Production of more durable and sustainable concretes using volcanic scoria as cement replacement

A.M. al-Swaidani*

Faculty of Architectural Engineering, Arab International (Formerly European) University, (Damascus, Syria)

*aydlswaidani@yahoo.fr; a-swaidani@aiu.edu.sy

1. INTRODUCTIONTOP

Production of Portland cement; the principal binder in concrete, is a major contributor to greenhouse gas emissions that are implicated in global warming and climate change. The worldwide cement production accounts for almost 7% of the total world CO₂ emissions (1, 2). Depending on the carbon content of fossil fuels used for clinkering, nearly 0.8 to 1.0 tonne of CO₂ is directly released from cement kilns during the manufacture of one tonne of clinker (1, 3). In addition, about 4 to 7 GJ of energy is required to produce one tonne of cement (1, 2). Furthermore, the aggressive environments may shorten significantly the service life of the concrete structure. So, finding a solution to increase the durability of concrete structures and thus the sustainability of the concrete seems to be desirable for sustainable development.

Probably one of the most effective means of decreasing both energy consumption and the production of greenhouse gases is to substitute supplementary cementitious materials (SCMs) for a portion of the Portland cement. There are a number of SCMs available, several of which are already used extensively in the industry (2). They are all pozzolanic materials; that is, fine siliceous materials that react at ambient temperatures with the lime released during the hydration of C₂S and C₃S to form a secondary C-S-H (2). Furthermore, the inclusion of these materials which can be categorized as natural or artificial enhances the concrete durability due to their filler effect and pozzolanic reaction (4).

Natural pozzolans are being widely used as substitute for Portland cement because of their ecological, economical and performance-related advantageous properties (5, 6, 7, 8, 9, 10, 11, 12). After thousands of years, natural pozzolan-containing concretes are still used today owing to their good mechanical and durability properties (13). Syria has important volcanic areas. More than 30 000 km² of the country is covered by Tertiary and Quaternary-age volcanic rocks (14), among which volcanic scoria occupies important volume with estimated reserves of about 0.75×10⁹ tonnes (15). However, its potential use in making concrete is not well established. The cement produced in the country is almost of CEM I, although an addition of volcanic scoria up to 5% was frequently used in most local cement plants. Hence, less than 300 000 tonnes of these pozzolans are only exploited annually (the annual production of Portland cement in Syria is about 6 million tonnes) (16).

Although there are numerous studies on using natural pozzolan as cement replacement (5, 6, 7, 8, 9, 10, 11, 12), few works on studying volcanic scoria were reported in the literature (7, 12). Furthermore, no detailed research was conducted in the past to investigate the potential use of volcanic scoria in production of blended cements in Syria. The present work reports a part of the first detailed research in the country to investigate the potential utilization of volcanic scoria as cement replacement in producing Portland-pozzolan cements. This investigation has been conducted through a series of experiments. Mechanical strength development, water permeability, chloride penetrability and porosity of concretes containing different scoria contents (10 to 35%) have particularly been investigated. Physical, chemical and mineralogical analyses of volcanic scoria and blended cements have been reported, as well.

The importance of the present study can be summarized in the following points:

2. MATERIALS AND METHODSTOP

2.1. MaterialsTOP

The volcanic scoria used in the experiments was quarried from Dirat-at-Tulul site, at about 70 km southeast of Damascus as shown in Figure 1. The mineralogical analysis showed the scoria is mainly composed of amorphous glassy ground mass, vesicles, plagioclase and olivine. Thin sections of the investigated volcanic scoria are shown in Figure 2. The chemical analysis of scoria used in the study is summarized in Table 1. This analysis was carried out by means of wet chemical analysis specified in EN 196-2.

Seven binder samples were prepared in accordance with EN 197-1; one plain Portland cement CEM I (control), three CEM II/A-P samples with three replacement levels (10, 15 and 20%), and three CEM II/B-P samples with three replacement levels (25, 30, 35%), respectively. According to EN 197-1, CEM I and CEM II refer to two different types of cement, i.e. ordinary Portland cement and Portland-composite cement, respectively; A-P and B-P refer to the percentage of natural pozzolan in cement, i.e. 6–20% and 21–35%, respectively. All replacements were made by mass. Five percent of gypsum was added to all these binder samples. The clinker was obtained from Adra Cement Plant, Damascus, Syria. Chemical analysis of clinker and gypsum is shown in Table 1. All samples were ground by a laboratory grinding ball mill of 25 kg raw mix capacity into a Blaine fineness of 3200±50 cm²/g which is a customary value in cement production. This grinding process was adopted because it requires less energy than separate grinding (17). The Blaine fineness was measured using air permeability method in accordance with the European standard EN 196-6. The particle size distributions of the binders were obtained using Malvern Mastersizer 2000; a laser particle size analyzer. Figure 3 shows the particle size distribution for some blended cements used in the investigation. The chemical and some physical properties of the binder samples produced for this study are shown in Table 2. CEM I (the control sample) was designated as C1/CEM I, whereas scoria-based binders were designated according to the replacement level. For instance, C2/10% and C7/35% refer to the binders containing 10% and 35% of volcanic scoria, respectively.

