Effect of fineness of ash on pozzolanic properties and acid resistance of sugarcane bagasse ash replaced cement mortars

Abstract

This paper addresses the potential use of Sugar Cane Bagasse Ash (SCBA) as a pozzolanic material for partial cement replacement in concrete mixtures. Cement mortars containing SCBA having five different particle size distributions at a replacement rate of 20% by weight were used to study the chemical and physical pozzolanic properties of SCBA. The durability of SCBA replaced mortars was also evaluated. SCBA with 0% retained on sieve No. 325 was used to replace 20% by weight of cement and create mortar specimens that were subjected to sulfuric acid attack of varying concentrations (1%−3% by weight of water). The tested samples were observed to check visual distortion, mass loss, and compressive strength loss at 1, 7, 14, 28, and 56 d of acidic exposure, and the results were compared to those for the control sample, that was lime water cured, at the same ages. The SCBA sets were found to meet the requirements for pozzolan class N specified by ASTM C 618. Mortars containing SCBA with 0% or 15% retention produced better compressive strength than the control mortars after 28 d. Additionally, X-ray fluorescence and X-ray diffraction analysis showed that the SCBA had favorable chemical properties for a pozzolanic material. Furthermore, SCBA replaced samples at all ages showed improved resistance against acidic attack relative to that of the control mortars. Maximum deterioration was seen for 3% concentrated solution. This study’s findings demonstrated that SCBA with an appropriate fineness could be used as a pozzolanic material, consistently with ASTM C 618.

Graphical abstract

Keywords

durability / cement replacement / sugarcane bagasse ash / fineness of ash / pozzolanic properties / mortar acid resistance

1 Introduction

One of the key issues of durability that reduces the service life and increases the maintenance cost of vital civil infrastructures is the degradation of concrete members when exposed to aggressive sulfuric acid environments. Acids in groundwater, chemical wastes, or oxidation of sulfur-based materials (e.g., pyrite) in backfill soils are a few examples of circumstances in which concrete structures can be subjected to sulfuric acid. Acid attack on concrete occurs in concrete pipes used for the disposal of sewage. The sewage is basic in nature but in the presence of oxygen it is converted to acidic nature, while the concrete is basic in nature. Acid rain is another source of acid attack on concrete structures, and it can be very harmful when acid concentration in rain is high.

Incorporation of innovative materials such as supplementary cementitious materials (SCMs) and the use of different mixture proportions can be effective in improving the concrete’s resistance to acids [1,2]. SCMs such as silica fume, fly ash, ground granulated blast furnace slag, sugarcane bagasse ash (SCBA), and rice husk ash help increase the chemical resistance of concrete for two reasons. Firstly, they replace the cement by a certain percentage hence reducing the cement content, and secondly, they utilize the C-H phase from cement to C-S-H phase more rapidly hence improving the chemical stability of concrete in an acidic environment [1].

