at 21-days curing age but the water- cement 0.8 variant finishes with the ultimate/highest strength at the end of 28days of curing. This behaviour may be due to the presence of more fines in this mix and since lesser water- cement does not favour strength in this case, it is evident that the water- cement ratio 0.8 is the preferred variant in this mix due to its value of compressive strength at the 28-days curing age in the 1:1.5:3 mix.
Trend from findings:
Critically examining the three plots for the cement control variant, between the 14 to 21days, water-cement ratio of 0.7 mixes predominantly developed the highest strength except for the 1:3:6 mixes.
Summary
In summary, for the three mixes, the water-cement ratio 0.7 had the highest compressive strength except for the 1:3:6 mix which has more pore spaces/voids in its structure, leading to low compressive strength.
It can thus be concluded that, using 20% bentonite blend, the mix 1:2:4 at water- cement ratio 0.7 is the optimal mix because it has the highest ultimate compressive strength at 28days.

VARIANT WITH 30% BENTONITE BLEND

Fig. 4.1.4.1.2K 30% bentonite blend 1:2:4

Fig. 4.1.4.1.2L 30% bentonite blend 1:3:6

Fig. 4.1.4.1.2M 30% bentonite blend 1:1.5:3
Fig. 4.1.4.1.2K shows the water-cement ratio 0.7 variant had the highest compressive strength all through the curing days. This behavior may be due to the water retention capacity of bentonite i.e. ability to store water in its molecules and with longer days of curing, may not favour compressive strength in the water- cement ratio 0.8 variant, hence reduced compressive strength.
Fig. 4.1.4.1.2L shows the water-cement ratio of 0.8 variant had the highest compressive strength all through the curing days. This may be attributed to the presence of more aggregates in the 1:3:6 mix, which has more pore spaces/voids in its structure, leading to the cementitious material (gel) filling up the pore spaces i.e. the presence of more water in the water- cement ratio 0.8 allows the free flow of the gel into the voids more than what is obtainable in the water-cement 0.7 variant.
Fig. 4.1.4.1.2M shows the water-cement ratio of 0.8 variant had the highest compressive strength all through the curing days. This may be attributed to the fact that the mix contains more bentonite, which needs more water to improve workability which increased the compressive strength.
Trend from findings
We can see that the lesser values of strength of the water- cement ratio 0.7 in the 1:3:6 and 1:1.5:3 mixes is due to loss of aggregate-cement bond in the mix, caused by internal shrinkage in the concrete cube, attributed to lesser water content.
Summary
At 30% bentonite blend, the optimal mix is 1:2:4 mix at water- cement ratio 0.7, which has the highest ultimate compressive strength at 28days.

