Prediction of Strength Properties of Concrete Containing Calcined Black Cotton Soil Using Response Surface Methodology

: This research predicts the strength properties of concrete containing Calcined Black Cotton Soil (CBCS) using response surface methodology. Cement production requires large amount of energy and emits greenhouse gases that have negative impact on the environment. Utilization of CBCS as cement replacement in concrete will reduce these negative impact. Experimental plan was designed using response surface method in Design Expert software to predict compressive strength, density and water absorption of concrete containing CBCS. The CBCS was varied from 5 to 20% while the curing period was varied and 7 to 28 days. Face-centered central composite design method of response surface was used. The design consists of two design factors at three levels (coded as -1, 0, +1) each. The factors are the curing period, and the CBCS contents. The results showed that CBCS is a pozzolana. CBCS increases durability of concrete by decreasing its water absorption. All the response surface models developed for the water absorption, density and compressive strength showed very good relationship between the predictors and the responses with coefficients of determination, R 2 ˃ 0.94 and p-values ˂ 0.05.


Introduction
Concrete is one of the most widely used construction material in the world due to availability of its constituents materials. Its major constituent materials are cement, gravel, sand and water. More than 20 billion tons of concrete is produced annually, which is the highest among all composite materials [1].
Cement represents 10-15% of total weight of concrete with annual production of about 2.8 billion tons worldwide [2]. Manufacture of cement is highly energy and carbon IV oxide (CO 2 ) emission intensive due to the extreme heat required during its production. The CO 2 emission has negative impact on the environment. Also, the extraction of raw materials for cement production damages the environment [3].
Under the strong demand for modern, economically viable and environmentally friendly materials, researchers have focused on the use of Supplementary cementitious materials (SCMs) as partial replacement of cement in concrete and mortar production. SCMs have been used in concrete production which yielded improvement in strength and durability of the concrete. These materials exhibit very good pozolanic activity due to high content of silicate and alumina. When used in right proportion, SCMs can improve the fresh and hardened properties of concrete [2]. SCMs save a significant amount of cement and give specific properties to cementitious products that help to meet the requirement of mordern construction [4]. Black cotton soils (BCSs) are found in the North Eastern part of Nigeria where they occupy an estimated area of 104,000 km 2 . They are dark colored expansive clays rich in montmorillonite clay minerals. BCSs are known to be problematic because of the presence of this montmorillonite, which is highly responsible for the shrinkage-swell behavior of the soil depending on the available moisture in the soil. Also, montmorillonite is the root cause of many problems such as pavement failure and excessive settlement associated with the BCSs [5].
Response surface methodology (RSM) is a collection of mathematical and statistical techniques for empirical model building. The major objective of RSM is to optimize a response (output variable) which is influenced by several independent variables (input variables). Results obtained from series of experimental tests, called runs, are used as the input variables in order to identify the reasons for changes in the output response. Empirical models are developed to predict the response under similar experimental conditions [6].
This research is aimed at predicting the strength properties of concrete containing CBCS as partial replacement of ordinary Portland cement using response surface methodology. Utilization of CBCS as partial replacement of cement in concrete production will reduce the cement content in the concrete and thereby reduces the embodied CO 2 emission and increases environmental friendliness of the concrete.
The calcination (thermal treatment) temperatures suitable to produce a good cement replacement material have been reported to be between 500°C and 900°C depending on the nature and type of clay. Calcination of natural pozzolana containing clay at this temperature range increases performance of the pozzolana. The thermal treatment destroys the crystal structure of the clay minerals and transforms it to a very reactive amorphous structure [7]. The advantage of using CBCS as pozzolan in concrete compared with other pozzolanic materials is the ease of access of the BCS in most parts of the world at low price. Grinding can also be advantageous by breaking up particle agglomerates and exposing additional surface area.

Materials
The various material used in the investigation are; ordinary Portland cement, fine aggregate, coarse aggregate, CBCS and water.

Cement
Ashaka brand of Portland cement (Grade 32.5) was used throughout the investigation. The cement was obtained from a local dealer and stored in a cool dry location. The cement conforms to EN 197:1 [8] specification.

Fine Aggregate
The fine aggregate (sand) used was obtained from a stream in Bauchi. Sand particles not larger than 4.75mm were used in the experiments. The specific gravity of the sand was found to be 2.65. The sand falls within zone 2 after conducting the particle size distribution test. The tests were conducted in accordance with BS EN 1097:6, BS 812:2 and BS EN 933:1 specifications respectively [9, 10, 11].

Coarse Aggregate
The coarse aggregate used was normal weight crushed aggregate of igneous rock origin with particle sizes larger than 4.75mm but less than 20mm. It was obtained from a quarry site in Bauchi. The aggregate has specific gravity of 2.69, aggregate crushing value of 26.7% and aggregate impact value of 12.9%.