Seven mortar specimens were prepared using the investigated binders and sand meeting the requirements of ASTM C778. In all mixtures, binder: sand ratio was kept constant as 1:2.75 by weight. Water-binder ratio (w/b ratio) was also kept constant for all mixtures at 0.5. After being kept in a curing cabinet (RH~95%) for 24 hours, the mortar specimens were demolded and kept in lime-saturated water at 20±2 ^oC until the time of testing.

Seven concrete mixes were prepared using a grading of aggregate mixtures kept constant for all concrete mixes. Aggregates used in the study were crushed dolomite with natural sand added. Chemical composition and some physical properties of the aggregates are illustrated in Table 1 and Figure 4, respectively. Their quantities in 1m³ concrete mix based on the oven-dry condition were as follows: 585.5 kg of coarse aggregate, 585.5 kg of medium-size aggregate, 467.5 kg of crushed stone sand and 312.5 kg of natural sand. All concrete mixes were designed to have a water-binder ratio of 0.5 and a slump of 150±20 mm. Superplasticizer of “type F” (ASTM C494) was added. Concrete cubes (150 mm) were cast for the determination of mechanical strengths (compressive and tensile splitting strengths) and water permeability. Concrete cylinders (100 mm×200 mm) and (75 mm×150 mm) were also cast for evaluation of the chloride penetrability and the porosity of the concretes, respectively.

2.2. MethodsTOP

Water requirements, setting times and soundness of all binder paste specimens were determined in accordance with EN 196-3. The compressive and flexural strength development of mortars was determined on 40×40×160 mm prismatic specimens, in accordance with EN 196-1, at ages of 2, 7, 28, 90 and 180 days. The compressive and splitting strengths of concretes were conducted on 150 mm cubic concrete specimens in accordance with ISO 4012 and 4108, at ages of 2, 7, 28, 90 and 180 days. Concrete permeability measured in terms of depth of water penetration was carried out as per the standard EN 12390-8. The results shown in this paper are the average penetration depth. The rapid chloride penetrability (RCP) test was conducted in accordance with ASTM C1202. The set-up of RCP test is illustrated in Figure 5. Three cylinder specimens of each concrete mix were tested after 2, 7, 28, 90 and 180 days curing. Porosity measurements were conducted using vacuum saturation method in accordance with RILEM CPC 11.3. The values reported in the results represent the average of six readings for compressive strength test and the average of three readings for all other tests.

3. RESULTS AND DISCUSSIONTOP

3.1. Properties of volcanic scoria and blended cementsTOP

As seen from Table 1, volcanic scoria is considered as suitable material for use as pozzolan. It satisfied the standard requirements for such a material by having a sum of SiO₂, Al₂O₃ and Fe₂O₃ of more than 70%, a SO₃ content of less than 4% and a loss on ignition (LOI) of less than 10% (ASTM C618). reactive SiO₂ content was more than 25%, as well (EN 197-1). In addition, it had a strength activity index with Portland cement (PC) higher than the values specified in ASTM C618. The chemical properties of volcanic scoria-based binders, Table 2, were also in conformity with the standard requirements (ASTM C595). Their contents of MgO and SO₃ were less than 6% and 4%, respectively. The loss on ignition (LOI) was also less than 5% as specified in ASTM C595.

3.2. Physical properties of cement pastesTOP

The results of water requirements, setting times and soundness are given in Table 2. As it can be seen from the Table 2, there was no significant change in the water content even for the binder containing 35% of volcanic scoria which increased only by less than 0.5 compared to C1/CEM I. This could be explained by the lubricant effect of volcanic scoria on paste when finely divided (18) and the effect of volcanic scoria shape which was characterized by sharp-edged grains (19) as seen in Figure 1(d).

The results shown in Table 2, revealed that the setting times increased with volcanic scoria replacement level. The effect was more pronounced when the volcanic scoria content was used at 35% as cement replacement. For example, the highest initial and final setting times at this percentage were 158 min & 188 min. This could be due to the pozzolanic reaction between pozzolan and CH liberated during hydration of C₃S and C₂S of clinker, which is usually slower than the hydration of cement (20). Some researchers related the delay in setting times when using natural pozzolan with the increase in water requirement (21). It is worthwhile to note that all the scoria-based binders complied with the standard requirements (initial setting time=45 min and final setting time=420 min), according to (ASTM C595).

