Concretes with binary mixtures of artificial pozzolans and concrete demolition waste

he objective of this study is to analyse the use of binary mixtures of pozzolanic materials and concrete demolition waste in concrete mixtures, especially the resulting mechanical properties and durability. A total of ten concrete formulations were produced distinguishing them in different types using different Portland cements, different artificial pozzolans and coarse aggregates from concrete demolition. The particular properties of each formulation were verified by testing the axial compressive strength, longitudinal modulus of elasticity and penetration of chloride under immersion. Substitutions were of 15% w.t. and 30% w.t. natural coarse aggregate substituted with concrete demolition waste and, in the case of binary mixtures, additional 25% w.t. of the binder agglomerate substituted with rice husk ash or fly ash. Results showed that the final strength to axial compression and modulus of elasticity of concrete mixtures were negatively affected by utilising demolition waste, but this effect was balanced by adding supplementary cementitious materials. Regarding the durability test, it was found that the lowest coefficients occurred in the mixtures using CP V-ARI, together with artificial pozzolans, in mixtures with 15% w.t. substitution of natural aggregate with recycled aggregate. It was concluded that using recycled aggregates in concrete is viable but conditioned to the concomitant use of pozzolanic materials.


Introduction
Using concrete in construction and engineering works is widespread and estimated at approximately 20 billion metric tons per year (WANG et al., 2017). This high use results in a substantial demand for cement which is its principal constituent. The global demand for cement is estimated at 4 billion metric tons per year, of which 40 million tons were the estimated demand in Brazil in 2019 (U.S. GEOLOGICAL…, 2019;SINDICATO…, 2019). According to Chen et al. (2017) and Huang et al. (2018), the cement industry generates about 8% of global carbon dioxide (CO2) emissions. As such, the carbon footprint of this cement demand is bound to have negative environmental effects.
Furthermore, Blankendaal, Schuur and Voordijk (2014) raise serious concerns regarding environmental damage and destruction of the ecological equilibrium of cement use. Additionally, if there is no appropriate recycling of waste, most industrial by-products are incinerated resulting in contamination and atmospheric pollution (XUAN; POON; ZHENG, 2018).
The International Energy Agency (IEA) and World Business Council for Sustainable Development (WBCSD) released a Cement Technology Roadmap in which four strategies are listed so that the cement industry can reduce CO2 emissions by 2050. These strategies are electrical and thermal efficiency; alternative fuels; clinker substitution and carbon capture and sequestering. Regarding electrical and thermal efficiency, Schneider (2015) states that kiln efficiency is already at a limit. On the other hand, clinker substitution can produce more immediate results with a potential reduction of 25% to 30% in CO2 emissions. However, if this strategy is taken to its maximum potential, sources of supplementary cement materials (SCMs) could become scarce (DIAZ-LOYA et al., 2019).
In the current scenario, alternatives have been used to reduce the environmental impact of cement use and improper handling of waste. One solution has been to use clinker substitution with several artificial pozzolans, for example: fly ash (FA), rice husk ash (RHA), active silica etc. Medina et al. (2018) points out that, in binary mixtures with distinct additions, SCMs are beneficial as long as there is a balance between substitution proportions and performance, which are in turn dependent on physio-chemical characteristics of the SCMs. These characteristics interact differently in the concrete paste and influence its microstructure so that the resulting concrete has distinct mechanical and durability properties (ISAIA; FURQUIM; GASTALDINI, 2009).
Concrete demolition waste (CDW) recycling can reduce the expenditure of natural raw materials. However, when used in concrete mixtures, CDW can reduce mechanical strength and increase porosity and water absorption. The degree of these effects depends on the level of substitution, production technique for the aggregate concrete and the inter-surface transition zone (KOU; POON, 2015;ISAIA et al., 2017). Despite these limitations, Santos and Leite (2018) demonstrated that concretes with up to 40% recycled coarse aggregate could be made viable with compressive strength in accordance to stipulated levels from mixing ratios, improved density and adherence between cement and aggregates. An alternative form or CDW reuse was demonstrated by Kalinowska-Wichrowska, Pawluczuk and Boltryk (2020). In this case recycled mortar is mechanically and heat-treated in an apparatus which produces high-quality recycled concrete aggregate.
Based on these facts, the objectives of this study were to analyze the mechanical properties of axial compressive strength and elasticity modulus in concretes with: (a) partial substitution of natural coarse aggregate (NCA) with aggregate from concrete demolition waste; and (b) partial substitution of different Portland cements with rice husk ash or fly ash.

