Consistency and mechanical properties of sustainable concrete blended with brick dust waste cementitious materials

The reuse of waste materials in civil engineering projects has become the topic for many researchers due to their economic and environmental benefits. In this study, brick dust waste (BDW) derived from cutting of masonry bricks and demolition waste which are normally dumped as land fill is used as partial replacement of cement in a concrete mix at 10%, 20% and 30% respectively, with the aim of achieving high strength in concrete using less cement due to the environmental problems associated with the cement production. To ascertain the effects of BDW on the consistency and mechanical performance of concrete mix, laboratory investigations on the workability of fresh concrete and the strength of hardened concrete were carried out. Slump and compaction index test were carried out on fresh concrete mix and unconfined compressive strength (UCS) test and tensile strength test were conducted on hardened concrete specimen after 7, 14 and 28 days of curing. The results showed high UCS and tensile strength with the addition of 10% BDW to the concrete mix, hence achieving the set target in accordance with the relevant British standards. A gradual reduction in strength was observed as BDW content increases, however, recording good workability as slump and compaction index results fell within the set target range in accordance with relevant British standards. Findings from this study concluded that BDW can partially replace cement in a concrete mix to up to 30% igniting the path to a cleaner production of novel concrete using BDW in construction work.


Introduction
Recent growth in the world's population has triggered an increase in the demand for concrete products for the construction of more building for use as homes and other infrastructure. This has led to the consumption of billions of tons of our natural resources such as clinker, water and aggregates by construction industries to produce cement; therefore, increasing the problems associated with cement production such as high greenhouse gas (GHG) emission and environmental pollution [1].
Cement consumption in 2014 in developed economies increased by about 9.2 million metric tons followed by 9 million metric tons in 2015 and is expected to grow from 2.3% in 2019 and 1.7% in 2020 [2]. The world cement vision predicted a rise in GHG emission in 2020 due to a rise in demand for cement worldwide [3]. According to [4] approximately one ton of CO 2 is produced in preparation of one ton of cement. The negative impact of cement production has raised concerns to push towards a more sustainable practice of using recycled waste materials. By-products pozzolanic materials such as marble dust has been used in concrete production [5]. Quarry dust, fly ash and billet scale are successfully used in a study as cement replacement in a concrete mix [6]. Cementitious materials such as fly ash, rice hush ground granulated blast furnace slag, and bagasse ash can be used in concrete mix due to their pozzolanic properties [7]. Clay bricks ground to cement fineness was partially replaced with cement at 0% 10%, 20% and 30% in standard mortar [8].
About 14% of construction and demolition waste is made up of bricks and reasonable results have been achieved when Brick dust waste (BDW) was used as a substitute for cement at 5%, 10% and 15% [9].
Brick dust waste is a pozzolanic material when mixed with cement produces calcium silicate hydrate gel (C-S-H) which determines the strength of concrete. Any materials with a siliceous and aluminous content that reacts with calcium hydroxide when in contact with water to form cementitious hydration product are referred to as Pozzolans [10]. Ancient structures in Rome and Egypt used pozzolans as part of the cement used in their construction [11]. Excessive dumping of construction and demolition waste as landfill has influenced current research to consider the use of construction and demolition waste as partial replacement for cement due to the negative environmental impact associated with landfill. Using waste materials in the construction sector will reduce the overall cost of construction and the negative environmental effects associated with cement production leading to a more sustainable construction industry. Bricks are manufactured by the calcination of alumina-silicate clay and ground to suitable fineness to possess pozzolanic properties and made brittle by subjecting it to a high temperature of about 1000 to 1100 °C and the creation of a liquid glassy phase when cooling indicate high pozzolanic properties [8]. Although kaolinite loses its structural water around 600 °C to form metakaolin, this new material is used as pozzolan in finely divided form in cement-based system [12]. In this study, detailed investigation into the consistency and mechanical properties of concrete made from partially replaced cement with brick dust waste at varying proportions is presented, and it highlights the tests conducted along with the appropriate standards adopted. The study will outline the experimental methods used, describing the samples preparation, design mix and various tests conducted on fresh and hardened concrete. Results and discussion section will focus on presenting and discussing the results obtained from the various tests conducted, which subsequently provides the key findings given in the conclusions and recommendations for future work.

Scope of the study
This study will contribute to the understanding of partially replacing cement with BDW focusing on the workability of fresh concrete and the strength of hardened concrete. Slump and compaction index test were carried out on wet concrete and unconfined compressive test and tensile splitting test were conducted on hardened concrete. The study focuses on the replacement of cement with brick dust waste and its effect on strength and workability. CEM II cement was used and a strength class of RC 35/45 was adopted. A target Class S3 was set for the slump test as

