Performance assessment of Alccofine with silica fume, fly ash and slag for development of high strength mortar

Abstract

Previous studies on concrete have identified silica fume (SF) as the most effective supplementary material, whereas fly ash (FA) and slag have been identified as economical materials with long term strength potential. Development of blended cement mortar referred to as blended mortar (BM) requires similar assessment. The present study explores the application of Alccofine (AL) as supplementary material and compares its performance with conventional materials namely SF, FA and ground granulated blast furnace slag (GGBS). The mortar specimens with binder to fine-aggregates (b/f ) ratio of 1:2 are prepared at water to binder (w/b) ratios of 0.4 and 0.35. The strength values and stress-strain curve for control and BM specimens are obtained at 7, 28, 56, and 90 d curing periods. The assessment based on strength activity index, k-value method and strength estimation model confirms that AL, despite lower pozzolanic activity, contributes to strength gain, due to reduced dilution effect. Assessment of stress-strain curves suggests that the effect of w/b ratio is more dominant on the elastic modulus of BM specimens than on control specimens. The observations from the study identify enhanced strength gain, improved elastic modulus and higher energy absorption as key contributions of AL making it a potential supplementary material.

Graphical abstract

Keywords

Alccofine / high strength mortar / efficiency factor / dilution effect

1 Introduction

The utilisation of supplementary cementitious materials as secondary binders has been well acknowledged due to their potential benefits in improving one or more properties related to strength, durability, economy and environment [1–3]. The prominent materials explored in the quest to reduce cement consumption and enhance the properties of blended cement concrete, include silica fume (SF), fly ash (FA) and ground granulated blast furnace slag (GGBS) [4–10]. For SF blended concretes, it is reported that the increment in strength (noted for an optimal percentage of 10%–15%) decreases with curing time accompanied by corresponding increase in shrinkage [10–17]. In comparison to SF, FA and GGBS blended concrete exhibit decrement in strength with increase in w/b ratio and replacement percentage [2,18,19]. Apart from these, other supplementary materials namely metakaolin (MK), quartz powder (QP), diatomaceous earth (DE) and limestone powder (LP) have also been explored in previous studies as secondary binder materials [10,18,20,21].

It has been reported that the high early strength observed in concrete with finer materials such as SF and MK can be attributed to their high ratio of specific surface area to volume, which enhances reactivity and packing density. Further, finer materials contribute to significant densification of interface transition zones (ITZ). Similar observations of improvement in compressive strength and microstructure densification have been reported on cement paste blended with cellulose nanocrystals [22]. On the other hand, FA and GGBS have ratio of specific surface area to volume comparable to cement, causing dilution resulting in low early strength. Further, the packing density is influenced by maximum and minimum particle size. As a result, the influence of supplementary materials on the properties of mortar or cement paste is observed to be different from that on the properties of concrete. Understanding the impact of supplementary materials on mortar is thus important for evaluation of their effect on mortar applications, repair grouts, rehabilitation of micro concrete and development of ultra-high-performance concrete.

For the applications discussed above, new supplementary materials can be explored with mortar specimens to compare their performance with conventional materials. The present study considers the application of AL as a potential supplementary material for development of BM. AL is a processed micro fine slag obtained through controlled granulation and is suitably modified with special chemicals to improve reactivity [23,24]. Its chemical composition is similar to GGBS and is thus expected to contribute strength development through latent hydraulic activity and also pozzolanic action. Further, the finer size of AL with surface area per mass of 1.2 m²/g is greater than GGBS and is expected to enhance packing density. Microstructural studies on AL blended cement paste have reported formation of calcium silicate hydrate (CSH) gel with reduced Ca/Si ratio. For replacement ratios in the range of 0%–10%, the Ca/Si ratio of CSH is reported to decrease from 2.56 to 1.26 [25]. The reduction in Ca/Si ratio is known to enhance compressive strength with increasing curing age [26]. The application of AL for geo-polymer concrete has been observed to contribute towards reduced water demand, increased compressive strength and workability [27]. The limited studies that have reported on AL blended mixes highlight the potential applications of AL as mineral admixture for development of high strength mortar. In order to evaluate the performance of AL against commonly used supplementary materials, conventional supplementary materials namely SF, FA and GGBS are also considered in this study.

