Investigation of Mineral Admixtures on Mechanical Properties of Alkali-Activated Recycled Concrete Powders Cement

Abstract

Recycled concrete powders (RCPs) are collected during the treatment of recycled aggregates using devices that suction dust. However, RCPs have not been effectively utilized as mineral admixtures in concrete due to their low activity and high capacity for water absorption. In this study, alkali-activated recycled concrete powders cement (AARCPC) was prepared using chemical activation, and then the composition, fluidity, setting time, strength and micro-structure of hydration products in AARCPC were studied. We found that the addition of mineral admixtures significantly improved the strength of hardened paste at various ages, and that the effect of slag powders on the mechanical properties was significantly better than that of fly ash. Replacing AARCPC with 50% slag caused the 28 d and 90 d compressive strength of pastes to reach 79.5 and 84.4 MPa, respectively. The optimal ratio of the various minerals that make up AARCPC was 60% recycled concrete powder, 20% slag powder and 20% fly ash. In particular, hydration with fly ash and slag of AARCPC promotes better fluidity and compactness. AARCPC showed higher strength and has the potential to replace Portland cement and be applied to concrete.

1. Introduction

The acceleration of urbanized construction has led to generation of large amounts of construction waste due to increased demolition and reconstruction activities [1,2,3]. More than 3 billion tons of construction waste are produced annually, with waste concrete accounting for more than 34% [4]. Recycled concrete aggregates and recycled concrete powders have been found to be effective and sustainable construction materials [5,6]. Currently, recycled aggregates are widely used in road engineering and building engineering based on set guidelines for their use. According to Tang’s results [7], the addition of recycled materials had an adverse effect on the concrete strength. The replacement rate of recycled aggregate was less than 50% generally. It was very likely to be different in the presence of different types and contents of construction and demolition waste [8]. After adding NS into RCP-cement pastes, Liu [9] found that nano-SiO₂ can compensate for the mechanical strength loss of mortar caused by RCP as supplementary cementing materials. As well as nano-SiO₂, nano-TiO₂ can also improve strength and durability compared with the control mix [10].

However, recycled concrete powders (RCP) have not been effectively utilized due to their uneven fineness and complex composition; only a small amount is used for supplementary cementitious material (SCM). Some concrete test methods have also been used for various types of concrete with different waste or by-products (steel slag, incinerator bottom ash) [11,12]. A review by Ye et al. [13] highlighted the increased interest in alkali-activated recycled concrete powders cement (AARCPC) as a sustainable approach for improving the properties of RCP. Improving the activity of RCP has the potential to increase its utilization in concrete mixtures, which would be of great social significance and provide economic benefits [14,15,16]. RCP from waste cement mortars is mainly composed of silicon and aluminum compounds, such as SiO₂ and Al₂O₃, which have potential alkali activity. Hence, granulated blast furnace slag (GBFS), fly ash (FA) and AARCPC can be synthesized to geopolymers [17,18,19,20,21,22] by regulating the compounds, fineness and activation technology. However, RCP has low pozzolanic activity and is composed of irregular particles, which limits its use in cement-based materials. Therefore, there is a need to use chemical methods to improve the solubilization and activation of RCP in order to improve its efficiency in building materials.

At present, there are studies using alkali-activated techniques to enhance the replacement rate of RCP in cement-based materials. However, no studies have been conducted to determine the effect of mineral admixtures on the mechanical properties of AARCPC. In this study, AARCPC was generated by using water glass to activate RCP. We then analyzed the effects of different proportions of GBFS and FA on geopolymers and the phase composition, mechanical properties and microstructure of AARCPC. The findings from this study highlight a valuable method for the high utilization of RCP and preparation of geopolymers with RCP [23].

2. Materials and Methods

2.1. Raw Materials

Recycled concrete powders were obtained from utilization of renewable resources in Yangzhou City, China. XRF analysis revealed that RCP, GBFS and FA were mainly composed of SiO₂, Al₂O₃ and CaO, as shown in

Table 1. Chemical composition of RCP, GBFS and FA.