Sugarcane bagasse ash is a waste product generated by the burning of sugarcane bagasse as an industrial fuel and it is being produced in considerable amounts around the globe. In some countries, such as Brazil and China, it is used as fertilizer [3,4]. However, in countries such as India and Pakistan, it is currently being disposed of in landfills which causes environmental pollution [5,6]. To incorporate this waste in the construction industry, several researchers have investigated the pozzolanic nature of SCBA [7–14]. From an analysis of the chemical composition of SCBA reported in the past studies (Tab.1), it can be observed that most sources of SCBA contain sufficient silica, alumina, and ferric content to meet the specifications for pozzolanic materials according to ASTM C618 [15]. Specifically, a Class N pozzolan requires the sum of contents of SiO₂, Al₂O₃, and Fe₂O₃ to be higher than 70%, while SO₃ and loss on ignition (LOI) must not exceed 4.0 and 10%, respectively. However, unprocessed SCBA discarded from sugar mills contains large fibrous particles having high porosity, which typically results in up to 90% retention on sieve No. 325 (45 µm opening size). While several studies have utilized this material as fine aggregates in concrete or mortar production [16,17], the high porosity of the SCBA results in increased water demand and reduced workability. A decrease was also observed with respect to mechanical properties, such as compressive strength, splitting tensile strength, and modulus of elasticity, as compared to control specimens. Therefore, to utilize SCBA as a pozzolanic material, its fineness must be increased. Cordeiro et al. [18] studied the effect of mechanical grinding of SCBA and checked the improved pozzolanic activity using the strength activity index (SAI), i.e., the ratio of compressive strength of samples containing the pozzolan to the compressive strength of control specimens of mortar at 7 and 28 d of age. The grinding time was varied from no grinding to 240 min, and it was concluded that after 15 min of grinding, 75% SAI was achieved and it reached up to 100% SAI at 120 min. Further increase in grinding time did not improve the pozzolanic activity. In another study by the same author [19], SCBA was passed through ultrafine grinding and D₈₀ (80% passing size) was used as an indicator to correlate the pozzolanic activity of SCBA. It was concluded that SCBA of D₈₀ below 60 µm and Blaine specific surface area above 300 m²/kg can be utilized in concrete as a pozzolanic material. Cordeiro et al. [19] also recommended an optimum value of 20% cement replacement to achieve improved concrete properties. Bahurudeen and Santhanam [20] utilized SCBA after grinding it to the Blaine specific surface area of 310 m²/kg. SCBA replacement level was varied from 5% to 25% to check various properties along with the pozzolanic activity. Results were found satisfactory in terms of pozzolanic activity at all replacement levels.

Several researchers have investigated the effect of incorporation of different SCMs on the acid resistance of concrete [54–56]. One such study [57], checked the effectiveness of natural pozzolan and silica fume for resisting the mass and compressive strength loss in 5% nitric acid (HNO₃) and 5% H₂SO₄ solutions. Specimens containing natural pozzolan showed increased resistance in both acidic environments because of acid ingress and reduced permeability, increased strength and stiffness, less alkali present because of replacement of cement, and conversion of C-H into C-S-H. Similar findings were noted by [58] on replacing cement with glass powder and checking its effectiveness against sulfuric acid attack. It was reported that increased strength was caused by the consumption of C-H hydrate by glass powder. Sulfuric acid starts decalcifying the C-S-H hydrate for a higher CaO/SiO₂ ratio of about 3 (conventional/control concrete). The dissolution of C-S-H occurs at a slower rate for a lesser CaO/SiO₂ ratio of about 1. Hence the decay of mortar containing pozzolan, having a lower ratio of CaO/SiO₂ than conventional concrete, was reduced and the resistance against acid attack improved.

A few studies in the literature investigated the effect of H₂SO₄ on SCBA replaced cement mortars. Arif et al. [17] evaluated the effects of 1% H₂SO₄ solution on SCBA replaced cement mortars exposed for 90 d. They concluded that control mortars showed an increased amount of mass loss and compressive strength loss at 90 d exposure, but lower values were reported with 20% SCBA-replaced mortars at the same age. This increased resistance could be explained by the pore refining property of small SCBA particles in mortars which directly reduced the permeability of sulfuric acid penetration. It has also been reported that a buffer zone was created on the surface of mortar that prevented the formation of ettringite and gypsum and reduced the mass loss of SCBA replaced mortars. The study by Singh et al. [39] concluded that mortar with 10% of cement replaced with SCBA showed improved resistance against H₂SO₄ because of C-H consumption in the pozzolanic reaction.

2 Research significance

Although previous studies have shown that ground SCBA can be utilized as a pozzolanic material in concrete, the optimal fineness and mean particle size remain uncertain. In addition, very few studies of sulfuric acid resistance of SCBA-replaced cementitious compounds have been reported. Moreover, the degradation trend of these compounds with increasing concentrations of acid at different ages needs to be checked for a constant dosage of SCBA as cement replacement. The optimum replacement level of 20% was based on recommendations from previous studies [20,22,36,38]. Thus, this study aims at investigating the effect of different levels of fineness of ground SCBA for a cement replacement level of 20% to determine the influence on the chemical, physical, and mechanical properties of cement mortar. It also adds to the currently limited body of work on the effect of using ground SCBA on mortar durability in acidic environments.