Calcined Black Cotton Soil
The BCS was collected from Baure town in Yemaltu-Deba Local Government area of Gombe State in the North Eastern part of Nigeria. The village is located on Latitude 10 0 13' N and Longitude 11 0 23'. The BCS is dark gray in colour and is called 'Kasan Kalari' in Hausa. The soil was calcined in a kiln using temperature range of 600°C -1000°C. The calcined soil was ground and sieved using 75 micrometer sieve. The specific gravity and pH for the CBCS was found to be 2.78 and 7.6 respectively. The oxides composition of the CBCS was determined using X-ray fluorescence (XRF) test in accordance with American Society for Testing Materials ASTM C311-11b and ASTM C618 specifications [13,14]. The test was conducted at Ashaka Cement Factory, Gombe State.

Experimental Design
A mix proportion of 1:2:3 and water-to-cement ratio of 0.5 were adopted throughout the experiment. The experiment was designed using response surface method in Design Expert software. The CBCS was varied from 5 to 20% while the curing period was varied and 7 to 28 days. Face-centered central composite design method of response surface was used. The design consists of two design factors at three levels (coded as -1, 0, +1) each. The factors are the curing period, and the CBCS contents.
The samples were produced and cured in accordance with BS EN 12390:1 and BS EN 12390:2 specifications [15,16]. Concrete cubes of size 100mm x 100mm x 100mm were cast and cured by water immersion for 7, 14 and 28 days respectively before testing.

Specimens Testing
The strength activity index test was conducted on 50mm x 50mm x 50 mm mortar specimens containing 0 and 20% CBCS replacing cement by weight. Compressive strength of the mortar specimens were determined after curing for 7 and 28 days. The test was conducted in accordance with ASTM C311-11b specifications [13].
The workability of the fresh concrete was determined using slump test in accordance with BS EN 12350:2 specifications [17]. The test was conducted on the specimens containing 0, 5, 10, 15 and 20% CBCS.
Compressive strength test was conducted on hardened concrete cubes. Three (3) cubes were tested for each curing period and levels of cement replacement in accordance with BS EN 12390:3 and BS EN 12390:4 specifications [18,19].
The saturated density of the hardened concrete cubes prepared for compressive strength test was calculated as per BS EN 12390:7. The water absorption was determined on the concrete cubes prepared for compressive strength test. The test was carried out in accordance with BS 1881:122 specification [20,21].

Oxides composition of CBCS
The oxides compositions of CBCS are presented in Table 1.

Strength Activity Index (SAI)
The Strength Activity index for batches of the test cube produced in accordance with ASTM C311-11b [13] as shown in figure 1. The results obtained is presented in table 2.

Workability of Fresh Concrete
Workability of the fresh concrete was measured using slump test ( figure 2). The test was conducted on concrete containing 0, 5, 10, 15 and 20% CBCS in accordance with BS EN 12350: 2 [17]. The results obtained are presented in the figure 3 which show that addition of CBCS decreases the workability of the concrete. The workability decreased from 40mm at 0% CBCS content to 22mm at 20% CBCS content. This is 29% drop in workability of the concrete when compared to the control. Therefore increase in CBCS contents affect the workability of the concrete.

Water Absorption
The test was conducted in accordance with BS 1881 part 122 [21]. Figure 4a shows the contour plot of curing period (CP) against the CBCS contents while figure 4b shows the surface plot. From figure 4b, the water absorption increased with increase in CP. Also the water absorption increases with increase in CBCS contents up to 10% replacement. Beyond 10% CBCS contents the water absorption decreases, this is because of high volume of CBCS (due to its lower density than cement) which blocked the pores in the concrete and durability of concrete increases with decrease in its water absorption.    (1): Water absorption = +1.65 + 0.34*A + 0.22*B + 0.06*A*B -0.39*A 2 -0.049 *B 2 (1) The model equation in terms of actual factors is presented in equation (2): Water absorption = + 1.64966 + 0.34*CBCS + 0.225*CP + 0.06*CBCS*CP -0.39379*CBCS 2 -0.048793*CP 2 (2)

Density
The test was conducted on hardened concrete containing CBCS in accordance with BS EN12390:7 [20]. Figure 5a shows contour plot of CP against CBCS while figure 5b shows surface plot. From figure 5b, the density decreased with increase in CBCS content and increased with increase in curing period. The density of the control concrete specimens increased from 2703 to 2756 Kg/m 3 . Concrete samples containing 20% CBCS have lowest density values compared to the other mixes, with density values of 2536 and 2570 Kg/m 3 at the age of 7 and 60 days, respectively. These lie within the range of 2200 to 2600 Kg/m 3 specified as the density of normal weight concrete [22].

Conclusion
The research examined the properties of concrete containing CBCS as partial replacement of Ordinary Portland Cement. Based on the results obtained, the following conclusions were made: 1. Calcined black cotton soil was characterized as a pozzolana. 2. The strength activity index of CBCS concrete at 28 days was greater than 75%. 3. CBCS increases durability of concrete by decreasing its water absorption. 4. All the response surface models developed for water absorption, density and compressive strength showed very good relationship between the predictors and the responses with coefficients of determination, R2 ˃ 0.94 and p-values ˂ 0.05.