No.	Chemical Composition/wt-%
	SiO2	Al2O3	Na2O	Fe2O3	CaO	TiO2	K2O	MgO	SO3	Others
RCP	34.3	8.2	1.0	6.2	42.1	1.0	1.2	4.5	0.9	0.6
GBFS	30.0	13.6	0.3	0.6	38.1	0.6	0.4	12.5	-	3.9
FA	61.9	28.8	0.3	2.5	2.4	1.0	1.5	0.8	-	0.8

; the element and oxide compositions have been obtained in detection of components. RCP was composed of coarse granules that require mechanical grinding using a planetary ball mill (QM–3SP2L) to refine the particle size and improve the activity of RCP. This refinement accelerated the alkali-activated reaction rate between activators and RCP. The specific surface area of RCP was 380 m²/kg, and the average particle size was 38.6 μm. The SiO₂/Na₂O modulus of water glass was 1.4, while its solid and Na₂O content were 41.1% and 17.1%, respectively. AARCPCs were activated using 6% water glass by weight of powder mass fraction (measured by Na₂O).

2.2. Methods

We prepared AARCPC pastes with different compositions of mineral admixtures (w/c = 0.35) and used them to generate cuboid specimens measuring 20 × 20 × 80 mm³. The fluidity and setting time of the different AARCPC pastes were determined, and then the pastes were cured at standard curing temperature of 20 ± 1 °C, above RH > 90% for 3, 28 and 90 d. The fluidity, setting time and mechanical properties were tested according to GB/T17671-2021 (ISO method), GB/T1346-2011 and GB/T8077-2012, respectively. The mineral admixture proportions of composite cement pastes are shown in

Table 2. Mix proportions of composite cement pastes.

Sample	RCP/%	GFBS/%	FA/%
R9F1	90	-	10
R8F2	80	-	20
R7F3	70	-	30
R6F4	60	-	40
R5F5	50	-	50
R9S1	90	10	-
R8S2	80	20	-
R7S3	70	30	-
R6S4	60	40	-
R5S5	50	50	-
R7S2F1	70	20	10
R6S3F1	60	30	10
R6S2F2	60	20	20
R5S3F2	50	30	20
R5S2F3	50	20	30

.

Hydration products of hardened pastes were cured for 3, 28 and 90 d, then analyzed using an X-ray diffraction (XRD, Smart Lab 9 kw, Rigaku, Tokyo, Japan) with a CuKα radiation (30 kV and 20 mA) at a scanning rate of 4 °/min, Fourier transform infrared spectrometer (FTIR, Nicolet 6700, Thermo Scientific, Waltham, MA, USA) with a KBr method at a resolution of 0.1 cm⁻¹ and field emission scanning electron microscopy images (SEM, Hitachis-4800 15 kV, Japan) with a working voltage of 15 kV to determine the chemical composition, morphology and pore structure of the hydration products cured for different lengths of time.

3. Results

3.1. Fly Ash

3.1.1. Fluidity

The fluidity of AARPC containing different amounts of fly ash is shown in Figure 1. The RCP particles were irregular, angular and very rough, which increased the resistance between the internal materials. The nature of the RCP particles as well as the high number of pores and higher water demand cause a significant decrease in the fluidity of the AARCP slurry. However, an increase in fly ash content from 10 to 50% caused a significant increase in fluidity (from 150 to 184 mm, 22.7%). This was mainly attributed to the “ball effect” of fly ash, which significantly improves the fluidity of the slurry. The spherical particles in fly ash and the vitreous beads of SiO₂ and Al₂O₃ can improve workability. The fluidity of the slurry increased with an increase in fly ash content.

3.1.2. Setting Time

The setting time of AARPC with different fly ash content is shown in Figure 2. An increase in fly ash content significantly prolonged the initial setting time and final setting time of the AARCPC slurry. The initial and final setting times of R9F1 were 210 min and 347 min, respectively. However, when the fly ash content was increased by 50%, the initial and final setting times increased to 302 min and 405 min, respectively. This is because the structure of fly ash was relatively stable at the initial stages of hydration, and it acts as an inert material in AARCPC. The poor solubility of pozzolanic material, such as beads, in fly ash leads to slow setting and hardening. There is a small gap between the activity of fly ash and that of RCP, and the addition of fly ash slightly accelerated the setting time of AARCPC.