3 Experimental program

The experimental program was divided into two phases.

1) Phase 1: Grinding of SCBA was done and chemical composition and physical properties of the ground SCBA were assessed according to Ref. [15].

2) Phase 2: Mortar mixtures were prepared using the ground SCBA at a cement replacement ratio of 20% and the mechanical properties and the acid resistance were evaluated for different particle sizes. Physical and chemical tests were carried out on fresh mortars similar to those of phase 1 for each particle size.

3.1 Materials

SCBA used in this study was obtained from the local sugar industry. The high carbon unburnt particles were removed by passing the unprocessed SCBA through ASTM sieve No. 100 (150 µm opening size) [20]. The fraction passing through ASTM sieve No. 100 was then divided into two groups, with one group ground in a ball mill to a fine powder such that no particles were retained on ASTM sieve No. 325 (45 µm). Five different sets of materials were prepared by mixing certain proportions of the material completely passing from sieve No. 100 and completely passing from sieve No. 325 as shown in Tab.2.

Lawrencepur sand, a locally available pit sand, was utilized as fine aggregate. OPC conforming to ASTM Type-I was used in this study.

3.2 Mortar specimens

Twelve 50 mm mortar cube specimens were prepared for testing of compressive strength [53] and strength activity index [59], with a cement replacement level of 20% for each of the five SCBA sizes, as well as one control mixture without SCBA. In addition, Tab.3 shows the four groups of cube specimens that were prepared from the mortar mixture, by 20% replacement of cement, and tested following exposure to three different concentrations of sulfuric acid for visual distortion, mass, and compressive strength loss [60]. Here CD denotes the control specimen for durability (acid resistance test) whereas SCBA01, SCBA02 and SCBA03 denote the group of specimens containing SCBA for acid attack with 1%, 2%, and 3% H₂SO₄ solution respectively. For the control specimen, the cement to aggregate ratio was 1:2.75 (as recommended by ASTM C109), and the water to cement ratio was 0.485 which resulted in a flow of 112 mm, in accordance with ASTM C1437 [61].

Trials were performed on the SCBA specimens to achieve a constant flow of 112 mm for all mixtures by varying the water to cementitious materials (W/C, including both cement and SCBA content) ratio, using a procedure similar to that followed by [62], while maintaining the cementitious material to aggregate ratio at 1:2.75 (as for the control specimen).

4 Test methodologies

4.1 Raw materials

The chemical composition of the cement and of the various sets of SCBA were obtained by performing X-ray Fluorescence (XRF) analysis. X-Ray Diffraction (XRD) analysis was also performed on the SCBA samples to determine the amorphous or crystalline nature of elements present in the material [19,63]. Tests were performed to determine physical properties of the cement and SCBA, including specific gravity [64], mean particle size (d₅₀), and grain size distribution curve by hydrometer analysis. Fine aggregates were also evaluated to determine their physical properties including sieve analysis [65], bulk density [66], specific gravity, and water absorption [67].

4.2 Fresh properties of mortar

Fresh properties were calculated for the control mortar, as well as the five SCBA mortars. These properties were standard consistency [68], initial and final setting time [69], workability [61], and water demand. The water demand was obtained by varying the W/C ratio to achieve a constant value of flow, using a standard flow table [70].

4.3 Hardened state properties of mortar

The pozzolanic nature of SCBA was assessed using the SAI [59] and compressive strength test [53]. Tests were performed using a 500 kN Shimadzu universal testing machine. Tests were conducted on the 7th, 28th, 56th, and 90th day after casting. Specimens were lime water cured until the testing day. The volume of permeable pores was calculated for the cube specimens of each mix at a testing age of 28 d following ASTM C642 standard procedure [71].