The Results of experiments conducted to see the swelling effects of compounds such as CaO and MgO that exist freely in cement are given in Table 2. According to these results, the volume expansions of samples containing volcanic scoria-based binders showed similar values to that of control. Interpretation of these results is currently beyond the understanding of the authors and needs further investigation, although all soundness values were much less than 10 mm as specified in EN 197-1.

3.3. Mechanical strength development of mortars/concretesTOP

3.3.1. Compressive strength development of mortars/concretesTOP

Strength of concrete is commonly considered its most valuable property, although, in many practical cases, other characteristics, such as durability and permeability, may in fact be more important (22).

Results of compressive strength development of the investigated mortars and concretes are given in Tables 3 and 4, respectively. As expected, all mortars and concretes showed an increase in strength with curing time. Mortar and concrete specimens containing C1/CEM I had higher compressive strengths at any curing time compared to volcanic scoria-based binder mortars and concretes. Also, it is seen that the compressive strength of volcanic scoria-based mortars and concretes decreased with the volcanic scoria replacement level for all curing times. For instance, the compressive strength of C1/CEM I and C7/35%-based mortars and concretes after 7 day curing decreased from 30.6 and 27.6 MPa to 20.0 and 17.3 MPa, respectively. This could be explained by (i) the reduction of cement content in the mix with the increase of volcanic scoria content; i.e. the dilution effect (23) and (ii) the slowness of the pozzolanic reaction (24). However, due to the continuation of this reaction and the formation of a secondary C-S-H, a greater degree of hydration was achieved resulting in strengths after 90 and 180 day curing which were comparable to those of C1/CEM I specimens. For instance, the compressive strength of C6/30%-based mortar and concrete was found to be 28% and 31% lower than C1/CEM I-based specimens after 2 day curing, but this reduction was only 7% and 12% after 180 day curing. It is also clearly seen from the results that the significant gain in strength in blended cement mortars and concretes occurred when moving from 28 to 90 day curing times while in C1/CEM I specimens this was noted during the first 28 days. This could be explained by the slow pozzolanic reaction and its progress with age in scoria-based mortars and concretes.

3.3.2. Correlation between compressive strength of mortar and compressive strength of concreteTOP

The correlation between the compressive strength of volcanic scoria-based mortars and the compressive strength of volcanic scoria-based concretes is given in Figure 6. The compressive strength of scoria-based concretes seems to have a close relationship with that of volcanic-scoria-based mortars. A linear regression analysis was conducted to determine the best-fit relationship between the all measured compressive strengths of mortars and concretes for all curing times. The following linear equation was derived based on the analysis with a regression coefficient of 0.99; a regression coefficient of more than 0.85 indicates an excellent correlation between the fitted parameters (25) [1]:

f_c=0.861f_cm+0.387        [1]

where f_c, f_cm are compressive strength of concrete and mortars in MPa, respectively. So, the compressive strength of concretes at a given curing time can be predicted from a knowledge of the compressive strength of mortar prepared with the same w/b ratio. This linear equation is in good agreement with that reported by Neville for plain mortar and concrete (22). Such similar relationships may need to be developed for other types of aggregates, different w/c ratios and other curing conditions.

3.3.3. Flexural and splitting tensile strength of mortars and concretesTOP

Results of flexural strengths of prismatic mortars and splitting tensile strengths of cubic concretes prepared from the produced binders and cured in water until the test date are arranged in Tables 3 and 4, respectively. A similar trend to that observed for compressive strength seems to be followed by the flexural and splitting tensile strength results. However, the results showed that flexural and splitting tensile strengths were less sensitive than the compressive strength to the volcanic scoria content.

3.3.4. Correlation between compressive and splitting tensile strength of concretesTOP

The relationship between the compressive strength (f_c) and splitting tensile strength (f_st) of concretes is given in Figure 7, and seems to fit well with the relations proposed by Oluokun et al. (26), JCI (27) and AIJ (28) for plain concrete [2]:

f_st=k(f_c)ⁿ        [2]

The correlation between the splitting tensile strength and the compressive strength results was calculated for the entire population of test results and hence the relation obtained is [3]:

f_st=0.195(f_c)^0.85        [3]

with a regression coefficient of 0.98. So, knowing the compressive strength (f_c) of concrete the splitting tensile strength (f_st) can be predicted by using Eq. (3).

