The Effect of Curing Conditions on Selected Properties of Recycled Aggregate Concrete

: The paper presents the influence of different curing conditions – wet, dry and protection against water evaporation on selected properties of concretes with different amount of recycled concrete aggregate previously subjected to atmospheric CO2 sequestration. Additionally, the eco-efficiency bi and ci indexes as well as eco-durability S-CO2 index were calculated. It was found that dry conditions deteriorate the properties of concrete, especially made of blast furnace slag cement, while protection against evaporation allows to achieve results comparable to wet conditions. Moreover, for series with the highest amount of coarse recycled aggregate and after longer period of curing, the difference between the effects of wet curing and protection against water evaporation disappears. The eco-efficiency and eco-durability indexes approach confirms the beneficial effect of blast-furnace slag cement used as a binder but on condition of proper way of curing.


Introduction
Sustainability has been becoming the main idea of modern technology in civil engineering.
With its increasing profits in human activity, despite time limitation in designing, it is capable to achieve continuous development in modern society. Such perception of the concept of sustainable development requires the fulfilment of the 4R principle, i.e.: reduction of material and energy consumption, reuse of products, thus extending their life expectancy, processing of products by recovering some part of raw materials as well as recovery of materials from waste products.
The construction industry is burdening the environment by, among others, exploiting deposits of natural resources and simultaneously producing large amounts of waste. The annual production of concreteas the most popular construction materialamounts to nearly 10 billion tons per year.
Undoubtedly, significant consumption concerns mineral natural and crushed aggregates resulting from crushing of rock raw materials. This leads, among others, to depletion of natural resources, violation of ecosystems, carbon dioxide emission; the latter also resulting from cement production (in 2017 with a 4% share in global anthropogenic CO2 emission [1], which, apart from aggregate, is a key component of concrete and despite a smaller share in the quantity of concrete compared to the share of aggregate, exceeds the emission from the exploitation of natural aggregates, production of crushed aggregates and transport of aggregates. According to [2], production of 1 m 3 of concrete with a strength of 32 MPa generates emission from 251 kg CO2 for CEM IIIblast-furnace slag (BFS) cementto 322 kg CO2 for CEM I -Portland cementwhile the production of 1 t of pure clinker cement is connected with the emission of 820 kg CO2, 1t of ground granulated blast-furnace slag -143 kg CO2, and 1t of aggregates between 35.7 kg (basalt) and 45.9 kg (granite).
Mostly, a lower w/c ratio leads to a more durable concrete. Porosity is related to the apparent density of aggregate [15] and is the cause of its water absorbability, which is in the range of 3-10% [16]. Water absorbability of recycled aggregates does not necessarily have to be a technological problem in the concrete manufacturing process if it is measured correctly. If it is underestimated, the water suction effect of water contained in cement paste by recycled grains may occur [16].
As a consequence, the workability of the concrete mix deteriorates, and the hydration processes are disturbed. Therefore, two ways of adding water to concrete mix are proposed, resulting from the absorption of recycled aggregate [17]: • compensation (use of additional water resulting from absorbability of recyclable aggregates), • pre-soaking of RCA with water.
It is also essential to take into account the absorption time. It is unlikely to be full saturation, achieved usually after 24 hours or more, sometimes even up to 120 hours [16]. According to [18], the absorption time should be 10 minutes. However, other authors [19,20] point to a longer time (20-30 minutes). The choice of a not very long period is justified by a study of [21], who concluded that both the use of dry recycled aggregate as well as the use of aggregate in the state of full saturation, worsens frost resistance of concrete. [22] states that the use of aggregate soaked in water significantly reduces water absorbability and sorption of concrete in relation to composites, where superficially dried aggregate was used (without pre-soaking). The latter type of aggregate contributes to a higher porosity of the contact zone of the aggregate -paste (ITZ).
The justification of this effect is the loosening of the contact zone structure, in which the development of C-S-H phase is blocked due to ettringite dominance.