Along with the control mix, the SCBA mortars were used to fabricate mortar prisms with dimensions of 40 mm × 40 mm × 160 mm to measure the modulus of rupture [72] at 7, 28, 56, and 90 d of curing.

4.4 Visual inspection, mass loss and compressive strength loss due to acid attack

For evaluating the durability performance, SCBA material was utilized for the preparation of 50 mm × 50 mm × 50 mm cubes with 20% replacement of cement and cured for 28 d. Acid solution of 1%, 2%, and 3% by weight of water was prepared in an airtight container. The control, as well as SCBA replaced cubes, were placed in the containers for 1, 7, 14, 28, and 56 d. pH of the solution was monitored twice a week and maintained to a constant value (pH = 1.8 for 1% acid, pH = 1.4 for 2% acid, and pH = 1.1 for 3% acid). The solutions were replaced with fresh ones at every testing age. Specimens were evaluated, by visual inspection, percentage mass loss, and percentage compressive strength loss, for the three basic parameters used for checking the extent of deterioration/damage. These parameters were evaluated at every testing age [60].

5 Results and discussion

5.1 Material characterization

Tab.4 shows the chemical composition of the OPC used in this study. The chemical compounds were well within recommended limits [73]. For phase composition calculation, Bogue’s equation was utilized.

XRF analysis of both SCBA size distributions (SCBA passing from sieve No. 100 and SCBA passing sieve No. 325) is also shown in Tab.4. According to the XRF results, both material sets had chemical compositions that could be regarded as good pozzolans according to ASTM C618. LOI value was reduced from 9.5% to 5% because of the grinding of the SCBA [27,73]. Similarly, the SO₃ percentage was also within the limit specified by ASTM C618.

XRD analysis of SCBA0 material which composes 100% ground SCBA was performed and is shown in Fig.1. It was observed that almost all the material was regarded as amorphous, with some fraction of crystalline silica in the form of quartz and cristobalite detected at the representative Bragg’s angle (2θ). A small proportion of Hematite was also observed due to the crystalline nature of ferric. These peaks of crystalline silica and ferric provided evidence of non-uniform burning of SCBA in the industrial plant [19,63].

The physical properties of the cementitious materials (cement and SCBA) were determined according to the relevant ASTM standards and shown in Tab.5. The specific gravity of the OPC was 3.12, while all size fractions of SCBA had specific gravity values less than that of the OPC. The highest value of 2.39 was observed for SCBA0 [38,74]. The percentage retained on sieve No. 325 showed that SCBA0 and SCBA15 were finer than cement. Similar findings can be observed from the mean particle size that was less for SCBA0 and SCBA15 than for the cement.

5.2 Fresh properties of mortar mixtures incorporating sugar cane bagasse ash

A mortar flow table was used to measure the flow value of the freshly mixed mortar (Tab.6). SCBA0 and SCBA15 mortar mixtures showed nearly the same flow as the control mortar for the same W/C ratio. To maintain a constant flow, the W/C ratios were slightly increased from 0.485 for mixtures SCBA30, SCBA45, and SCBA60. A maximum W/C ratio of 0.51 was required for SCBA60, resulting in a 5% increase in water demand. A similar increase in water demand was reported in previous studies [75].

Consistency and setting time tests were carried out on the pastes of all the mixtures using ASTM C187 [68] and ASTM C191 [69], respectively. It was observed that consistency values increased with the increase in mean particle size of SCBAs [38,39]. A major increase in consistency values was obtained for SCBA45 and SCBA60 due to their coarser and hygroscopic nature. Conversely, the increase in values for SCBA0 and SCBA15 was relatively small. Smaller SCBA particles, which were finer than those of the cement, helped achieve the required workability.

Initial and final setting times also increased with the incorporation of SCBA as cement replacement. This increase is attributed mainly to two reasons.

1) Increasing the water cement ratio which means increasing the mixing water content, and decreasing the ordinary Portland cement content.