3.4. Water permeabilityTOP

Permeability of concrete to water is closely related to the durability of concrete. Permeability is the rate at which aggressive agents penetrate through concrete. Water penetration depth can be considered as an indication of permeable and impermeable concrete (22). A depth of less than 50 mm classifies the concrete as impermeable and a depth of less than 30 mm as impermeable under aggressive conditions (22). Figure 8 shows the water penetration depth test results for all binder types. Concretes containing volcanic scoria-based binders are supposed to have lower permeability than plain Portland cement. But in this research, it has been observed that after 2 day curing, water penetration depths of C2/10% and C3/15% were higher than that of CEM I. Increasing the moist-curing period of concrete from 28 to 90 days reduced water penetration depths of all concretes by a factor ranging from 2.11 to 3.03 for concretes containing scoria-based cements and a lower factor of 1.44 for plain Portland cement concrete. This result is in good agreement with the values reported by Neville for plain concrete (22). None of concretes was found to be impermeable before 28 day curing. However concretes containing 20 to 35% scoria can be considered as impermeable after 28 day curing, and as impermeable under aggressive environments after 90 day curing, according to Neville (22). However, the concretes containing 10% and 15% volcanic scoria needed 180 day curing to achieve this level of impermeability.

3.5. Chloride penetrabilityTOP

From the rapid chloride penetrability test results, as illustrated in Figure 9, it should be noted that C1/CEM I-based concrete permitted almost 2 to 3 times the coulombs charge, compared to the concrete containing C5/25%, C6/30% and C7/35% in spite of the fact that all concretes were made with similar cementitious content and water content. None of the concretes had a total charge less than 2000 coulombs after 2 or 7 day curing. However, the concretes containing binders with volcanic scoria contents of 25, 30 and 35%, showed the best performance among all the specimens. According to ASTM C1202, these concretes can be considered as low and very low chloride permeable after 90 and 180 day curing. The improvement in the resistance to chloride penetration may be related to the refined pore structure of these concretes, their reduced electrical conductivity (29) and the secondary pozzolanic reaction which contributes to make the microstructure of concrete denser. This improvement was confirmed by many researchers who investigated natural or other pozzolans, particularly at later curing times (12, 30, 31, 32).

3.6. PorosityTOP

Porosity of concretes containing different levels of volcanic scoria at various curing times is presented in Figure 10. Porosity of all mixes decreased with curing time. As volcanic scoria is incorporated porosity decreased significantly. Porosity of the concretes containing binders with high replacement levels of volcanic scoria demonstrated much lower porosity as compared to the plain concrete. The rate of decrease in porosity was faster for volcanic scoria-containing concretes as compared to the plain concrete. The reduction in the porosity could be attributed to the pozzolanic reaction between the glassy phase in volcanic scoria and the CH liberated from hydration of C₃S and C₂S (12).

The experimental data, depicted in Table 4 and Figs. 8–10 has been statistically analysed to develop estimation equations to correlate the concrete properties with the curing time and the replacement level of volcanic scoria.

The equations can be expressed by the following single formula [4]:

CP=a₁+a₂×VS+a₃×lnt+a₄×VS×lnt        [4]

where CP is the concrete property (i.e., compressive strength, water permeability, chloride penetrability and porosity); VS is the percentage of volcanic scoria in the binder; t is the curing time; a₁, a₂, a₃ and a₄ are constants.

The constants have been obtained through the regression analysis of the data in Table 4 and Figs. 8–10. The best-fit values of constants a₁, a₂, a₃ and a₄ and the regression coefficients R² of the correlation between the measured and the proposed values are presented in Table 5. The results indicated excellent relationships between the curing time and the properties of concrete with different replacement levels of volcanic scoria.

It is to be noted that the correlation equation developed in the present work would be helpful for concrete mix designers. However, it should be noted that the relationship reported in the paper have been developed for concrete containing dolomite aggregates, having w/c ratio of 0.5 and cured at ambient temperatures. Such similar relationships may need to be developed for other types of aggregates, different w/c ratios and other curing conditions.

From the results obtained, volcanic scoria showed significant improvement which clearly indicated that the volcanic scoria was efficient in the refinement of pore size distribution. This has been reflected in the reduction of water permeability and porosity.

The results have been confirmed using the SEM/EDX techniques, as shown in Figs. 11–13. The microstructural and EDX analysis of 7 day cured C6/30% paste, Figure 11, revealed a non-compacted structure. On the other hand, after 28 and 90 day curing, Figures 12 and 13, the microstructural analysis of C6/30% paste showed denser and more compacted structure due to the progress of the cement hydration. In addition, the EDX analysis results clearly indicated the formation of cementitious phases, such as calcium silicate hydrates (C-S-H) and calcium alumino-silicate-hydrates (C-A-S-H). These might be formed through the continuation of cement hydration and the progress of the pozzolanic reaction between the amorphous phases in volcanic scoria and CH (calcium hydroxide) released during cement hydration (33).

4. CONCLUSIONTOP

ACKNOWLEDGEMENTTOP

The authors gratefully acknowledge the technical and financial support of this research from the management of General Organization for cement & Building Materials/Adra Cement Plant. Thanks are also expressed to Prof. Tamer al-Hajeh, Vice-President of AIU, for his appreciated help.

REFERENCESTOP