3.1.3. Strength

Figure 3 shows the development trend of compressive strength of AARCPC with different fly ash content. There was only a slight difference in the compressive strength among AARCPCs with different fly ash content. The compressive strength of R9F1 and R5F5 was 18.8 MPa and 19.2 MPa, respectively. Our findings suggest that fly ash content and RCP have similar effects on the compressive strength, which was consistent with results obtained from other studies [24]. Fly ash has insufficient activity and reacts slowly in an alkali-activated system, so the compressive strength results with different dosages of fly ash and RCP were close. During the alkali-activated reaction, Si-O-Si, Si-O-Al and Al-O-Al bonds in fly ash were destroyed by OH⁻ with high alkalinity, and then condensed to form an oligomeric sol. Finally, the sol particles were connected using metal cations, such as Na⁺ and Ca²⁺, to form a hydration product with a three-dimensional network structure. In addition, long age strength has been proven to be faster and stronger than early age strength in AARCPC with different fly ash content. This was partly because “the volcanic ash effect” of fly ash is released in the late hydration period and a large amount of C(N)-A-S-H is generated on the surface of particles.

3.2. Slag

3.2.1. Fluidity

The fluidity of AARPC with different slag contents is shown in Figure 4. An increase in slag content from 10 to 50% caused a significant increase in fluidity (from 148 to 170 mm, 14.9%). Although the fluidity can be improved, it was less effective than fly ash. This was mainly because of various shapes of slag during slurry mixing, which led to large friction assistance. The fluidity modestly increased with an increase in slag content.

3.2.2. Setting Time

The setting time of AARPC with different slag contents is shown in Figure 5. An increase in slag content significantly accelerated the initial and final setting times of AARCPC slurry. The initial and final setting times of R9S1 were 153 min and 282 min, respectively, but, when the slag content was increased by 50%, the initial and final setting times decreased to 73 min and 128 min, respectively. This is because the OH⁻ group breaks the Si-O and Al-O bonds of glass in slag to release aluminosilicate ions and Ca²⁺, which condense and form C(N)-A-S-H gels with a strong structure. Therefore, the higher the slag content, the faster the slurry setting speed. There is a significant difference in the activity of slag and RCP when the same amount is used in the alkali-activated reaction, with the addition of slag accelerating the setting time of AARCPC by a greater degree compared with RCP or fly ash.

3.2.3. Strength

Figure 6 shows the development trend of compressive strength of AARCPC with different slag content. The figure shows that an increase in slag content increased the compressive strength of AARCPC at a constant rate. An increase in slag content from 10% to 50% increased the compressive strength at 3 d, 28 d and 90 d from 9.1 MPa to 38.6 MPa (324%), 14.4 MPa to 79.5 MPa (453%) and 29.7 MPa to 84.4 MPa (184%), respectively. We found that the growth trend in compressive strength slowed down as the curing duration increased, indicating that the slag content played a significant role in the development of high strength. Although slag has superior activation strength to RCP, RCP can be used as rigid support since it has ‘micro aggregate’ effects. RCP can be mixed with slag at a certain ratio and used as ultra-fine aggregate to form intercalation. It can be used as micro aggregate in AARCPC pastes to prevent the generation of cracks and form a better micro-skeleton structure. Our findings indicate that the optimum slag content ranges from 40% to 50%.

3.3. Composite Fly Ash and Slag

3.3.1. Fluidity

The fluidity of AARPC with composites composed of different fly ash and slag contents is shown in Figure 7. The results show that fly ash and slag can increase the fluidity of AARCPC pastes. However, excess RCP particles can cause surface adsorption of water and reduce the fluidity of the mixture. As shown in Figure 7, changes in AARCPC composition initially increases and then decreases its fluidity, with R6S2F2 attaining the highest fluidity of 174 mm. These findings demonstrate that the mixture of fly ash and slag has a significant effect on the fluidity of AARCPC.

3.3.2. Setting Time

The setting time of AARPC with composites composed of different fly ash and slag contents is shown in Figure 8. The setting time initially increased and then reduced with an increase in RCP contents. The setting time of R6S2F2 reached the highest initial setting time of 130 min and the highest final setting time of 310 min, an indication that unreacted active silicate materials, such as C₂S, and hydration products, such as C-S-H gels, play a very important role in the AARCPC system. Since these minerals react faster in the form of slag, an increase in fly ash and slag content decreased the setting time. In addition, AARCPC slurry prepared from a mixture of fly ash and slag performed better than slurry composed of either component alone, suggesting a synergistic effect between fly ash and slag in alkali-activated system.