The performance of AL blended mortars (B-AL) compared with those of conventional supplementary materials is evaluated in terms of fresh and hardened properties, i.e., consistency, setting time, compressive strength and stress-strain response. Contribution of supplementary materials in development of compressive strength is further analysed through strength activity index (SAI), k-value method and strength estimation model [28–34]. The chosen models not only help to evaluate the overall effect and efficiency but also provide insight into the filler and pozzolanic contribution of the mineral admixtures in strength development. In addition, quantification of stress-strain curves through evaluation of elastic modulus and toughness index is performed to study the impact of mineral admixtures on the material behaviour.

2 Research significance

The strength performance of supplementary materials is often quantified for concrete specimens instead of mortar. For the supplementary material under consideration, variability in strength gain for mortar and concrete reported in literature [35,36]. It can be observed that the early age strength increment/decrement is lower for mortar specimens than concrete specimens. Further, it can be observed that FF blended mortar (B-FF) follows the same trajectory as that of its concrete counterpart. In comparison, SF blended mortar (B-SF) exhibits substantial variability between mortar and concrete specimens in the trajectory of strength gain, which suggests that the effectiveness of supplementary materials for mortar can be substantially different from its concrete counterpart and requires separate investigation. Such assessment of supplementary materials is useful for development of ultra-high-performance concrete, 3D printed concrete and grout applications for tunneling, as well as for repair and rehabilitation activities [37–41].

Thus, the present study attempts to compare and quantify the performance of AL with conventional supplementary materials like SF, FA, and GGBS for development of high strength mortar.

3 Experimental procedure

3.1 Materials and methods

Normal Portland Cement (NPC) used as primary binder confirms to IS 12269 and ASTM C150 [42, 43]. Supplementary materials namely high calcium fly ash (FC), low calcium fly ash (FF), GGBS, SF and AL have been used to partially replace proportional weight of cement. FC, FF, GGBS and SF are by-products of industrial processes and are widely used as supplementary materials for development of BM and concrete [27,44,45]. Alccofine 1203 (AL) is a commercially available micro fine mineral additive manufactured by Counto Microfine Products Pvt Ltd (Goa-India). It is manufactured with controlled granulation resulting in D₁₀, D₅₀, and D₉₀ values of 1.6, 4.3 and 9 µm respectively. AL is finer than cement and GGBS with specific surface area of 12000 cm²/g and specific gravity of 2.86. Apart from improvement in packing density, the addition of AL is expected to influence microstructure of concrete by decreasing Ca/Si ratio of CSH and thus contributing to higher strength gain. The chemical composition of supplementary materials and reference binder (cement) utilized in the study are shown in Tab.1. As evident from Tab.1, AL and SF have specific surface area larger than other supplementary materials and can potentially influence packing density and also the rate of pozzolanic activity. Accounting for this variation in particle size, two different replacement ratios have been considered. AL and SF have specific surface area greater than that of cement and are used to replace 10% of cement whereas FC, FF, and GGBS have specific surface area less than that of cement and are used to replace 20% of cement.

In addition to binder materials, river sand confirming to gradation limits defined in zone-IV as per IS 383-2016 and ASTM C136M-19 with maximum size of 2.36 mm is used as fine aggregate [46,47]. Its physical properties i.e. water absorption, specific gravity, and fineness modulus are 1.61%, 2.64, and 2.62, respectively.

