Design of Concrete Made with Recycled Brick Waste and Its Environmental Performance

: In addition to the known uses of natural clays, less publication attention has been paid to clays returned to the production process. Industrially recovered natural clays such as bricks, tiles, sanitary ceramics, ceramic rooﬁng tiles, etc., are applicable in building materials based on concrete as an artiﬁcial recycled aggregate or as a pozzolanic type II addition. In this way, the building products with higher added value are obtained from the originally landﬁlled waste. This paper details the research process of introducing concrete with recycled brick waste (RBW) up to the application output. The emphasis is placed on using a RBW brash as a partial replacement for natural aggregates and evaluating an RBW powder as a type II addition for use in concrete. A set of the results for an RBW is reported by the following: (a) an artiﬁcial RBW ﬁne aggregate meets the required standardized parameters for use in industrially made concrete, (b) a RBW powder is suitable for use in concrete as industrially made type II addition TERRAMENT showing the same pozzolanic reactivity as a well-known and broadly used pozzolan-ﬂy ash, and (c) such an RBW as aggregate and as powder are, therefore, suitable for the production of industrially made TRITECH Eco-designed ready-mixed concrete.


Introduction
It is impossible to use natural clays in concrete due to their intensive water absorption, associated deterioration of performance, and obvious volume changes associated with outer humidity changes (e.g., wet-dry outer environment). This disadvantage is eliminated in the case of industrially recovered natural clays, which are applied in concrete either as an artificial aggregate or as a pozzolanic type II addition. In this way, space is open for the active use of industrially processed natural clays in concrete with the effective recovery of landfilled construction and demolition (C&D) waste into a material with a higher added value. Such treated clays can be homogenized with recycled concrete waste to be recycled together to gain a material usable again in the construction practice. The research to date has shown the evolution of preparing eco-friendly mortar with various fineness levels and replacement ratios of RBW coming from C&D waste. Recycled fine aggregates and waste powders can be ground into ultrafine powders, defined as recycled powders for effective utilization of C&D waste. However, the performance of recycled powders relies on their sources, the complex components, which hinder their application [1]. Due to the pozzolanic activity and filler effect of RBW, the use of high-fineness RBW improves the hydration degree and refines the pore network of cementitious materials; the pozzolanic reactivity of RBW increases with an increasing fineness [2]. The results showed that the irregular and rough shapes of RBW required higher water consumption to achieve a standard consistency of a cement-based paste. The higher alkali-content and lower content of SO 3 of recycled powder contributed to the higher heat evolution rates of the pastes with RBW in the first 30 min, but the pastes with recycled powder exhibited lower cumulative hydration heats than pure cement after 1 day. The additional use of RBW decreased the flowability, density, and strength of mortar. Incorporation of recycled fine aggregates at 15 wt % does not negatively affect the mechanical performance of the mortars. The mixed brick and clay brick powders had a relatively positive impact on the mechanical properties of mortar also at later ages. The maximum reduction of strength was about 20 wt %. The addition of RBW reduces the number of hydration products. Incorporating RBW increases the water demand and decreases the slump of the prepared mixture. The incorporation of RBW results in a decrease in drying shrinkage as well. When the RBW fineness is higher than that of the cement, the compressive strength increases with increasing RBW content up to 15 wt %, and the maximum activity index of RBW is approximately 90%. The flexural strength decreases linearly with increasing RBW content. The flexural strength of mortar with high-fineness RBW is higher than that of mortar with low-fineness RBW [1,[3][4][5][6][7]. The use of RBW decreases the resulting expansion due to the alkali-silica/aggregate reaction (ASR).
The CO 2 -curing treatment is more suitable for RBW modification because RBW absorbs the CO 2 gas while also improving the properties of the prepared concrete, and thus, is more eco-friendly. RBW-containing concrete has a lower preparation cost than plain concrete, and the energy consumption and CO 2 emission of concrete preparation decrease with an RBW incorporation. The use of RBW in cement concrete products shows good recycling, economic and environmental benefits [8]. The results show that such a replacement reduces the chloride migration, but, contrary, increases water absorption, water sorptivity, drying shrinkage, and carbonation. The above-mentioned properties can be minimized by the water content reduction using a superplasticizer. The pore structure of RBW-containing concrete deteriorates with increased cement replacement because of the RBW porous structure. Contrary, due to the pozzolanic reactivity of RBW, the Ca(OH) 2 crystals in the formed binding hydrate phase are consumed to generate to a larger extent gel-like hydration products (C-S-H and C-A-H). This, in the final effect, results in a denser interfacial transition zone and enhanced adhesion between the RBW and the cement matrix [9]. It was also revealed that the recycled concretes incorporating ceramic waste as secondary clay materials have a comparable performance level to the one exhibited by the conventional concrete at 28 days, in part due to their pozzolanic characteristics, but also due to a lower effective water-cement ratio [10]. The application of recycled clay brick in concrete can solve the disposal problem of demolished solid waste. There are two main utilization purposes of such material: (1) recycled clay brick powder has a pozzolanic reactivity and can be used as a cement replacement, and (2) recycled clay brick aggregate can attain suitable strength and can be used in the production of medium-and low-strength concrete [11]. A case study conducted to verify the applicability of RBW in Shenzen of China subjected to massive urban renewal pressure shows the following types of recyclable products: 18.41 million tons of recyclable bricks, 7.02 million tons of mortar, 28.36 million tons of aggregate, and 4.16 million tons of lightweight wallboard [12]. At present, 200,000 tons annually of industrially processed RBW waste with recovered natural clays and recycled concrete content of approx. 51:49% by weight are manufactured by Hasenöhrl GmbH, St. Panthaleon (Austria) for reuse in TRITECH Eco-designed ready-mixed concrete C 25/30 strength class.
Today's state-of-the-art issues of using recycled material also include other products, such as beer green glass, waste tile, asphalt [13], glass as fine aggregate [14] and ceramic [15] waste. Each waste, by its presence in concrete, affects its properties. Attention is focused mainly on strength characteristics and long-term service life than concrete made only of natural aggregates.
This paper also aims to show the feasibility of using a recycled brick waste as a partial alternative of natural fine aggregate and a fine powder in the Eco-concrete production (TRITECH) [16] and as the fine powder as pozzolanic type II addition (TERRAMENT) [17] for use in concrete. Unlike existing laboratory experiments and relevant findings to date, this paper upgrades the existing knowledge with information about the complete research process necessary to implement recycled brick waste in practice as the certified construction products.