Furthermore, the presence of water in aggregate grains is an additional source (apart from classical curing) of water during hydration processes. For this reason [23] recommend the Two-Stage Mixing Approach (TSMA), which is an effective way of ensuring the stability in the time of consistence of concrete mix, and achieving a higher compressive strength of concrete.
In the context of the mentioned RCA defects as well as RAC defects, it is particularly important to obtain the firm and tight microstructure of concrete, which is, among others, determined by the conditions of its curing. Improperly cured concrete achieves lower strength and durability.
For this reason appropriate curing conditions of RAC seems to be more important than in the case of ordinary concrete, especially because of the different characteristics of their interfacial transition zones [24][25][26].
However, this does not mean that studies on the influence of curing conditions on the properties of concrete made of natural aggregates have been neglected. One of the most recent studies concerns the effect of curing conditions on properties of such concretes but produced from alkali-activated cements as binders which enable reduction of the carbon emission footprint in comparison with plain Portland cement. Special attention is focused on the influence of curing conditions on shrinkage, which to some extent determines the durability of concrete. [27,28].
The data in the literature concerning the influence of curing conditions on the properties of concrete with waste aggregates, including concrete recycled aggregate, are not numerous, and the results of researches conducted with different material assumptions as well as curing conditions may only contribute to the knowledge on this subject. [29] have studied the effects of curing conditions in four ways: laboratory curing at 100% relative humidity and 20°C, outdoor natural curing (with a variable temperature and air RH from 25% to 88%), indoor storage (45%-65% relative humidity) and tap water storage. After using concrete recycled aggregate in 20%, 50% and 100% as a substitute for natural aggregate, case of 100% RCA, only the external natural conditions of curing contributed to a significant decrease in strength (by 7%). In the case of modulus of elasticity, its reduction was more related to the increase in the amount of concrete recycled aggregate than to the curing method, although slightly better results were obtained for samples stored respectively: in tap water and climate chamber with RH = 100%.
Interesting insights are provided into the curing of recycled aggregate concrete in steam curing, which proved to have an adverse effect on the compressive strength of concrete. According to [30], steam treatment for 4 hours after concrete mixes were made, combined with the subsequent curing in chamber with high level of RH (>95%), resulted in a reduction of 90-day compressive strength of concrete. Although, 1-day compressive strength proved to be higher by approx. 20%, no difference was observed after 28 days. In turn, thanks to steam curing, the modulus of elasticity of recycled aggregate concrete slightly increased; such a trend was also observed in the case of splitting tensile strength.
Researches of [31] focused on the effects of steam temperature and its application time, conducted on a 28-day recycled aggregate concrete, indicated an upper temperature limit of low-pressure brewing at the level of 50°C and a steam curing application time of no more than 1 hour in order to avoid the reduction of compressive strength. The application within more than 2 hours significantly reduced the strength.
The strength and durability of concrete as key parameters determining its quality depend on the amount of cement, which should be optimised in terms of eco-efficiency. The proposal to optimise the cement content in accordance with the requirements of the designed concrete by taking into account two environmental impact factors when determining the composition of the concrete, is related to strength [32]. Index bi (binder intensity index), which expresses the mass production process of such quantity of cementwill allow to achieve the strength of concrete of 1 MPa (kg/MPa).
In both cases, the lowest possible values should be obtained. The optimal solution is to produce concretes of higher strength, because if the compressive strength is higher than 50 MPa, the bi coefficient can reach 5 kg/m 3 /MPa, while in low strength concretes (up to 20 MPa) bi increases even up to 13 kg/m 3 /MPa. For the ci coefficient, the minimum value is assumed at the level of 1.5 kg/MPa (in case of using mineral additives in the production of cements), whereas in pure clinker cements it is not possible to achieve a value lower than 4.3 kg/MPa.