GENERAL SUMMARY
Slump as an indicator of quality
Slump values may not be used as a guide for strength development prospects, hence reliance will be on the CSH-gel or inherent formation status and the curing conditions exposed to. Slump is not adequate as a good indicator. Beyond 10%, slump cannot be used as an indicator because for up to 10% for all mixes, there seems to be a consistency in rhythm of slump values for both water-cement variants.
Hence, the 10% bentonite blend can be suitable for placement and can be recommended for use in the industry.
Compressive strength as an indicator of quality
The graph of compressive strength against curing days shows that the compressive strength of the 150mmx150mmx150mm concrete cube increase as the age increases. Majority of the graph for 1:2:4 mix design shows a progressive increase in compressive strength of the concrete. The cement control mix graph reveals there was not much increase in strength at the 28-days curing age in comparison with the early strength at 7-days curing age. For some cases in the 1:3:6 mix design, the graph shows the compressive strength increase, from the 7 to 21-days curing age was not too significant but it sharply increases thereafter to a high value at 28-days of curing. In other words, the graph shows the compressive strength was almost constant between the 7 and 21-days curing age, but eventually increases at 28-days of curing. Also this reveals that the bentonite blend retard the increase in compressive strength at the early age, but increase in strength with time is a possibility. The more the bentonite blend was introduced to the mix, the lower the compressive strength achieved.
The bentonite control graph shows an almost uniform low compressive strength at all ages. From the graph, conclusion could be drawn that the bentonite control mix cannot be used for construction in comparison with the cement control mix and other bentonite blend mix. The change in water –cement ratio is as a result of the workability of the mix not been achieved at water-cement ratio 0.8. Only one bentonite control mix of 1.5 water-cement ratio was cast for each mix design.
The 30% Bentonite blend graph had a progressive increase in the compressive strength of concrete although the strength achieved was low. There is a possible increase in strength over long period of time.
The graph of compressive strength against the three mix designs for the cement control mix at both water – cement ratio, shows that 1:3:6 mix has and develops the lowest compressive strength, followed by 1:1.5:3 mix and 1:2:4 mix has and develops the highest compressive strength.
The graph of compressive strength against the three mix designs for the 10% bentonite blend mix at water–cement ratio 0.7 shows that 1:3:6 mix has and develops the lowest compressive strength, followed by 1:1.5:3 mix and the 1:2:4 mix has and develops the highest compressive strength, while the graph of compressive strength against the three mix designs for the 10% bentonite blend mix at water–cement ratio 0.8 shows that the 1:2: mix has and develops the lowest compressive strength, followed by 1:3:6 mix and 1:1.5:3 mix has and develops the highest compressive strength.
Majority of the other bentonite blends follows the strength pattern as the cement control mixes. Majorly, deviation of the strength pattern for the mixes is either due to the seven or twenty eight day's strength pattern.
Conclusively, 1:3:6 mix has the lowest compressive strength, followed by 1:1.5:3 mixes and the 1:2:4 mixes has the optimal compressive strength for bentonite blends of up to about 20%.
Other factors that influenced quality of concrete
Control of loss of water should be a major emphasis on trends in strength values.
Hand mixture method used also had considerable effect on workability and strength characteristics.
Ease of placement of the 1:2:4 mix will be lost without adding much water which contrasts the trend observed in the 1:1.5:3 mix.
The presence of more coarse aggregates in the 1:3:6 mix than in the other two mixes causes mortar to flow more to the edges of the concrete structure, reducing its resistance to shear forces.

CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION
5.1 CONCLUSIONS
The 1:2:4 mix appears to be apparently better on the basis of having the highest compressive strength value all through this experiment. This can be attributed to errors from batching systems (errors that occurred when adding water) and the unexplainable effect of using single sized aggregate and other general factors resulting from manual mixing methods employed in mixing. The high compressive strength values of the 1:2:4 mix can also be attributed to large volume of coarse aggregate, which mass to provide resistance to shearing.
Based on the discussion of results in the previous chapters that clay content, when present in any concrete mix has detrimental effect to the concrete compressive strength, the result shows that bentonite blend can be used in-place of ordinary Portland cement if not more than 20% substitution to give an acceptable and required compressive strength. It can also be inferred that bentonite control mix cannot be used for construction work due to its relatively high hydration properties, low compressive strength and shrinkage. These properties are as a result of the fact that clay materials have ability to store water in their pores for a long time, and this water retention capacity does not favour compressive strength as it causes concrete to fail easily when subjected to compressive or shear forces.
The 1:3:6 mix has the lowest compressive strength due to the presence of voids in the internal structure of the concrete cubes, filled by the bentonite-cement gel (since bentonite swells when mixed with water), which expands in the voids, causing reduction in the cohesion between aggregates. The low values of compressive strength could also have resulted from inadequate compaction and reduced workability due to a higher aggregate-cement ratio.
According to standard (when using ordinary Portland cement), the 1:1.5:3 mix should have the highest compressive strength for the cement control mix but the high water-cement ratio used affected the compressive strength. Other factors that affected the general concrete cube strength were the non-uniform compaction, use of sub-standard mixing tools, and single sized coarse aggregate used (angular). Therefore, we can use the 1:1.5:3 mix with better aggregate grading and adequate water-cement ratio, since it has closer compressive strength values to that of the 1:2:4 mix.
Concrete compressive strength achieved in this thesis is lesser than that required by the British Standard code for materials and construction.
Bentonite blend not more than 20% substitution will provide desired result as a blend with ordinary Portland cement although early strength may be very low but with time, considerable values of compressive strength will be achieved. There is the case that higher durability and less pollution to the environment from cement production may be achieved thereby. Also, for up to 20% bentonite blend mix, 0.7 water–cement ratio gives the optimal compressive strength.
Bentonite is more economical as it is now cheaper than cement, hence when used as a blend; it produces more cost effective concrete.