Research Significance
The related research was focused on the examination of the suitability of such recycled brick waste in concrete. The RBW substitutes a part of 0/4 mm fraction of natural aggregate (further abbreviated as NA) with equivalent grain-sized 0/4 mm (RBW-0/4) and/or cement as a powder of the specific surface area 300 m 2 /kg (RBW-P300) and 500 m 2 /kg (RBW-P500). The utility properties of RBW as an aggregate and those of NA of 0/32 mm size and as a blend of 0/32 mm NA and RBW 0/4 mm (NA + RWB blend) according to the specific concrete mixture composition were tested in compliance with the requirements of STN EN 13055-1 [18] and STN EN 12620 + A1 [19]. As the subject of innovative know-how, the mentioned concrete mixture compositions are confidential. The RBW powder was alone studied as an innovative type II addition for use in concrete, partially replacing the cement. Pozzolanic reactivity according to the Frattini test [20] and the suitability of an RBW powder properties, as specified in STN EN 450-1 [21] defining the chemical, physical, and quality control requirements for fly ash used as type II addition following STN EN 206 + A1 [22], were determined. The article introduces the methodological and experimental process of technology-oriented investigation with the final output in two innovative concrete products into building practice. The industrial outcome is the TRITECH Eco-designed ready-mixed concrete C 25/30 strength class and the type II addition TERRAMENT for use in concrete [23,24].

Casting and Testing
The consistency of fresh concretes was determined either by the flow-table or the slump-test methods [25]. After demolding (24 h), concrete specimens as 150 mm cubes and (100 × 100 × 400) mm prisms were stored in water at (20 ± 1) • C. Utility properties were determined at 28 and 90 days of concrete age. The k-value for the RBW powder with a specific surface area of 300 m 2 /kg and 500 m 2 /kg at the 10 wt % CEM II/B-S 42.5 N replacement was determined. Following the determined k-value of the RBW powder, the mandatory Aspects of durability were verified [26]: the 28-and 90 days cube compressive strength [27], suction capacity [28], and depth of water penetration under pressure [29]. Subsequently, the resistance to carbonation by the partially accelerated test according to the TSÚS methodology [30] (TSÚS is Slovak abbreviation for Building Testing and Research Institute in Bratislava) was determined based on the own carbonation resistance assessment criteria [31] exceeding those of recommended in TNI CEN/TR 16639 [27].