3.2 Sample preparation and testing

Mortar specimens are prepared with a binder to fine aggregate ratio of 1:2. In order to evaluate the effect due to supplementary materials, control samples are prepared with NPC as a primary binder. For blended specimens namely with FC, FF and GGBS, 20% of NPC by weight has been replaced with an equivalent proportion of the respective supplementary material. To account for improvement in packing density and higher reactivity due to increased specific surface area, AL and SF blended specimens are prepared by replacing 10% cement with equal proportion of respective material. Mortar mixes are prepared at water to binder (w/b) ratios of 0.4 and 0.35. For w/b ratio of 0.4, cement is mixed with mineral admixture for about two minutes followed by dry mixing with fine aggregates for another 3–5 min. Water is then slowly added and mixing is continued till a homogeneous paste is formed. For w/b ratio of 0.35, 0.5% high range water reducing admixture (HRWR) by weight of cement is used to enhance workability. Post dry mixing, water is first mixed with the admixture and 70% of the solution is added to the dry mix prepared earlier. The mixing is continued for a period of 5 min followed by addition and mixing of remaining solution for another 2–3 min. The details of the mix composition are given in Tab.2.

Mortar specimens confirming to IS: 4031 (part-6)-1988 and having internal dimensions of 70.6 mm × 70.6 mm × 70.6 mm are prepared. Specimens are duly vibrated to achieve desired compaction levels and are demoulded after 24 h [48]. The demoulded specimens are then cured in water tank till their desired testing age (viz., 7, 28, 56, and 90 d).

A compression testing machine of 3000 kN capacity with ±0.5% accuracy has been used to perform load and displacement control uniaxial compression tests. For load control testing, loading rate of 2.9 kN/s as specified in IS: 4031 (part-6)-1988 has been adopted. For displacement control testing, a constant displacement rate of 1.5 μm/s is applied and the resistance offered is measured till the displacement of 3000 μm has been attained.

4 Quantification methods

Owing to difference in replacement percentage considered for AL and SF compared to other supplementary materials, it is pertinent to quantify the performance of each supplementary material in terms of its overall effect, efficiency of replacement and contributions from packing density and pozzolanic activity. The following quantification methods have been adopted to evaluate the effect of mineral admixtures on compressive strength.

4.1 Strength activity index

SAI value at a given curing period represents the ratio of compressive strength of blended specimen with that of control specimen. ASTM C618 and EN 450-1 identify the mineral admixture as pozzolanic for SAI value greater than 0.7. For a given replacement ratio, SAI value higher or lower than 1 indicates higher or lower reactivity with reference to cement [28,49].

4.2 k-value method

This quantification of the role of supplementary material (FA) has been proposed by Smith and is based on effective water to cementitious material ratio as {\displaystyle \frac{w}{(c+kf)}} [34]. Further, Schiessl has proposed the strength equivalence between control and blended specimen and introduced the concept of water reduction (∆w) as in Eq. (1) [50].

(1)\mathrm{\Delta }w=\left(w/c\right)\left[\frac{1}{1+k{\displaystyle \frac{f}{c}}}-\frac{1}{1+{\displaystyle \frac{f}{c}}}\right].

The comparison of strength between control and blended specimen for given w/c ratio and curing period can be used to predict water reduction/addition which in turn can be used to evaluate an efficiency factor. The step-by-step procedure for determination of k-value is provided in the Supplementary materials. Also, the determination of k-value is influenced by parameters A and B of Abram’s law, described in the Supplementary materials. The k-value reported for FA and GGBS blended concrete specimen at 28 d curing period ranged from 0.25–1.2 and 0.7–1.29, respectively [33,51,52]. In comparison, SF blended concrete exhibits k-values ranging from 2.5–0.5 for replacement percentage in the range of 5%–40% [32]. Assessment of k-value concept to evaluate the effect on durability of blended specimens has been proposed by Gruyaert et al [53]. The efficiency factor for mineral admixtures, i.e., FA, GGBS, SF, etc., is evaluated mostly for concrete specimen. However, the data for BM specimens is limited. Information on the efficiency factor for BM is critical for understanding its role in bulk of matrix and design of high strength and ultra-high strength mortar mixes.