3.3.3. Strength

Figure 9 shows the compressive strength of AARCPC with composites composed of different fly ash and slag content. Generally, an increase in curing time increased the compressive strength of AACPC pastes with different proportions of the mineral admixtures. Only R7S2F1 increased relatively slowly from 3 d age to 90 d age. When the proportion of the mineral admixtures was 50%, R5S3F2 and R5S2F3 had a similar strength development trend from 3 d to 90 d. The compressive strength of R5S3F2 reached the highest strength of 28.7 MPa, 44.8 MPa and 55.7 MPa at 3 d, 28 d and 90 d, respectively. When the proportion of the mineral admixtures was 60%, the compressive strength of R6S2F2 increased to 39.5 MPa at 28 d, but its strength was lower than R6S3F1 at 90 d. These findings indicated that the calcium in slag activated RCP, which not only enhanced the strength but also increased the fluidity. Fly ash induces strong pozzolanic activity and longer setting time. RCP with optimal dosage generated a relatively dense structure, which was reflected in the increase in concrete compressive strength. Therefore, the optimal ratio of the components of AARCPC was 60% recycled concrete powder, 20% slag powder and 20% fly ash.

3.4. Micro-Structure

3.4.1. FTIR

The FTIR spectra of R5S5, R5F5 and R6S2F2 pastes were compared in Figure 10. An increase in fly ash content strengthened the Si-O-R stretching vibration peak and caused it to move to the high wavenumber region, and the 28 d age pastes moved to 974 cm⁻¹. However, the 28 d age pastes of R5S5 moved to 954 cm⁻¹. These findings indicated that fly ash had some highly polymerized Si-O-R and that the alkali activator had a poor depolymerization effect at 28 d age. On the contrary, the Si-O-R bond in slag was easily depolymerized by the alkali activator to generate new cementitious products and provide strength. As the FTIR spectra of R6S2F2 show, the 28 d age pastes moved to 968 cm⁻¹ in the range of 974 cm⁻¹ and 954 cm⁻¹; it can be found that the effect of composite mixing was similar to the effect of single mixing of fly ash.

3.4.2. XRD

Figure 11 shows the XRD patterns of 28 d AARCPC pastes with different mineral admixtures contents. Long diffraction peaks, similar to steamed bread, were observed at 20–40, indicating that there were many amorphous phases in the hydration product. There were mainly C-S-H gels obtained from RCP. The main products were silica, calcite and dolomite, and the characteristic peak of Ca(OH)₂ was not found in the patterns, which was consistent with the FTIR analysis. An increase in fly ash content significantly enhanced the corresponding diffraction peak of silicon dioxide since fly ash contained more silicon dioxide. An increase in slag weakened the calcite and silica peaks, with the diffraction peak of R5S5 being significantly weaker than that of R5F5 and R6S2F2. Only the main peaks of silica, calcite and dolomite were detected in the R6S2F2 diffraction patterns. This was because most of the slag and fly ash were amorphous, and the alkali-activated products, such as C(N)-S-H, would not appear in the form of sharp peaks.

3.4.3. SEM

Figure 12a,b shows SEM images of 28 d AARCPC pastes with different mineral admixtures contents. The differences between R5R5 and R6S2F2 were that the microstructure of R5R5 was loose at 28 d age, the bonding between C(N)-S-H gels was not tight and there were many cracks. In contrast, the products of R6S2F2 cured for 28 d were more closely connected with each other, and the loose structures were reduced.

Figure 12a,b also shows that the R6S2F2 at 28 d age has many three-dimensional blocks, globules and round pits. Further analysis indicated that the three-dimensional blocks came from slag, and the globules and round pits came from fly ash. The presence of a large number of blocks in Figure 12a indicated that the setting time was shortened with the increase in slag content, and the slag had fully participated in the reaction, resulting in significant appearance of shrinkage cracks. However, the flocculent and layer gels wrapped the spherical particles of fly ash in pastes (Figure 12b), suggesting that fly ash can also act as RCP and have the "micro aggregate" effect, which has been reported by Xie [25]. In summary, fly ash in the AARCPC pastes played two structural roles at 28 d age. First, fly ash participated in the reaction and enhanced strength development. Secondly, fly ash improved the skeletal structure, which enhanced the micro-structure development and reduced cracks. Slag has high activity and can fully react with alkali to form gels and develop strength quickly. However, the hydration of slag and RCP is not a continuous process. Without fly ash, the strength of AARCPC cannot be intensified in the later stages.