Materials and methods
Four types of materials were used in this study: CP IV -32, CP V -ARI, RHA and FA. CP V -ARI cement is particularly suitable to test the effect of partial substitution of binders because it has a low level of mineral admixtures. On the other hand, substitution was not applied to CP IV as it already contains pozzolans in its composition. Supplementary cement materials were chemically characterised according to NBR 12653 (ABNT, 2015a) and ground in a ball mill for 1 hour. The physio-chemical characteristics of the materials are presented in Tables 1 and 2, respectively.
4.89 2.27 --Furthermore, the aggregates used in this study were: (a) coarse aggregate from basalt (gravel); (b) natural fine aggregate (sand); and (c) concrete demolition waste (CDW) aggregate from the production of interlocking concrete pavers.
The physical characteristics of the aggregates are shown in Table 3.
The water used fo concrete mixtures was municipal tap water. A plasticiser chemical admixture -MasterGlenium54with chemical characteristics shown in Table 4 was added to the water to improve workability of the concrete paste.
The mineralogical characterisation of the supplementary cementitious materials was obtained through X-ray Diffraction (XRD) as shown in Figure 1.
Concrete mixing ratios were based in theoretical and experimental studies following the methods proposed by Helene and Terzian (1992) in order to obtain suitable results for workability, additive ratio, mixing time and Abrams law curve. Chosen parameters were characteristic axial compressive strengths of 25 MPa and 35 MPa, two types of Portland cement with water/binder ratios (w/b) of 0.30, 0.475 and 0.65 and slump test results of 100 ± 20 mm. Furthermore, the water/dry materials ratio was defined as 8.22% and the dry mortar ratio as 0.50 so that mass-based mixing ratios in the format (cement:sand:gravel: w/b) could be determined.
According to NBR 12655 (ABNT, 2015b) and assuming a standard deviation of 4 MPa, average strengths for the mixtures after 28 days were 31.6 MPa and 41.6 MPa for the pre-defined characteristic strengths of 25 MPa and 35 MPa, respectively. Each cement mixture was moulded into cylindrical test specimens (Ø10cm x 20cm) following NBR 5738 (ABNT, 2016). There were 3 mixture ratios for each concrete mixture and 3 samples were prepared for each mixing ratio. Samples were water cured for 28 days after which axial compressive strength tests were performed according to NBR 5739 (ABNT, 2018) procedures. The Tab;e 5 shows these results.
180  Based on the results shown in Table 5, Abrams Law curves were plotted and exponential expressions were determined as shown in Figure 2 and Table 6. Table 6 also displays the coefficient of determination R 2 and the water to binder ratios for each axial strength value.
Abram's law curves allowed the determination of the definitive concrete mixture ratios shown in Table 7. Samples prepared according to these ratios were subjected to axial compressive strength tests in accordance with NBR 5739 (ABNT, 2018), modulus of elasticity tests in accordance to NBR 8522 (ABNT, 2017) and penetration from immersion in chloride ion in accordance with Otsuki, Nagataki and Nakashita (1992) and Wee, Suryavanshi and Tin (2000). In addition, the statistical analysis for assessing the durability of concretes was performed using the parametric ANOVA test (two-way). 181