Verification of RBW Powder as the Active Pozzolan
The RBW powder was tested on a pozzolanic reactivity by Frattini test performed on two sets of the samples with a specific surface area of 300 m 2 /kg and 500 m 2 /kg. The Frattini test consists of mixing 1 g RBW with 75 mL of saturated lime solution at (40 ± 1) • C for 1, 7, and 28 days. At the end of each period, CaO-remaining concentrations in the solution are measured according to  and re-calculated to mass percentage consumption of CaO by the RBW.

Applicability of RBW Powder as a Pozzolanic Type II Addition
The RBW properties were determined according to STN EN 450-1, which complies with STN EN 206 + A1. This closest standard related to fly ash was chosen to evaluate the RBW powder as a pozzolanic addition, as the tested RBW does not have its standard. The RBW powder may not meet the parameters required for fly ash; these serve as indicators of the quality of a pozzolan under consideration.

Performance of Aggregates
Relevant aggregate tests according to the EN 12620 +A1 requirements were carried out on three aggregate systems: fine RBW with declared grading: D ≤ 4 mm, NA having declared grading: D between 0 and 32 mm, and a blend of NA and RBW, in which a part of NA was substituted by fine RBW 0/4 mm in the experimental concrete (EXP-1) according to the specified concrete mixture composition. The aggregates were tested asreceived from the producer. According to STN EN 13055-1, density in the dried state did not exceed 2000 kg/m 3 (2.00 Mg/m 3 ) or whose bulk density of loose aggregate did not exceed 1200 kg/m 3 (1.20 Mg/m 3 ). Concerning the values of bulk density of loose aggregate 1.20 Mg/m 3 and grain density of dried aggregate 2.12 Mg/m 3 , fine RBW 0/4 mm was included in the category of common aggregates with property criteria according to STN EN 12620 + A1 and not as a light aggregate according to STN EN 13055-1. Table 1 provides the list of verified properties necessary for the certification process and relevant standards, as well as the tests needed to verify the properties of RBW 0/4 mm, NA 0/32 mm and a (NA + RBW 0/32 mm) blend applied in the TRITECH ecotechnology concrete.

Performance of Cement and Superplasticizer
The cement properties were analyzed according to EN 197-1 [38] and STN 72 1179 [39], and those of superplasticizer according to EN 934-2 + A1 [40]. The required range of tests is reported in Table 2.

Performance of Concrete
The REF-1 concrete and EXP-1 concrete made with RBW as a 0/4 mm aggregate and as a powder was tested according to valid European (STN EN) and Slovak (STN) standards. The necessary standard tests for the certification process are revealed in Table 3. Table 3. Range of standard tests for fresh and hardened concrete.

Property of Concrete Test Rule
Consistency of fresh concrete-flow

Determination of k-Value of RBW Powder
The k-value calculation of the RBW powder is based on the determination of 28 day compressive strengths of the REF-2 and EXP-2 concrete. The 30 various concrete mixtures differ with two dosages of the cement (260 kg/m 3 and 360 kg/m 3 ), two specific surface areas of RBW powder (300 m 2 /kg and 500 m 2 /kg) at 10 wt % cement replacement by the RBW powder and 5 water to cement ratios (0.40, 0.45, 0.50, 0.55, and 0.60), were prepared. The consistency of fresh concrete mixtures was recognized by the slump-test method. The 28 days cube compressive strengths were determined, and the respective k-values were calculated from the regression of the observed dependencies according to the TNI CEN/TR 16639. The k-value determines the strength equivalence between REF-2 and EXP-2 concrete. The STN EN 206 + A1 prescribes the determined k-value for each pozzolan in conjunction with the used cement. Until a pozzolan has not determined the k-value, it must not be applied in any construction concrete.