2) The fine bagasse ash particles surrounding the cement particles delaying the hydration of the Tricalcium silicate. This creates a layer around the cement particles and its thickness is directly proportional to the measured SCBA content.

The increase in setting time was greater for SCBA samples with larger mean particle sizes. Similar results were observed in previous studies [38–40].

5.3 Hardened state properties

Compressive strength values of all the mortar mixes are shown in Fig.2. After 90 d, the compressive strength of the SCBA0 mortar was 9% higher than that of the control mix. This increased value of compressive strength at early ages may be attributed to the filling effect of particles that were finer than cement, and at later ages the pozzolanic reaction of SCBA. SCBA15 mortar samples only showed an increase in compressive strength over the control mix at 90 d, with nearly equal strength values at 28 and 56 d. This is due to the presence of larger particles that are less effective in filling spaces between cement and developing pozzolanic reactions at early ages. A decrease in strength was observed for the remaining SCBA groups that contained larger fractions that inhibited the hydration reaction [40].

The results also showed that all the SCBA mortars, with an exception of SCBA60, surpassed the ASTM C311 [59] limit of 75% SAI at early ages, which suggests their potential use as SCMs in concrete.

Mortar prisms were fabricated using the SCBA0 mix since it exhibited the best overall performance in the compressive strength tests. The results of the modulus of rupture tests at 7, 28, 56, and 90 d are shown in Fig.3. An 11%−13% increase in the modulus of rupture for the SCBA0 samples compared to the control group was observed from 28 d up to 90 d after casting.

The volume of permeable pores was calculated for all the mixtures at 28 d of curing to indicate the porosity and permeability characteristics of the mix. Tab.7 shows the volume of permeable voids of all the mixes. SCBA0, SCBA15, and SCBA30 cubes showed 9.2%, 10.1%, and 11.0% permeable voids, respectively, which were less than for the control cubes. SCBA45 and SCBA60 cubes presented 12.4% and 13.4% voids which were higher than in the case of the control cubes. Lower values of porosity, generally associated with improved durability characteristics, also correlated well with the improvements in mechanical properties discussed earlier.

5.4 Samples exposed to acid solutions

The typical surface appearance of Control and SCBA0 replaced mortar cubes labelled as CD and BA respectively, subjected to Sulfuric Acid solutions of different concentrations was observed at different days and its specimen images are shown in Fig.4 to Fig.6. The extent of deterioration and surface scaling was clearly visible on control samples as well as SCBA0 replaced samples. Concrete subjected to 1% and 2% H₂SO₄ demonstrated an aggressive environment similar to that in sewer pipes. An increased percentage of 3% H₂SO₄ was also evaluated to check the effect of extreme acidic conditions. It was seen that SCBA0 replaced cubes, subjected to 1% H₂SO₄, showed a milder deterioration at all ages as compared to deterioration of the control one. Minor surface scaling and pore formation were observed at less ages of exposure and these effects increased at later ages. Size reduction of the SCBA0 specimen was also observed at 28 and 56 d of exposure. Heavy deterioration was visible on cubes subjected to 2% H₂SO₄ solution. Major surface scaling and reduction in dimensions were observed at 28 and 56 d (see Fig.5 and Fig.6). The cement slurry was slightly lost, and the sand particles were visible on the control as well as SCBA0 replaced cubes. The surface scaling and deterioration level were more serious in the case of 3% H₂SO₄ exposure conditions. Complete removal of cement slurry from the outer layer along with the separation of sand particles from the mortar cubes was observed. Unlike the first two exposure conditions, here, the deterioration started immediately after contact of cubes with the acidic solution.