Materials and method
The materials used in this study include brick dust waste, Portland cement, limestone aggregate (coarse) 10 mm (10/4) and 20 mm (20/10) grade, natural sea-dredged sand (fine aggregate). Brick dust used in the study is a waste from cutting of fired bricks supplied by Brick Fabrication Ltd, Gemini works, Pontypool, South Wales, UK. Portland cement type BS EN 197-1 CEM II/B-V 32.5R with a minimum compressive strength of 32.5 N/mm 2 was used in accordance with BS EN 197-1:2011a and supplied by Lafarge Cement UK through a local contractor. Fine and coarse aggregate used in the study was supplied by a local contractor in line with PD 6682-1-:2009. Sieve analysis was conducted by the local supplier in accordance with BS EN 12620:2002+A1:2008 and BS EN 933-1:2012. Consistency limit and particle size distribution of brick dust waste (BDW) are shown in Table 1 and Fig. 1, Chemical composition mineralogy and physical properties of BDW and PC are shown in Table 2 and Particle size distribution (Figs. 2, 3, 4), chemical composition and other properties of sand and stone are shown in Table 3.

Mix design and sample preparation
Based on BS 8500-2:2015+A1-2016, concrete type RC35/45 was adopted and four batches of concrete mix were prepared with ratio 1:2:3 and water-cement ratio (w/c) 0.5, cement was partially replaced with BDW based in the control mix. Nine (9) cubes (100 mm × 100 mm × 100 mm) and two (2) cylindrical specimens (100 mm diameter and 200 mm height) were made per mix. Based on the design criteria for control mix in Table 4, Mix composition (MC)1-3 were designed by replacing cement with BDW at 10%, 20% and 30% by weight of various materials.
Based on the mix design, dry materials were mixed using a concrete mixer and a measured amount of water added. An oiled steel cube and cylindrical moulds filled with fresh concrete and vibrated for up to 40 Hz for 5 s using a vibrating table until no further settlement was observed, and a compact and air free concrete achieved. The surface was levelled and store without any seal in a room for 24 h with a temperature of about 20 ± 5 °C. With the help of a powered tool, the dry concrete samples were de-moulded and cured in water to be tested after 7, 14 and 28 days.  and two fool rest were used. The cone was placed on a non-absorbent surface with the top diameter 100 mm facing up. With a firm grip of the handle and footrest, the mould was filled with fresh concrete in three layers. Each layer of concrete was tapped 25 times with a standard steel rod (16 mm diameter and 600 mm length). The rod was rolled horizontally over the top of the mould to level the concrete. The mould was lifted slowly using the handle leaving unsupported concrete to slump. The metal cone is placed next to the slumped concrete and the 600 mm long rod was placed on top of the empty cone mould to set as a target. The distance between the set target and the slumped concrete was measured using a measuring ruler. The value of the measurement is the slump value of fresh concrete. Figs. 5 and 6 show the slump test and the types of slump. where, F = Maximum load at failure (N), L = length of specimen (mm), d = diameter of specimen (mm).

Results and discussion
The results obtained from the laboratory test conducted on the consistency and mechanical properties of concrete composed of varying brick dust waste proportions show good consistency and workability for fresh concrete and improved strength for hardened concrete. A true slump and good compaction index values were achieved. However, low slump was recorded for mix composition MC2 composed of 20%BDW and MC3 composed of 30% BDW and low compaction index value recorded for MC 3 composed of 30% BDW. A gradual reduction in slump and compaction index was observed with an increase in brick duct waste content. High compressive and tensile strengths were achieved for all mix compositions which signify strength improvement with the addition of brick dust waste as partial replacement for cement. Compared to the control mix, no significant difference in splitting tensile strength was achieved indicating good performance. The overall results obtained from this experiment prove that brick dust waste has the ability to improve the strength and consistency of concrete.

Slump and Compaction Index
A variation in slump was observed for each mix composition and a drastic reduction in slump of 110 mm (MC 1) to 40 mm (MC 2) was observed. However, a true slump was     [13]. In a similar study where BDW was added to a concrete mix at 10%, 20% and 30%, similar trend of results were observed by [9]. The study indicates that every percentage increase in ground clay bricks results in a gradual reduction in slump value [12]. However, the reduction in slump cannot be attributed to the increase in BDW content alone, changes in watercement ratio (w/c) and rate of water absorption by BDW in different mix compositions also play a major role. A study conducted by [8] revealed that 25% of PC replacement by  ground brick in the concrete mix had no significant effect on water demand. An increase in absorption properties of concrete increased with an increased coconut shell content. In [14], it's observed that water absorption in concrete mix increased when brick dust waste was used as a partial substitute for cement from between 5 and 30%. Fluidity in the mix composition generally reduced under the same w/c for varying mix compositions of brick dust waste, leading to higher stiffness and subsequent reduction in slump. In [15] it's shown that BDW has 25% more fineness compared to cement, and therefore requires a high amount of water during hydration when ground waste clay is used as a partial replacement. The gradual increase in percentages of BDW between 10% and 30% in fresh concrete influenced the 'S' value in their study. 'S' value is the mean value of the four sides distance from the edge top of the steel container to the surface of the compacted concrete. It thus follows that the lower the slump the higher the 'S' value and vice versa. Clear factors affecting compaction index include w/c, cement content, mix proportions, aggregate shape and size. All compaction index values achieved in the current study are within the target range as shown in Fig. 14 with the highest compaction index of 1.18 mm recorded for MC 1.