4.3 Strength estimation model

A strength estimation model based on co-relationship between strength of control (S_c) and BM specimen (S_p) has been proposed by Razak and Wong [34]. The model enables determination of strength contributions due to pozzolanic and filler action of the material. The model as shown in Eq. (2) is independent of w/b ratio and the replacement ratio.

(2)S_{\mathrm{p}}=a{S}_{\mathrm{c}}+b.

In Eq. (2), the slope ‘a’ represents the strength contribution of pozzolanic action, whereas, y-intercept ‘b’ represents strength increment/decrement due to change in packing density. The strength estimation model attempts to distinguish the strength contribution due to pozzolanic and filler behaviour. Since the formulation of the equation relies on the strength of reference mortar and BM, the values of pozzolanic and filler contribution predicted by the model are influenced by the curing period.

Apart from quantification methods based on compressive strength, variability in stress-strain curves and related parameters are also quantified.

5 Results and discussion

5.1 Fresh cementitious material properties

Fresh properties of cement and blended pastes are evaluated in this study following the guidelines of IS 4031, 1988 (part-4 and part-5) and ASTM C-191, 2008 [54–56]. The obtained results along with the designation for control and blended specimens are given in Tab.3. The observed values of consistency and setting time for control and blended specimens considered in the study are within the prescribed limits. The contribution of FA and GGBS in reducing water demand, thereby contributing to reduction in consistency, is well reported [57]. It has been observed that AL exhibits similar reduction in consistency and its usage can also contribute to reduction in water demand. The reduction in initial and final setting time for AL is in line with its higher specific surface area. However, the reduction in water demand for consistency reported in the study could be due to special chemicals added during the manufacturing of AL.

5.2 Compressive strength

The variation in compressive strength with curing age for control and blended specimens at the water to binder (w/b) ratios considered in the study is shown in Fig.1.

For w/b ratio of 0.4, the strength of control specimen attains plateau at 28 d, whereas, blended specimens continue to exhibit some strength gain with curing age as shown in Fig.1(a). Owing to this, blended specimens achieved strength greater than control specimens at higher curing periods despite low early strength. The observed trends in strength development indicate that the process of strength gain for blended specimen is not complete at 28 d and continues with curing age. The mortar blends of SF, FA and GGBS suggest that the role of specific surface area is less significant in mortar due to absence of dominant interface transition zone (ITZ).

For w/b ratio of 0.35, the variability in strength of blended specimens is low for curing periods of 7, 28, and 56 d, whereas it increases by the 90 d curing period as shown in Fig.1(b). The reduction in w/b ratio decreases the interparticle distance, which in turn can restrict the impact on packing density. This reduction in packing density might contribute to reduction in variability of strength. Post 56 d curing, both strength and its variability increase significantly, mainly due to the variation in pozzolanic contribution of different supplementary materials.

Comparison of strength evolution for w/b ratio of 0.4 and 0.35 suggests that the variability in early age strength amongst blended specimens decreases with water binder ratio. The increment in strength of B-AL with reference to control specimen decreases with curing period for w/b ratio of 0.4, whereas, it increases with curing period for w/b ratio of 0.35. Similarly, other blended specimens also exhibit the effects of w/b ratio and curing period. This makes generalization of the strength behaviour of these materials a difficult task. Unlike blends of SF, FF, FC and GGBS that exhibit low early strength, B-AL (representing blend of AL) exhibits higher strength at all curing periods and w/b ratios considered. Though the exact mechanism on strength gain of AL blended specimens is still not well understood, microstructure studies on AL blended paste have demonstrated formation of CSH gel at lower Ca/Si ratio [26]. Since the reduction in Ca/Si ratio of CSH gel is associated with increasing compressive strength, the strength increment observed for B-AL in the present study can be considered to be an effect of the reported reduction in Ca/Si ratio of CSH gel.