4. Discussion

The results of this study indicated that RCP, fly ash and slag have a marked effect on AARCPC paste across the rheological and plastic to hardening states. Particularly, the results indicated that fly ash and slag have synergistic effects on the hydration and micro-skeleton of AARCPC, which were attributed to two factors.

4.1. Effect of Fly Ash on Hydration and Hardening of AARCPC

Fly ash has a significant effect on Portland cement and alkali-activated cementitious material. Notably, “fly ash effects” are a combination of morphological active and micro-aggregate effects [26,27]. The active effect in Portland cement is more significant than that in the alkali-activated system when the fly ash content ranges from 10% to 50%. In the present study, water glass provided Na⁺, OH⁻ and [SiO₄]⁴⁻ as alkali activators. Since fly ash does not accelerate the alkali-activated reaction rates of C-S-H gels and the aluminum phase in RCP, the addition of fly ash does not improve the compressive strength. However, the morphological and micro-aggregate effects induced by fly ash improved the compactness and workability of slurry as well as its effect in Portland. As shown in Figure 13, there were floating beads, sinking beads, magnetic beads, carbon particles, etc., in fly ash. There was a contribution to workability by these beads in recycled pastes, and some spongy carbon particles would also increase the water requirement, so the water requirement of normal consistency was more than that of cement. Our findings suggested that fly ash acted as a workability regulator, where it protected AARCPC from quick setting, but it did not improve the strength.

4.2. Effect of Slag on Hydration and Hardening of AARCPC

The strength of AARCPC depends on the slag content of the original materials [28]. AARCPC pastes prepared from RCP containing 10% to 50% slag results in the highest compressive strength, which can reach 84.4 MPa. When the slag content of RCP exceeded 40%, the initial and final setting times decreased by 52.3 and 53.9%, respectively. Therefore, it is evident that a combination of glass phase in slag can significantly accelerate hydration of AARCPC and produce a higher strength of AARCPC pastes. However, during the dissolution of water glass, [SiO₄]⁴⁻ and [AlO₄]⁵⁻ ions were released from the broken Si-O-R bonds, dissolved into the slurry phase and promoted the original C-S-H gels, C-A-H and Ca²⁺, to form new hydrates, such as C(N)-S-H gels with a large specific surface. These effects are undesirable since cracks usually appear in calcium-rich areas covered with Na+ ions, which results in lower alkalinity. Moreover, the proper proportion of CaO-SiO₂-Al₂O₃ can generate a new and renewable cementitious material with a more compact microstructure. The appropriate proportions of RCP, fly ash and slag have improved the hydration efficiency and mechanical properties.

5. Conclusions

AARCPC pastes were prepared using water glass as an activator and controlling the proportion of RCP and mineral admixtures. The following conclusions were drawn after analyzing the fluidity, setting time, strength, micro-structure and mechanical properties of the pastes:

(1)

The addition of fly ash prolongs the setting time of cementitious materials to a certain extent and increases fluidity but does not significantly improve the strength of the pastes at various ages. When the fly ash content was 50%, the 90 d compressive strength was 19.2 MPa.

(2)

The addition of slag can also increase fluidity, and it shortens the setting time. Most importantly, slag significantly improves the strength of the pastes at various ages. When the slag content was 50%, the 3 d, 28 d and 90 d compressive strength values were 38.6 MPa, 79.5 MPa and 84.4 MPa, respectively.

(3)

The mechanical properties and workability of the pastes were the strongest when 60% RCP was mixed with 20% slag and 20% fly ash. The fluidity was 174 mm, the initial setting time was 130 min and the final setting time was 310 min. The 3 d, 28 d and 90 d compressive strength values were 20.7 MPa, 39.5 MPa and 42.9 MPa, respectively.

(4)

The strength and workability of AARCPC can be controlled by adjusting the proportion of mineral admixtures content. AARCPC has the potential to replace Portland cement and can be applied to concrete.