Resistance to Carbonation
Verification of the correctness of the RBW powder k-value was performed for use in XC2. Resistance to carbonation was assessed on the mortars by the accelerated carbonation test (ACT) in the carbonation chamber according to the TSÚS Methodology [30,31]. The resistance of EXP-2 mortar containing RBW powder having a specific surface area of 300 m 2 /kg and a k-value of 0.5 was compared with that of the REF-2 mortar. The k-value found for RBW powder at the previous concrete tests was used in determining the material composition of the EXP-2 mortar (Table 4). This step was taken to apply the ACT results to the concrete. The TNI CEN/TR 16639 enables performing the accelerated tests to determine the durability of smaller samples when the mortars of related components are used for this purpose. The mortars (40 × 40 × 160) mm were conditioned to constant weight before transferring to the climate chamber. The ACT was performed for 28 days in air saturated with 20% vol. CO 2 at (20 ± 1) • C and 50% relative humidity (RH). The second half of mortar specimens were treated only in reference (20 ± 1) • C/50% RH air.

Suitability of RBW Powder as a Pozzolanic Type II Addition
Pozzolanic reactivity of RBW powder with other pozzolans is compared in Figure 1. Both RBW powder modifications show pozzolanic reactivity similar to broadly used fly ash and ground blast-furnace slag. RBW powders are classified as medium active pozzolans and, therefore, suitable for use in concrete.

Performance Properties of Aggregate
The chemical criteria for the suitability of (NA + RBW 0/4 mm) aggregates for use in concrete are specified for chloride ion content, sulfur compounds, and other constituents that alter the rate of setting and hardening concrete and affect the volume stability. The maximum water-soluble and acid-soluble chloride ion content must be 0.1 wt % Acidsoluble sulfate content is given by the value ≤0.2%. Total sulfur may not exceed 1 wt %. The influence of water-soluble materials occurring in (NA + RBW) blend aggregates on the initial setting time of cement paste is less than ≤40 min. The proportion of organic material may be such that they do not increase the setting time of mortar test specimens by more than 120 min and decrease the compressive strength of mortar test specimens by more than 20% at 28 days.
Tables 5-12 compare the test results of three tested aggregates: RBW 0/4 mm, NA 0-32 mm, and (NA + RBW 0/4 mm) blend, which are important for assessing the suitability of use as aggregates in concrete.