Materials
Two types of cement were used, Portland cement CEM I 42.5R and blast-furnace slag (BFS) cement CEM III/A 42.5N -LH/HSR/NA. Properties of binders are given in Table 1. Recipes and compressive strength of parent concretes (PC) are given in Table 2. Recycled concrete aggregate (RCA) was the mix of these three parent concretes, and was applied for investigations as 0%, 50% and 100% replacement of natural coarse aggregate. The final sieve curves of natural and recycled concrete aggregate compositions used in the experiment are presented in Figure 1. • waiting for 2 hours for pre-soaking aggregate samples in water, • removing aggregate samples from the bucket (using sieve) and placing it on the towel for drying, • waiting until the surface of aggregate will be still in wet state but without visible layer of water on the grains surface, • rotating grains of aggregate in order to ease evaporation of surface layer of water, • weighting aggregate in wet state.
Before measurements of water absorption RCA was superficially dried by keeping it at relative humidity of 50-60% for 2 weeks. Results of water absorption measurements of recycled concrete aggregate are presented in Table 3.

Concrete Recipes
Concrete recipes applied in the experiment are presented in Table 4. The components of concretes were mixed using a paddle-type 0.05 m 3 mixer. Mixes were prepared using the two stage mixing approach proposed by [23]. The approach involves adding 50% of water (which amount was calculated according to the mass of total amount of aggregate) only with the aggregate leaving it for 30 minutes to saturation, while the remaining water is added in a traditional way. According to the authors, such method improves significantly the concrete quality. After a 30-minute break, cement was added and the composition was mixed for a 60-second period. Then the water remainder with total amount of superplasticiser was added and mixed for a 3-minute period.

Properties of Concrete Mixes
The slump of concrete mix was measured according to the method specified in the European Testing of sorption as a useful parameter for assessment of concrete durability is proposed, among others, by [33] and where: Msat is saturated mass of concrete (g) and Mo mass of concrete sample dried at 50 o C (g).

Eco-efficiency and Eco-durability Indexes
The average emission value of CEM I cement taken to calculation of bi and ci [32] is 761 kg CO2/ton while CEM III is 360 kg CO2/ton (data from production in one of Polish cement plants). Furthermore, for CEM III series, the ci coefficient was also calculated in an alternative configuration, i.e. taking into consideration the emission associated with the production of ground granulated blast-furnace slag (143 kg CO2/tonaccording to [2]).
The authors of this paper have used a method proposed by [33] and described in chapter 2. Higher index values indicate higher eco-durability.

ANOVA and Tukey's Test
Analysis of variance (ANOVA) was performed for density and compressive strength results.

Fresh Mix Properties
Slump and air content measurements for concrete mixes made of CEM I and CEM III in relation to RCA participation is presented in Table 5. Slump of concrete mix CEM I and 100% of RCA content was the lowest and significant loss of workability compared to concrete mixes made of CEM III was observed while at 0% and 50% of RCA results of slump measurements are similar in both cases. And air content results show that more RCA content results in the higher air content. This is clearly linked to the presence of air pores in the structure of the recycled aggregate. However, the differences are not significant, which can be justified by the use of carbonated aggregate, which has a lower porosity than the material obtained immediately after crushing.

Density
Density of concretes made of CEM I and CEM III after 90 days of hardening in relation to RCA participation is presented in Figure 2. Density is slightly higher for RCA concrete series made of both CEM I and CEM III in the case of wet conditions of hardening comparing to dry conditions of curing. The trend refers to both 0% and 50% RCA. However, concretes including 100% RCA presented similar density in the case of both WET and PEV conditions.

Sorption
Sorption results of concretes made of CEM I and CEM III in relation to RCA participation is presented in Figure 5. slag cement follows the denser microstructure due to using just this binder, but only under the condition of proper curing (WET or PEV). DRY way of curing should be definitively excluded.

Saturation degree
Saturation degree of concretes made of CEM I and CEM III in relation to RCA participation is presented in Figure 6. Wet curing condition were more suitable for concrete made of both CEM I and CEM III. Moreover, CEM III are more susceptible to dry curing conditions than CEM I.

Eco-efficiency and eco-durability indexes
The values of bi as well as ci and S-CO2 are presented in treated under WET conditions. Only slightly higher were the values of the jest ci coefficient, when the emission associated with the production of ground granulated blast-furnace slag was taken into consideration. When considering the S-CO2 index, the use of CEM III cement, regardless of whether the CO2 emission associated with the production of ground granulated blast-furnace slag is taken into account or not, is more advantageous.

Conclusions
Based on the obtained results the following conclusions have been formulated: 1. Concrete mix made of CEM I turned out to be more sensitive to slump loss at the maximal content of RCA in comparison to concrete mix with blast furnace slag cement as a binder.
However, the worst influence of RCA presence on air content in concrete mix occurred when blast-furnace slag cement was used.
2. Conditions of hardening influenced sorption of concrete being definitely better for wet curing and protection against drying (water evaporation) and in both cases for CEM III than CEM I as a binder.
3. Dry conditions of hardening were perceived as more hazardous for compressive strength in the case of concretes made of both cement CEM I and CEM III.
4. The protection against drying (water evaporation) can be sufficient for concrete with high amount of RCA than for ordinary concrete taking into account compressive strength and in comparison to wet conditions of curing.
5. Statistical analysis showed the influence of RCA participation on compressive strength was less meaningful than dry curing conditions, which in obvious way are improper.
6. Binder intensity index bi occurred to be slightly higher for CEM III than CEM I. However carbon dioxide emission index ci was better in the case of blast furnace slag cement used as a binder.
7. In terms of durability, the authors of this paper propose eco-durability index S-CO2, especially for composites with recycled aggregate. In the conducted researches, more favourable values of this index were obtained for concrete with blast-furnace slag cement, but on condition of proper curing.