Despite higher strength, B-AL exhibits strength variability is consistent with those of other blended specimens. The high early strength and comparable long-term strength exhibited by B-AL is a favourable behaviour of B-AL mixes, which may be beneficial for many high strength applications.

5.3 Stress-strain curve comparison for control and blended specimen

The stress-strain curve for control and blended specimens evaluated at 28, 56, and 90 d curing periods for 0.4 and 0.35 w/b ratios is shown in Fig.2. The deviation in pre-peak response of BM specimens as compared to control specimen is higher at w/b ratio of 0.4 and reduces for w/b ratio of 0.35. For w/b ratio of 0.4, the variability in pre-peak response is highest at 56 d curing period whereas for 0.35 w/b ratio it is highest for 90 d curing period. The variability is also evident in the peak stress and the corresponding strain for control and BM specimens.

In general, the peak stress achieved by control and BM specimens exhibit variation for curing period of 28 and 56 d at w/b ratio of 0.4 and 0.35. Amongst BM specimens, peak stress corresponding to 90 d curing period shows slight reduction in variability for w/b ratio of 0.35. Further, except for B-AL all blended specimens exhibit softened response in comparison to control specimens. However, at 0.4 w/b ratios, the response for B-SF and B-GGBS improves and is comparable to B-AL at 90 d curing period. Also, the post peak response at 90 d curing period is comparable for blends of AL, SF and GGBS. At 0.35 w/b ratio, the pre-peak and post-peak response of control and BM specimens corresponding to 28 d curing period vary in a narrow range. For increasing curing periods of 56 and 90 d, the variation in pre-peak and post-peak response increases. At 56 d, B-SF in comparison to B-AL and B-FC shows softened response. However, the 90 d response of B-SF exhibits an improvement with stiffened response that is followed by B-AL and B-FC.

For BM specimens, comparison of stress strain response with curing period and w/b ratio is shown in Fig.3. The softening behaviour of control specimen for decreasing w/b ratio is evident in Fig.3(a). Similar behaviour is reflected by B-AL as shown in Fig.3(b) with curing effect also contributing to softening behaviour. For other blends, transition from softened to stiffened response is evident over the observed curing periods. The variability in pre-peak response observed can be attributed to distribution of hydrated and unhydrated products, whereas, the post-peak variability can be attributed to heterogeneity in distribution and the dominant fracture mechanisms.

Stress-strain response for blended specimens exhibit significant effect of curing period and w/b ratio. For decreasing w/b ratio, the variation in pre-peak response decreases, whereas, it increases in post-peak response. For a given curing period, decreasing w/b ratio decreases the degree of hydration and consequently reduces the degree of reaction. This enhances the heterogeneity of paste due to micro-aggregate effect generated by unreacted pozzolanic materials.

6 Performance assessment based on compressive strength

The quantification of performance of supplementary materials considered in the study is provided in the following subsections. The SAI method, k-value (efficiency factor) and strength estimation method provide different perspectives on the strength contribution of blended specimens. The average values of the compressive strength as obtained from the experimental data are used for deriving the values of SAI, efficiency factor and development of the strength estimation model.

6.1 Assessment based on strength activity index

The SAI values for blended specimen at w/b ratios of 0.4 and 0.35 are shown in Fig.4. In general, incremental change in SAI value is higher when curing period changes from 28 to 56 d at w/b ratio of 0.4, whereas, for w/b of 0.35 incremental change in SAI is higher for change in curing period from 56 to 90 d. The experimental results suggest that the time period required for attaining SAI of 1 decreases with decreasing w/b ratio, thus indicating higher reactivity.