Performance Properties of Aggregate
The chemical criteria for the suitability of (NA + RBW 0/4 mm) aggregates for use in concrete are specified for chloride ion content, sulfur compounds, and other constituents that alter the rate of setting and hardening concrete and affect the volume stability. The maximum water-soluble and acid-soluble chloride ion content must be 0.1 wt % Acidsoluble sulfate content is given by the value ≤ 0.2%. Total sulfur may not exceed 1 wt % The influence of water-soluble materials occurring in (NA + RBW) blend aggregates on the initial setting time of cement paste is less than ≤ 40 min. The proportion of organic material may be such that they do not increase the setting time of mortar test specimens by more than 120 min and decrease the compressive strength of mortar test specimens by more than 20% at 28 days.
Tables 5-12 compare the test results of three tested aggregates: RBW 0/4 mm, NA 0-32 mm, and (NA + RBW 0/4 mm) blend, which are important for assessing the suitability of use as aggregates in concrete. Figure 2 compares the mineral composition of RBW 0/4 mm and (NA 0/32 mm + RBW 0/4 mm) blend, while the difference in their phase composition is illustrated in Figures 3 and 4.   Chemical properties seem to be most unfavorable for fine RBW. Any negative impact of the (NA 0/32 mm + RBW 0/4 mm) blend on the properties of concrete is eliminated.
The fines content of the (NA + RBW) aggregate is considered non-harmful because the standard describes ≤ 3 wt % of fines. The addition of RBW to NA to create (NA + RBW blend) only slightly increases the fines content. However, fine RBW alone is characterized by a high proportion of fine grains. This fraction does not meet the condition of the maximum limit of fines ≤ 3. The results show that replacing a part of the NA 0/4 mm fraction with the 0/4 mm RBW fraction does not cause any obvious change in the grain composition of the resulting (NA + RBW) blend.                Chemical properties seem to be most unfavorable for fine RBW. Any negative impact of the (NA 0/32 mm + RBW 0/4 mm) blend on the properties of concrete is eliminated.   Both aggregate types show a decisive loss on ignition by releasing CO 2 bound in calcite and dolomite between 700 • C and 850 • C. This is confirmed by DTA plots by an endothermic double deflection, with a smaller endotherm in the first left indicating the thermal decomposition of dolomite and a second larger dissociation of calcite. The material similarity of both aggregate types is recorded by XRD and TG-DTA methods. The addition of recycled brick waste on a change in the mineral and phase composition compared to pure natural aggregates is negligible. A fine depression, indicating a weak endotherm with a maximum at about 300 • C, is attributed to the loss of bound water from the marginally present osumilite. Highly similar material compositions and dominant representations of quartz, dolomite, and calcite in the studied aggregates are confirmed by both techniques. None of the aggregates contains minerals potentially evoking the alkali-silica reaction of the aggregate with the cement or water-active natural clays, which harm the concrete performance. The influence of marginally present muscovite is negligible. One would state that industrially processed clays after calcination eliminate this dangerous disadvantage for concrete volume stability.
The basic properties of aggregates concerning the protection of human health and the outer environment, which are an integral part of the certification of aggregates intended for use in concrete, are presented in Tables 10-13. The values of the monitored properties are the most unfavorable for RBW 0/4 mm, but they all meet the legislative conditions in Slovakia for building materials intended for the constructions with living spaces (mass activity index must be <1). The addition of RBW 0/4 mm to NA 0/32 mm as a partial replacement for the natural 0/4 mm fraction confirms the applicability (NA + RBW) blended aggregate in concrete from the viewpoint of the danger caused by the radioactive radiation.
The measured values in the water extract are suitable for leachability class I in all monitored parameters.
The waste can be placed in a landfill for waste that is not dangerous. The test results of the (NA 0/32 mm + RBW 0/4 mm) blend declare: (a) the values of bulk density of loose aggregate, bulk density, and absorbency correspond to the values of natural mined aggregate (gravel); (b) no organic substances are detected-the test for the presence of humus was negative, (c) the content of water-soluble chloride ions in natural aggregates is considered not to be hazardous if its value is below 0.01% by mass-water-soluble chloride ions were not detected in the (NA + RBW) aggregate; (d) the aggregate contains acid-soluble sulfates 0.11% by weight, thus meeting the requirement for the strictest category (≤0.2% by weight)-a slightly increased value compared to the pure NA was caused by the presence of recycled brick waste, (e) the total sulfur does not exceed the permitted value of 1 wt %; (f) the presence of reactive forms of aggregates, which could cause the concrete to expand, is not found-6 month dilatometric test unambiguously confirms the (NA + RBW) aggregate as non-susceptible for alkali-silicate reaction; (g) the aggregate meets the requirements of the said decree for building materials intended for the construction of buildings with residential areas; (h) the water extract of the aggregate meets the requirements of the valid legislation designation as inert waste, and its composition does not harm the environment or toxic effects on biotic systems; (i) the aggregate does not contain substances that could negatively affect the volume stability of the made concrete, The overall evaluation of (NA 0/32 mm+ RBW 0/4 mm blend) aggregate is as follows: the tested aggregate meets all requirements of STN EN 12620 + A1 as well as requirements for radionuclide content, mass activity index, and on the properties as inert waste according to the values of selected indicators in the native state, in the water extract of waste and ecotoxicity. All values of monitored indicators are suitable for inert waste. The set of tests confirms the mechanical, geometrical, chemical, and ecotoxicological suitability of the (NA 0/32 mm, an RBW 0/4 mm blend) aggregate for use in concrete production.

Verification of TRITECH Eco-Designed Ready-Mixed Concrete
Basic properties of the used Portland-blast furnace slag cement, superplasticizer, and the utility properties of fresh and hardened concrete for TRITECH ecotechnology are listed in Tables 13-18. Both concrete (REF-1 and EXP-1) are characterized by fully comparable strength parameters at the age of 28 and 90 days. The chosen replacement does not deteriorate in any way, the rheological properties of fresh concrete, but also the strength. This finding is a realistic assumption for a comparable long-term service life of both concrete types. The obtained results prove the applicability of TRITECH Eco-designed ready-mixed concrete as an equivalent qualitative substitute for REF-1 concrete.