5.2 RECOMMENDATIONS/AREAS FOR FURTHER STUDY
Based on the results obtained during the project work, the followings are recommended;
Up to 20% bentonite-cement blend is recommended for use in mass construction works for economical and durability purposes as a replacement for cement.
Nigeria code standard for materials and construction should be developed considering environmental factors.
Quality assurance and control should be put in place in every civil engineering construction to ensure needed test are carried out effectively.
Standard equipment for carrying out of experimental works should be used, if standard results are to be obtained.

REFERENCES
American Society for Testing and Materials, ASTM C618 Standard Specification for Coal Fly Ash and Raw or calcined Natural pozzolan for use in concrete (Section 4, volume 04.02 October 2006)
Ahmed et al, Indian wonder clay to replace cement, retrieved from www.aggregate research.com, 01/03/2011. Article published in January 2011 by Rashmi Kalia (ARI-C NEWS)
Bensted, J. and P. Barnes, Structure and Performance of Cements, Second edition 2002,Spon Press London and New York.
British Standard 1377: PART 2; 1990-Methods of testing of civil engineering soils, British standard institution, London.
British Standard 1377: Part 4; 1990-Guidance on the description of Aggregates, British standard institution, London.
British Standard 1881: Part 1 (1982) – Methods for making test cubes from fresh concrete; British standard institution, London.
Detwiler R.J., J.I. Bhatty, G. Barger and E.R. Hansen , Durability of Concrete Containing Calcined Clay, Concrete International, 2001, pp.43-47.
Goldbeck, A. T., "Nature and Effect of Surface Coatings on Coarse Aggregates" American Highways, V. 12, No. 3, 1933, pp. 9-13.
MSDS Properties Technology, Primary Information Services,Ullagaram, India.)
Nawy, E. G. Fundamentals of High-Performance Concrete, 1st ed., Ch. 12. Longman,
United Kingdom, 1996.[2nd ed., John Wiley & Sons, New York, 2000.]
Neville A.M (1973)- Properties of concrete; Second edition, Pitman Publishing Ltd., United Kingdom, Pg. 114-177, 189, 236, 251-260, 461,581,582, 592-597.
Sabir B.B. et al, Metakaolin and calcined clay as pozzolans for concrete: a review, Cement & Composites 23, 2001, Pg.441-454.
Souza P.S.L.and D.C.C.D. Molin, Viability of using calcined clays, from industrial-products, as pozzolans of high reactivity, Cement and Concrete Research 35, 2005, pp.1993-1998.
S. Ahmad, S. A. Barbhuiya, A. Elahi and J. Iqbal, Clay Minerals; March 2011; v. 46; no. 1; p. 85-92.

Days
Mix 1:3:6, w/c = 0.8

Slump (mm)

mix 1:1.5:3, w/c = 0.7

Slump (mm)

mix 1:1.5:3, w/c = 0.8

Slump (mm)

slump for all mixes at w/c = 0.7

Slump (mm)

slump for the mixes at w/c = 0.8

Slump (mm)

Mix 1:3:6, w/c = 0.7

Slump (mm)

Mix 1:2:4, w/c = 0.8

Slump (mm)

Mix 1:2:4, w/c = 0.7

Slump (mm)

Cement control mixes with w/c = 0.7
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10% bentonite blend with w/c = 0.7
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Cement control mix 1:1.5:3
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Cement control mix 1:3:6
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cement control mix 1:2:4
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Bentonite control mix with w/c = 1.5
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Control mix with w/c = 0.8
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Days

Compressiv e strength (N/mm2)

Days

EFFECTS OF ORDINARY PORTLAND CEMENT BENTONITE BLEND ON COMPRESSIVE STRENGTH OF CONCRETE MIXES USING

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