The SAI values of B-SF are observed to be in the range of 0.83–1.09 and 0.92–1.0 for w/b of 0.4 and 0.35, respectively, and are comparable with those of other blended specimens at both w/b ratios. This suggests that the higher reactivity of SF reported for concrete specimens is largely a result of improvement in packing density and not an effect of pozzolanic activity variation. In comparison, the near constant but higher value of SAI for B-AL (1.21–1.17 and 1.07–1.29 for w/b of 0.4 and 0.35, respectively) exhibits its higher strength contribution over other supplementary materials. For 0.35 w/b ratio, the SAI value of B-AL is comparable with those of other blended specimens, but increase in strength gain beyond 56 d curing period allows B-AL to exceed SAI of other blended specimens. In comparison, B-FC and B-FF exhibit steady but continuous increase in SAI value, with values at 90 d curing comparable to B-AL. The values are expected to increase further as pozzolanic reaction progresses slowly with maturity period expected to exceed one year.

Though the 90 d SAI value of B-AL is comparable with those of B-FC and B-FF, consistently higher values at lower curing periods favour the choice of AL as an effective supplementary material.

6.2 Assessment based on k-values

The k-value for a mineral admixture reflects its reactivity with respect to cement and offers a way to compare its relative effectiveness. This is achieved through determination of effective water reduction evaluated by comparing the strength of control and blended specimens. For the supplementary materials considered, the efficiency (k-value) values evaluated using Eq. (1) as a function of curing period and w/b ratio are shown in Fig.5. The variation in k-value with curing period and w/b ratio reflects the net effect of filler and pozzolanic behaviour on strength development of blended specimens. The negative values of k corresponding to 7 d curing for SF, FC and FF indicate significant dilution effect that slightly improves with decreasing w/b ratio. In contrast, substantial reduction in k-value with reduction in w/b ratio is noted for AL.

For 0.4 w/b ratio, the k-value for all supplementary materials except AL increases with curing period. The variability in k-value, evident at 7 d curing, decreases with increasing curing period to 90 d in the range of 1.8–2.7.

The effect of mineral composition is evident in terms of initial variability of k-value and the same is also evident in its evolution with curing period. However, at 0.35 w/b ratio, increase in k-value with curing age is evident only for FC and FF, whereas, SF and GGBS exhibit near constant k-values after 56 d curing. A drop in k-value is observed for AL at both w/b ratios for increase in curing period from 7 to 56 d. Thereafter, the value increases with more substantial improvement at w/b ratio of 0.35 relative to that of 0.4. Despite the variability in k-value for AL with w/b ratio and curing period, it exhibits higher reactivity amongst all mineral admixtures considered.

The k-values of 90 d cured specimens at w/b ratio of 0.4 and 0.35 lie in the range of 1.8–2.7 and 1.7–3.8, respectively indicating higher reactivity than cement. The increase in upper bound values and slight reduction in lower bound values suggest that the variability in reactivity of mineral admixture as function of w/b ratio and curing period is difficult to generalize. The higher reactivity value justifies utilization of mineral admixtures for development of high strength mixes. However, the variability in k-values highlights the importance of the right choice of mineral admixture for achieving the desired strength performance. The variability in k-values obtained for finer materials like AL and SF relative to those obtained for coarser materials like FF, FC and GGBS suggest that the dominant role is played by mineral composition and not by the specific surface area.

The k-value for the mineral admixture chosen can be a useful indicator in finalizing the binder composition and for design of high strength and ultra-high strength mixes. Though FF and FC contribute to higher k-value at 90 d curing period, the consistency in higher k-values for B-AL suggests its benefits in achieving early strength gain.