Air bubbles were generated, as cement slurry started to dissolve in solution. It was the result of this fast reaction that after 24 h, the smooth faces of mortar cubes subjected to 3% H₂SO₄ were rough and showed the effect of concentrated acid attack as shown in Fig.4. Regular edges were converted to rounded because of the separation of loose material. At 28 d of exposure, major surface scaling happened as shown in Fig.5, and inner particles’ bonding was breaking. It was the result of this bond breaking that a chunk of control mortar specimen separated from the cube, reducing its one face dimension from 50 to 36.8 mm. As compared to control cubes, SCBA0 replaced mortar cubes were not showing this type of chunk separation even at the later age of 56 d exposure. That increased bonding power between particles was because of the hydration reaction of SCBA with calcium hydroxide. Therefore, SCBA0 replaced mortars performed well as compared to control mortars even under the most extreme exposure conditions of 3% H₂SO₄ and can be effectively utilized in cementitious materials as partial replacement of cement.

5.5 Fineness of sugar cane bagasse ash

The fineness of pozzolanic materials is one of the major factors that affect the rate of pozzolanic reaction. Finer particles will react more rapidly than coarser particles and are generally more suitable for use in concrete and mortar, as demonstrated in this study. Moreover, water requirements and strength activity index are also affected by the fineness of the material. For natural pozzolans of class N, ASTM C618 [15] requires that less than 34% of particles be retained on sieve No. 325, with a maximum water demand of 105% and a minimum strength activity index value of 75% at 7 or 28 d of age as compared with the control mix. In this study, SCBA45 resulted in a water demand of 103% compared with the control mix, and SCBA60 required 105% for the same flow. Moreover, the SAI was higher than 75% for SCBA45 at 28 d. Considering these various factors, it is reasonable to suggest that SCBA particles having an appropriate chemical composition, with less than 40% retained on sieve No. 325, may be used as a good pozzolan material in cementitious mixtures. Furthermore, SCBA with less than 15% retention on sieve No. 325 may produce a mortar with nearly 100% SAI values and can exceed 100% when no particles are retained on sieve No. 325. The lower porosity associated with the finer material also suggests that the resulting concrete or mortar would possess good durability characteristics and may be utilized to produce high strength concrete with SCBA0 material, and this presents a topic for future research.

5.6 Mass and compressive strength loss

Along with the visual observations, percentage mass and compressive strength loss were also measured for control and SCBA replaced mortars at different ages exposed to three concentrations of the acidic solution. Mass and compressive strength loss were measured and compared to actual mortar specimen free from acidic exposure at the same testing age. Percentage reduction in mass, as well as compressive strength (C.S), is shown in Fig.9 and Fig.10.

Maximum mass loss in 1% H₂SO₄ solution was observed for the control specimen at 56 d of exposure, which was about 3%. In contrast to that, at the same age and exposure conditions, the mass loss observed for SCBA0 replaced mortar cube was recorded as 1.4%, which was more than 50% less than that of the control cube. On all other exposure days, the decrease in mass was almost negligible for SCBA0 replaced cubes as compared to control ones. Similarly, mass loss increased with the increase of time, in terms of exposure days, and of acid concentration. The maximum mass loss recorded was about 17% for control mortars and nearly half of that for SCBA0 replaced mortars. A nearly linear trend between mass loss and exposure days was observed for both mixtures.

Sulfuric acid at first reacts with calcium hydroxide, one of the hydration products, to produce gypsum and ettringite [17,39]. They cause an expansion in concrete and surface chunks start separating from the specimen, resulting in mass loss. This C-H phase of hydration reaction starts more rapidly if the concentration of acid increases [57]. This was the reason for the mass loss of mortar cubes when exposed to an acidic medium [76]. In the case of SCBA0 replaced mortars, the hydration reaction took place at a slower rate, therefore, less C-H phase was generated to react with acid. Moreover, at later stages, the resistance of acid attack also improved because of the consumption of the C-H phase by SCBA0 in pozzolanic reaction, hence providing lesser mass loss.