Axial compressive strength
Axial compressive strength is one of the most important properties to be measured in hardened concrete. Satisfactory strength levels are a reflection of the quality of materials used. Figure 3 shows average results from 80 test specimens after 28 and 91 days of curing. Figure 3 shows that concrete mixtures aged 28 days reached axial compressive strengths within predicted parameters. These corresponded to values within an interval of f 28 = 31.6 ± 5% and f 28 = 41.6 ± 5% with the best results for samples that contained artificial pozzolans in the mixture. Increasing the curing time to 91 days resulted in increases in strength with an average increase of 14.82%. This result suggested that increases in strengths could be related to the grinding process of the ashes since smaller particle sizes tend to favour nucleation of hydration compounds. Strength of mixtures which contained only CDW and CP V -ARI decreased with increasing aggregate substitution for both 28 days and 91 days. Similar reductions in strength were observed for mixtures which contained only CDW and CP IV, despite this type of cement containing pozzolanic material in its composition. According to Leite (2001) and Ryu (2002), the reduction in strength could be attributed to:  Erhart et al. (2014) also related that increases in strength can also be attributed to the high pozzolanic nature of RHA and FA while Mehta and Monteiro (2014) noted that artificial pozzolan addition reduced the porosity of the cement paste, increased its compaction and further refined calcium hydroxide grains.

Elasticity modulus
Elasticity modulus is affected by CDW having higher porosity while at the same time having a lower deformation modulus than natural aggregates due to its low capacity to resist matrix deformations. Consequently, NBR 8522 (ABNT, 2017) guidelines stated the need for determining static elasticity and deformation to compression. For this study, a total of 40 test specimens were tested after 28 days curing. From these, only concrete mixtures with elasticity modules higher than 37.78 GPa and 43.34 GPa were within the parameters, considering the defined characteristic of 25 MPa and 35 MPa (31.6 MPa and 41.6 MPa after 28 days, respectively). Results are shown in Figure 4 and Table 8. Table 8 shows that most concrete mixtures were not able to meet the elasticity modulus of 37.78 GPa and 43.34 GPa predicted by NBR 6118 (ABNT, 2014). This could be attributed to the larger deformation characteristic of CDWs when compared to natural aggregates. This result was also reported by Angulo (2005) and Poon, Qiao and Chan (2006). Substitution of Portland cement with 25% w.t. of supplementary cementitious materials improved results, where the substitution by rice husk ash resulted in differences of +4.91% and -13.20% in elasticity modulus when used with 15% w.t. and 20% w.t. CDW substitution, respectively. In comparison, the substitution by fly ash resulted in differences of -3.35% and -16.38% in elasticity modulus when used with 15% w.t. and 20% w.t. CDW substitution, respectively. However, 184 Almeida (2005) stated that elasticity modulus variation was not proportional to increases in compressive strength.
The CP V -ARI reference sample had an elasticity modulus difference of +3.46% with respect to NBR 6118 (ABNT, 2014). Samples with 15% w.t. and 30% w.t. CDW substitution had differences of +2.09% and -14.53%, respectively. On the other hand, no CP IV cement mixture was able to meet the predictions: the reference sample had a difference of -14.29% while samples with 15% w.t. and 30% w.t. CDW substitution had differences of -21.81% and -29.46%, respectively. These results were in accordance with Hui-Sheng, Bi-Wan amd Xiao-Chen (2009), Padmini, Ramamurthy and Mathews (2009) and Safiuddin et al. (2011) and were a reflection of recycled aggregates which were more porous and resulted in decreases in modulus of elasticity as the substitution rate increased. Gómez-Soberón (2002) also verified that a decrease in elasticity modulus was related to an increase in its porosity. Fonseca, Brito and Evangelista (2011) stated that the elasticity modulus of concrete was highly dependent on the rigid phases (aggregates, cement paste and transition zone) while Dal Molin (2005) considered aggregate characteristics the limiting factor.