6.3 Assessment through strength estimation model

The strength estimation model as discussed in Eq. (2) is used to quantify the contribution of pozzolanic and filler effect in the strength development process of blended specimens. Post 56 d curing, control specimens attain plateau in strength development whereas blended specimens experience considerable strength gain beyond this period. Owing to this, the contribution of pozzolanic and filler effects in blended specimens are influenced by curing period with higher curing period indicating higher pozzolanicity. It can be observed from Tab.4 that with curing age, the pozzolanic parameter ‘a’ and dilution effect (indicated with negative value of ‘b’) increase. Further, B-FC followed by B-FF exhibit high pozzolanic contribution; however, accompanied dilution effect decreases the net gain in strength. In comparison, dilution due to SF is lower, whereas, AL exhibits negligible dilution. Higher pozzolanic activity accompanied with higher dilution contribution for B-FC and B-FF suggest that the strength gain will considerably exceed that of control and other blended specimens for sufficiently long periods of curing. For finer materials, SF with higher pozzolanic activity than AL makes a lower contribution to strength due to increased dilution effect. Further, similarity in chemical composition of AL and GGBS is evident in their pozzolanic contribution. However, the dilution effect reduces the net strength gain for B-GGBS as compared to B-AL.

For consistent performance, the contribution from both filler and pozzolanic action is significant and will dictate the overall strength gain of the blended specimen. The understanding that higher pozzolanic activity implies higher strength gain is true only if the effect on packing density effect is insignificant. However, in case of mortar as well as concrete, positive or negative contributions to packing density will dictate the strength at all curing period.

7 Assessment of stress−strain curve for control and blended specimens

The comparison of stress-strain curves shown in Fig.2 provides a qualitative assessment of the behaviours of control and blended specimens. In order to quantify elastic modulus, the secant modulus method with stress values ranging from 5 MPa to 40% of peak stress has been adopted. Apart from elastic modulus, other parameters of interest, namely peak stress and corresponding strain, are computed and listed in Tab.5. The average stress-strain response obtained at specified curing period is used to compute values of peak stress, corresponding strain and modulus of elasticity. The same values are further adopted to compute pre-peak toughness, post-peak toughness and Toughness Index.

It can be observed that, the peak stress and corresponding strain for control sample increase significantly with decreasing w/b ratio, while the increase is not that significant with increasing curing period. On the other hand, elastic modulus values decrease more significantly with increase in w/b ratio than curing period. For blended specimen at 0.4 w/b ratio, peak stresses at 28 and 56 d are lower than those of control specimens, whereas, they exceed that of control specimens for w/b ratio of 0.35. The 90 d peak strength of blended specimens is greater than control specimens at both w/b ratios. Amongst blended specimens, B-AL is an exception with strength higher than control specimens at all curing period and w/b ratio. Peak strain values for control and blended specimens increase with curing age for w/b ratio of 0.4, whereas, it indicates mixed trend for w/b ratio of 0.35.

The elastic modulus value for control specimens decreases with w/b ratio and curing period. For blended specimens, the value exhibits no clear trend with either w/b ratio or curing period. At maturity period, considered as 56 d for control specimens and 90 d for blended specimens, elastic modulus values for blended specimens are considerably lower than the control specimens for w/b ratio of 0.4, whereas, it is considerably higher for w/b ratio of 0.35. The control specimens exhibit decrement in elastic modulus with decreasing w/b ratio, whereas, the blended specimens demonstrate increment in elastic modulus with decreasing w/b ratio. Amongst blended specimens, B-SF and B-GGBS represent highest and lowest elastic modulus at w/b of 0.4, whereas, for w/b of 0.35, B-AL exhibits highest elastic modulus and B-GGBS exhibits lowest elastic modulus. Thus, unlike control specimens, the variation in properties of blended specimens doesn’t follow a clear trend with either w/b ratio or curing period. Understanding the trend in variation of these properties is critical for structural applications and may need further exploration. It is generally acknowledged that with decreasing w/b ratio, concrete tends to more brittle behaviour. The quantification of ductility or brittleness in response can be measured through the toughness index (TI) defined as the ratio of total toughness to pre-peak toughness. TI values for control and BM specimens are shown in Fig.6.