Unconfined compressive strength (UCS)
A high compressive strength was achieved at later stages for all mix compositions in the current study. It's observed that at the early age of curing, a low compressive strength was generally observed but was followed by a predominate increase in compressive strength for all mix composition after curing age 7, 14 and 28 days as shown in Fig. 15. But in a similar study conducted by [4] at different replacement of 5%, 10% and 15%, a higher compressive strength was obtained with 15% brick dust waste. Another study by [16] revealed that a high compressive strength could be achieved when sand is replaced with up to 15% of BDW. Hardened concrete with BDW generally shows lower compressive strength at early ages, but a comparable strength with cement only concrete at later ages [12]. In the current study, MC 1 achieved the set target with a compressive strength of 36 N/mm 2 , while desirable strength values of about 75% and 65% of the control strength were achieved with MC 2, MC 3 composed of 30% less cement did not achieve the desired compressive strength at even at 28 days. The results showed an increase in strength as curing age increases as the formation of calcium silicate hydrate gel (C-S-H) developed during the hydration process shown in Fig. 16. The presence of pozzolans reacts with the calcium hydroxide during hydration and forms calcium silicate hydrate gel (C-S-H) [17]. The pozzolanic characteristics are responsible for high compressive strength at 50% brick waste replacement ratio [5]. The chemical reaction of brick dust in concrete shows a reaction of pozzolana with lime in the presence of water to form hydraulic compounds. The pozzolanic behaviour of ground clay brick (GCB) is similar to conventional materials such as fly ash and calcined clay at later age of 28 days [17]. The continuous of C-S-H gel within a pore structure contribute to strength development in concrete [14]. Results achieved in recent study show an increase in compressive strength as curing age increased due to the formation of C-S-H gel [18]. In line with this current study; development of additional C-S-H gel was observed when clay brick waste was used in mortar and concrete in an experimental study [19]. The high strength recorded for MC 1 can be attributed to the presence of pozzolanic material (brick waste) which contribute to high strength at a later age due to the rapid increase in silicon dioxide (SiO 2 ) leading to dicalcium silicate (2S) which is responsible for high strength at later age. This is because MC 1 has a lower cement replacement level which has resulted in high strength. Cement is responsible for the early strength of concrete and pozzolana from brick dust waste is responsible for strength at later ages by providing an extra amount of C-S-H gel [4]. Figure 17 shows the compressive test result. 3

Tensile splitting strength (TSS) test
MC 1 which is composed of 10% BDW achieved the highest TSS of 3.6 N/mm 2 in Fig. 18 compared to MC 2 and MC 3. The results show no significant difference in tensile strength for all mix compositions. However, a gradual reduction in strength as BDW percentage increase was observed for MC 2 and MC 3. High tensile strength was recorded when cement was replaced with 15% brick dust [4]. The close interval in tensile strength between mix compositions indicates a significant contribution to the increase in strength with the addition of BDW as a partial replacement for cement. The high tensile strength observed could be attributed to the angular shape of the aggregate coupled with the presents of pozzolans from BDW responsible for the production of high C-S-H gel during hydration [19]. Properties like tensile strength were significantly enhanced when waste materials were used to replace cement [4]. According to [17] tensile strength increased when brick waste was added to a concrete mix. The slow early strength observed in TSS is due to the presents of brick dust, however, the strength gain is higher compared to traditional concrete mix [17].

Conclusion and recommendation
The consistency and mechanical behaviour of concrete with varying proportions of brick dust waste content was investigated, discussed and the results showed high strength and good workability for all mix composition. MC 1 composed of 10% BDW achieved the highest UCS and TSS after 28 days of curing. MC 1 achieved all targets set in accordance with the relevant standard. A reduction in strength was observed with an increase in BDW percentage, However, the strengths achieved in this study are usable in construction. The strength pattern of concrete with BDW is similar to conventional concrete with cement only. A drastic reduction in slump was observed in MC 2. However, the concrete is still usable for construction. Results showed that brick dust waste has high water absorption abilities which can lead to variation in slump compared to traditional concrete. An increase in strength with increase in curing age was observed for all mix compositions. Some limitations associated with using brick dust waste in concrete mix is the process of harvesting. Brick dust waste are mostly contaminated with various construction and demolition materials such as concrete, steel and wood, this makes it difficult to and time-consuming during separation. Grinding bricks into powder using heavy crushing machine generate lots of noise, dust, heat and carbon dioxide (CO 2 ) which affects the environment. However, this is low compared to the problems associated with cement production. After careful consideration on the results, it's recommended that at least 10% of cement can be replaced with BDW in concrete mix in real-life construction, in order to reduce overall construction cost. Future investigation into the effect of BDW on the durability and rheological properties of fresh concrete and steel reinforcement in concrete can be carried out.

Conflict of interest
The authors declare that they have no conflict of interest associated with this publication and no financial support has been given to influence the outcome of this work.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.