Chloride Penetration under Immersion
Chloride penetration under immersion was studied by immersing the specimens in saline solution similar to seawater (31.94g NaCl per litre of distilled water) for four distinct periods: 28 days, 56 days, 84 days and 112 days. Four surfaces were analysed from each specimen to obtain an average penetration depth. Results are shown in Table 9. Table 9 does not show significant differences in penetration depths between mixtures of CP IV -32 and CP V -ARI cement. However, the ANOVA analysis shown in Table 10 proves no significant statistical difference occurs in the penetration depth with respect to the cement type.   Considering the concrete mixtures that did not include cement CP IV -32, Table 9 shows that mixtures with artificial pozzolans achieved better penetration results over all ages, with some mixtures outperforming the reference samples. Medina, Frías and Rojas (2012) and Thomas et al. (2013) hypothesised that concrete with CDW and pozzolans had improved interfaces between old and new concrete. This was a result of better adhesiveness between artificial pozzolans (such as RHA or FA) and recycled aggregates, creating a sealant able to fill pores and allow greater compaction of the mixture. It should be noted that Table 9 shows that RHA and FA had similar results even if these had distinct physio-chemical characteristics both with respect to fineness and with respect to oxide content. Rice husk ash had 0.39% w.t. of Al 2 O 3 and 0.54% w.t. of Fe2O3 while fly ash had 25.89% w.t. of Al2O3 and 3.15% w.t. of Fe2O3. Regarding fineness and the potential to fill pores, the BET specific area of RHA was 18.71 m²/g while FA was 3.55 m²/g, meaning that FHA fineness compensated its lower aluminium oxide content and vice-versa for FA. Furthermore, Wee, Suryavanshi and Tin (2000) stated that chloride penetration depended on the microstructure of concrete and its modifications, such as fixing chloride by the presence of aluminates and iron aluminate phases. Thus, substituting cement with pozzolans using low aluminate content such as RHA decreased the ability of chloride to combine with aluminates and could result in deeper penetration coefficients. However, mineral additions altered the porous structure of concrete and reduced chloride penetration and infiltration of humidity and oxygen. This increased concrete durability and contributed to chloride corrosion prevention (DAL MOLIN, 2011).

Conclusions
Reference CP V -ARI cement mixture had a measured axial compressive strength of 4.17 MPa with a w/b ratio of 0.54. When natural coarse aggregate was replaced with CDW in 15% w.t. and 30% w.t., average 186 strengths were measured as 30.59 MPa with a w/b ratio of 0.55 and 30.25 MPa with w/b ratio of 0.42, respectively. Concerning CP IV cement, the reference mixture had a measured strength of 30.32 MPa with a w/b ratio of 0.53 and the equivalent mixtures with 15% w.t. and 30% w.t. CDW substitution had average strengths measured as 30.44 MPa with a w/b ratio of 0.50 and 30.23 MPa with a w/b ratio of 0.42, respectively. These results showed that CDW substitution was detrimental to axial compressive strength, as well as longitudinal elasticity modulus.
Results also showed that the decrease in axial compressive strength from CDW use could be circumvented by using supplementary cementitious materials. Concrete mixtures with 15% w.t. and 30% w.t. CDW substitution, which had 25% w.t. cement-to-RHA substitution had measured strengths of 40.64 MPa with a w/b ratio of 0.62 and 40.25 MPa with w/b ratio of 0.53, respectively. Similarly, 15% w.t. and 30% w.t. CDW substitution mixtures, which had 25% w.t. cement-to-FA substitution, had measured strengths of 40.80 MPa with a w/b ratio of 0.57 and 39.89 MPa with w/b ratio of 0.47, respectively. These results clearly showed the corrective effect of pozzolan substitution in axial compressive strength, which was also extended to longitudinal elasticity modulus, even if the effects were not proportional between these properties.
Concrete durability was evaluated through chloride penetration under immersion. Concrete mixtures with CP V -ARI cement and partial pozzolan substitution achieved the best results. Similar to natural coarse aggregate substitution with CDW which was detrimental to axial compressive strength and longitudinal modulus of elasticity, both RHA and FA substitution were shown to compensate increases in chloride penetration. These results are repeatable considering previously used materials and mixing ratios.