It can be observed that for 0.4 w/b ratio, TI values of control specimens decrease with curing period, whereas, the same increases with curing period for w/b ratio of 0.35. The transition could be due to the fact that with decreasing w/b ratio the process of hydration gets delayed. For blended specimens, the TI values exhibit strong dependence on the curing period and also the w/b ratio. For curing periods of 28 and 56 d, the transition in TI values is more substantial with w/b ratio of 0.35 than for 0.4. For 90 d curing period, TI values for blended specimens decrease with decreasing w/b ratio indicating reduction in relative ductility. Thus, the transition in TI values for blended specimens could be the result of fluctuations in the pozzolanic reaction. TI values at 90 d curing period indicate that relative to control specimens, blended specimens indicate brittle behaviour, which increases with decreasing w/b ratio.

The results reported in the present study suggest AL as an effective mineral admixture with potential to enhance both early and late strength gain. Further, the stress strain characteristics of AL blended specimens are superior to those of control specimens. The above observations are reported with BM specimens and generalization of concrete may not be straightforward. The variability in packing density contributions of supplementary materials for mortar and concrete may be different and would affect the conclusions related to the effect of these materials on properties of mortar or concrete. The conclusions from the present study are significant for development of high strength mortar and for quantifying the performance of mortar region in concrete specimens. The results may be extended for concrete only after performing necessary studies on blended concrete in future.

8 Conclusions

Based on the study, the following conclusions have been obtained.

1) The fresh properties of blended mix with AL are consistent with those of other supplementary materials like FA and GGBS, and thus can be used with reduced water consumption, higher workability and lower dosage of plasticizers. Further, for all curing periods and w/b ratios considered, strengths of B-AL specimens exceed those of control and other blended specimens. For w/b ratio of 0.4, the relative increase in strength of B-AL, corresponding to 7 and 90 d curing periods, is 20.8% and 16.9%, respectively, whereas, for w/b ratio of 0.35, the increase is 6.7% and 29%, respectively.

2) For mortar specimens considered in the study, reduction in 7 d strength of B-SF with reference to control specimen is 17% and 8%, for w/b of 0.4 and 0.35, respectively. In comparison, B-FC exhibits reduction of 13.6% and 12.6%, for w/b ratios of 0.4 and 0.35, respectively. Further, the rates of strength gain for B-FC and B-FF are higher than for B-SF. For instance, strength increases at 90 d for B-SF are 9% and 1%; whereas, strength increases for B-FC are 11.7% and 20.6%, for w/b ratios of 0.4 and 0.35, respectively.

3) The assessment based on k-value method suggests higher reactivity for mineral admixtures that increases with decreasing w/b ratio. For w/b of 0.4, the variation in k-value ranges from 2.95–3.5, whereas it ranges from 1.92–3.9 for w/b of 0.35. The consistently higher k-value for AL indicates its benefits for achieving high early strength without compromising on the time dependent strength gain.

4) The strength contribution through pozzolanic and filler action in blended specimens demonstrate that for the replacement percentages considered, AL exhibits negligible improvement in packing density (0.08) and has lower pozzolanic contribution (1.14). In comparison, FF and FC have higher pozzolanic contributions (1.43 and 1.59, respectively), but increase dilution effects (–27.42% for B-FF and –35.83% for B-FC) restrict strength gain due to addition of these supplementary materials. Further, similar pozzolanic contribution for AL and GGBS confirm their similarity in chemical composition. The resulting higher strength for B-AL is due to its reduced dilution effect.

5) The control specimens exhibit reduction in elastic modulus with curing period and w/b ratio. In comparison, blended specimens exhibit higher elastic modulus with decreasing w/b ratio, whereas, mixed trends are observed with curing period. For decreasing w/b ratio, the reduction in elastic modulus (56 d) of the control sample is 14.7%, whereas, the increase for B-AL (90 d) is 22.7%.

6) The comparison of toughness index (TI) values reveals that the reduction in w/b ratio has more influence on the ductilities of blended specimens, as compared to those of control specimens. The brittleness in control specimens increases with decreasing w/b ratio, whereas, the effect of curing period shows a mixed trend. In comparison to SF, AL blended specimens indicate higher ductility at 0.4 w/b ratio and lower ductility at 0.35 w/b ratio.

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