Determination of k-Values for RBW Powder
Tables 19 and 20 reveal 28 days compressive strengths of 30 different concretes. Kvalues were calculated from the regression of the observed dependencies based on the same strength conditions [27].
The estimated k-values for the CEM II/B-S 42.5 (N and R) contents are specified in Tables 21 and 22. K-values refer only to using CEM II/B-S 42.5 in concrete by the TNI CEN/TR 16,639. Another cement requires separate testing. The universal valid k-value of industrially made type II RBW addition TERRAMENT for use in concrete is k = 0.5. This k-value is applicable in the whole range of water to cement ratios, except that of 0.65 and higher, the content of the cement CEM II/B-S 42.5 260 kg/m 3 and more, and a limit fineness of RBW powder between 300 m 2 /kg and 500 m 2 /kg. The RBW powder, as a pozzolanic type II addition, records higher k-values for individual water to cement ratios below 0.6, even up to that of 0.8. TERRAMENT meets the condition of using concrete with the specified k-value of type II addition prescribed in STN EN 206 + A1 and, therefore, can be applied industrially in the construction concrete.

Verification of the Durability Aspect-Resistance to Carbonation
To verify the resistance of the RBW powder of 300 m 2 /kg fineness (TERRAMENT) against aggressive CO 2 , the cement mortars (REF-2 and EXP-2) with the already calculated value k = 0.5 were used for testing (see Table 4). Figures 5-8 show decisive REF-2 and EXP-2 mortar properties. The degree of carbonation was determined by the accelerated carbonation test (ACT) based on the TSÚS Methodology [30,31] from the chemical tests performed. Quantitative evaluation of the parameters determining the extent of carbonation in mortars is listed in Table 23. The TSÚS method of evaluation includes six individual change criterion ∆k based on the relevant standards for the design of concrete: STN EN 206 +A1, STN EN 1992-1-1 Eurocode 2 [54] and own diagnostic practice in the area of structural concrete [55,56].                 The study of changes in monitored REF-2 and EXP2 mortar properties focusing on the evaluation of carbonation resistance shows that EXP-2 mortar has the potential for resistance to aggressive CO 2 against the influence of the XC2 environment of STN EN 206 + A1.

Conclusions
Based on a series of the performed tests, the following conclusions are drawn: (1) The powdered recycled brick waste (RBW) together with 0/4 mm fraction partially replacing a part of cement and natural aggregate in the specified concrete mixture composition are declared as materials suitable for the production of industrially made TRITECH Eco-designed ready-mixed concrete of C 25/30 strength class; (2) The type II RBW powder, as industrially made addition TERRAMENT, for use in concrete is characterized by the pozzolanic reactivity fully comparable to the standardized fly ash; (3) The universal valid k-value of TERRAMENT is k = 0.5; this k-value applies to the whole range of water to cement ratios, except that of 0.65 and higher, the cement content of CEM II/B-S 42.5 (N and R) from 260 kg/m 3 and more at the 10% weight percentage replacement of the cement by TERRAMENT. The k-values based on the specifically determined water to cement ratios vary from 0.5 to 0.8. One must consider that STN EN 206 + A1 does not allow using type II additions in concrete without a specified k-value; (4) Experimental cement mortar EXP-2 (with calculated k-value 0.5) made with CEM II/B-S 42.5 and TERRAMENT, and at the same water to cement ratio as concrete that is confirmed by the respective k-value 0.5, is resistant to the influence of aggressive CO 2 on the XC2 environment as specified by STN EN 206 + A1. This means that concrete with TERRAMENT is, in the same way, durable to carbonation in the XC2 environment; (5) The issued SK-technical assessments [20,21] for TERRAMENT type II addition and TRITECH Eco-designed ready-mixed concrete means at the national level the documents with the legitimacy equal to the Slovak technical standard (STN); (6) Research to date carried out globally, especially in China, but also ours, proves that the secondary recovered natural clays are becoming a strategic raw material in the production of eco-friendly concretes.
TRITECH presents a new ecotechnology that enables using industrially processed recycled brick waste in the construction sector. TRITECH was certified in TSÚS, Bratislava and subsequently defended at the Austrian Standard Institute, Vienna, for using recycled 0/4 mm aggregate and powder in concrete. TRITECH ecotechnology and type II addition TERRAMENT based on RBW are the results of industrial and research cooperation between Austria and Slovakia.