A similar trend was also observed in the case of the compressive strength values. Least loss in compressive strength was for 1% H₂SO₄ exposure conditions and the maximum was observed for control mortars exposed to 3% H₂SO₄. Nearly 30% compressive strength loss of control cubes subjected to 3% H₂SO₄ at 56 d was observed, and this was the maximum loss for all cubes. In contrast to that, almost 18% loss in compressive strength was seen for SCBA0 replaced mortar at the same conditions. Cubes exposed to other concentrations at different exposure days showed the same trend. In every case, the SCBA0 performed well as compared to control mortar. This agrees with what was reported in Ref. [17] where the effects of 1% H₂SO₄ solution on SCBA replaced cement mortars were evaluated and control specimens showed an increased amount of mass and compressive strength loss at 90 d exposure compared to the SCBA replaced cement. Similar improvement in sulfuric acid resistance of cement mortars was reported with the addition of other SCMs such as glass powder & limestone powder [58]; natural pozzolan & limestone fine [57]; fly ash, silica fume & lime sludge [54]; blast furnace slag & natural pozzolan [55]; fly ash & ultrafine fly ash [56] and for 10% replacement of SCBA studied by Singh et al. [39].

The reason for this decrease in compressive strength is explained by the production of gypsum and ettringite [17,39]. The expansion caused by these two compounds produces microcracks and breaks the bonding between the particles. Due to this, particle-to-particle interface is disturbed and contribution towards resisting compressive load is decreased [58,77]. The same phenomenon of decreased interlocking because of microcracking happened in the case of control cubes that experienced significantly reduced strength. The increased resistance of SCBA0 is explained by resistance to the production of ettringite and by the pore-filling effect of smaller particles. These particles retard the hydration reaction and provide the reinforcing and bridging effect by blocking the pores. A similar phenomenon resulted in the improved behavior of SCBA0-replaced mortars against compressive strength loss due to acidic attack.

6 Conclusions

A comprehensive experimental program was carried out to investigate the effect of different particle size distributions of SCBA at a replacement rate of 20% by weight of cement on the pozzolanic properties of cement mortars. The recommended particle size was found to be 0% retained SCBA on sieve No. 325. SCBA with this particle size was used to fabricate cement mortar cubes that were subjected to acid solutions with various acid concentrations for 56 d to assess the effect of the SCBA on the acid resistance and overall durability of the resulting mortars. The main conclusions to be drawn from this study are as follows.

1) All the sets of SCBA sources used in this study were found to meet the chemical and physical criteria of ASTM C 618 [15] for natural pozzolans of class N. SCBA0, which had the smallest mean particle size, was found to be the most effective.

2) Strength activity index values were well above the limiting value for all the mean particle sizes of SCBA at all the testing ages except SCBA60 at 7 d. The best result was shown by SCBA0 at 90 d which achieved an increase of 10% in compressive strength. The modulus of rupture of SCBA0 replaced mortar improved by 12% at 90 d compared to the control mix.

3) SCBA shows good potential as a natural pozzolan, provided that uniform burning is ensured to produce a material with good chemical composition. It is recommended to grind the material such that 15%−40% is retained on sieve No. 325, depending on the strength and durability requirements of the concrete or mortar.

4) Mortar specimens replaced with SCBA0 were subjected to sulfuric acid of different concentrations and visual observation of deterioration was reported. At each concentration, SCBA replaced mortar showed the least surface disfiguration at a particular testing age. This was because of consumption of the C-H and reduction of cementitious content because of replacement of cement.

5) Mass loss was also used as an indication of the extent of the acid attack. Control mortar cubes subjected to 3% sulfuric acid at 56 d showed the maximum mass loss. Conversely, the SCBA replaced mortar at the same testing age subjected to the same concentration showed improved resistance.

6) Compressive strength loss showed the same trend as mass loss. This was also an indication of hydration product consumption because of chemical reaction between acid and hydration products to start the disintegration of the intact mass of mortar.

7) Based on the past studies and discussions, it can be concluded that by providing a good visual appearance, resistance against mass and compressive strength loss, SCBA0 can be utilized as the partial replacement of cement due to its improved efficiency in an acidic environment.

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