Lime-Stabilized Solid-Waste Blends as Alternative Building Blocks in Construction

Housing has been one of the very fundamental requirements of humans since the dawn of civilization. With advancements in engineering and technology, modern building materials have been developed to cater to the needs of larger, taller, longer and stronger constructions. Modern materials also have an impact on the cost of construction with recently developed materials being costlier than conventional materials in many instances. However, the need for housing still remains, especially for the economically weaker sections in developing and underdeveloped countries in Asia and Africa. The development of cheaper alternatives, hence, becomes a major prerogative of the government as well as the private sector for ensuring housing for the poorer sections of society. India has embarked upon an ambitious “Housing for All” programme by 2022. Such initiatives from the government, though appreciable, become ineffective when the cost of construction stays relatively high. This, in turn, puts hurdles in achieving housing targets due to budget constraints and availability of cost-efficient materials. Thus, the need for cost-efficient building materials still remains, especially in developing nations like India where there is still a huge requirement in terms of housing, especially in rural areas.

Valorisation of solid wastes in building materials seems to be an effective means of bringing down the cost of construction. This also achieves effective waste management in a country like India which is still aggressively pursuing development activities to put itself in the big league. Thus, the valorisation of solid wastes in building materials provides a dual advantage of lower cost and waste management [1]. Conventional clay bricks form the fundamental building unit in housing requirements for the economically weaker sections. However, the use of soil in burnt clay bricks is an energy intensive process and has a heavy environmental cost in terms of consumption of energy [2]. In fact, compressed stabilized earth blocks consume 4.9 and 15.1 times less energy compared to wire-cut bricks and country-fired bricks, respectively [3]. Traditional soil-based blocks have become more popular in recent times due to their economic attractiveness [4]. Cement is the most common binder adopted in soil stabilization [5], however, cement has a heavy carbon footprint due to CO₂ emissions [1]. Despite the heavy carbon footprint of cement, the pollution emission of compressed stabilized earth blocks is 2.4 and 7.9 times lower than wire cut and country-fired bricks, respectively [3]. Valorisation of solid wastes can be attempted at two levels in the development of construction blocks. They can be used partially as a supplementary additive to the conventional stabilizers in stabilizing soil for the manufacture of blocks [6,7,8,9,10,11,12,13,14]. The other way is to completely use solid wastes as the raw material for the matrix of the blocks, stabilized using conventional stabilizers [15,16,17,18,19]. The latter methodology can especially be adopted when there is an abundance of solid wastes being generated which needs to be effectively managed. Extensive research has been done at both levels of utilizing wastes in development of bricks and blocks. However, there still remains a need to identify alternative bricks due to (1) generation of different types and increasing quantities of solid wastes (2) rising demand for housing due to an increase in population. A significant number of earlier investigations have predominantly adopted cement for the stabilization process. This present investigation, however, attempts to altogether avoid the use of cement in stabilization to reduce polluting emissions. Thus, this investigation attempts to develop an alternative construction unit by using a combination of solid wastes viz. fly ash (FA), steel slag (SS) and phosphogypsum (PG) stabilized using lime.

The present investigation focused on the valorisation of solid wastes in the manufacture of construction units or blocks. In order to maximize the utilization of wastes, the complete matrix of the block was manufactured using only solid wastes without the use of any soil or clay. The solid wastes used include FA (class C), SS and PG stabilized using hydrated lime. FA is generated during the burning of coal for the generation of power. Class C type FA is generated when subbituminous and lignite coals are burned for power generation. SS a by-product of steel making, is produced during the separation of the molten steel from impurities in steel-making furnaces. PG is the calcium sulphate hydrate formed as a by-product of the production of fertilizer from phosphate rock. FA and PG were obtained from a local supplier of waste by-products whereas SS was obtained from Jindal Steel Limited in Salem, India. The SS used in the investigation was aged over a period of five years. High-quality hydrated lime procured from M/s. Shiyal Chemicals, Chennai was used for the stabilization of the different combinations of the solid wastes for the manufacture of the brick. Potable quality tap water was used to prepare the block mixes. The typical chemical composition of the solid wastes generated in India and the lime used for the stabilization of the solid wastes is given in Table 1. Figure 1 shows the various materials used in the investigation.

Figure 1.

The different waste materials were first sieved through a 75-micron sieve and the fine fractions passing through the sieve were used for preparing the block mixes. Following this, the three different solid wastes were taken in different proportions. The quantity of PG was fixed at 10% as too much PG is detrimental to the development of strength [24, 25].

The proportions of FA and SS were varied within the remaining 90% to obtain various mixes. Three combinations of FA:SS were considered viz. 30:60 (LFSPX12), 45:45 (LFSPX11) and 60:30 (LFSPX21). “LFSP” refers to the blend of lime, FA, SS and PG. The X indicates the variation in lime content whereas the numbers refer to the proportions of FA and SS. The three solid wastes were manually mixed in dry conditions thoroughly using trowels. To this blend, lime was then added in proportions of 2%, 4% and 6% by dry weight of the whole blend and mixed further using trowels in dry conditions manually. After obtaining a uniform dry mix, a sufficient quantity of water was added to obtain a cohesive wet mix. The water-to-blend ratio varied between 0.34 and 0.43 for the different mixes based on the quantity of water required for standard consistency. A similar methodology was also adopted earlier by Vijayaraghavan et al. [14]. This wet mix was then packed in a cube mould of dimensions 70.6 mm × 70.6 mm × 70.6 mm. The mix was placed in two layers and tamped using a tamping rod uniformly to ensure full compaction of the mix. The excess mix above the top of the mould was struck off and the top of the packing in the mould was finished to a smooth surface. ASTM [26] states that any curing period can be used for stabilized soil but commonly adopted curing periods include 7, 28 and 90 days. According to Uzoegbo [27], stabilized soil blocks must be cured in a humid environment for at least 7 days. Based on this, the test specimens were placed under damp sacks for a period of 24 hours following which they were demoulded and cured in water for a period of 7 days. In order to study the effectiveness of the combination as a potential mortar for the building of masonry walls, the same combinations were also cast in the form of mortar cubes. One part of the solid waste blend mixed with lime was mixed with three parts of well-graded river sand in dry conditions. Then, a sufficient quantity of water was added to obtain a uniform wet mix. The water-to-blend ratio varied between 0.30 and 0.37 for the different mortar mixes. This mix was then packed in cube moulds using the same procedure as mentioned above. They were moist cured for 24 hours under damp sacks and then immersed in water for 7 days for curing. At the end of the curing period, the stabilized solid waste blocks as well as the mortar blocks were tested for their compressive strength based on the procedure recommended by Bureau of Indian Standards (BIS) code [28]. Three cubes were cast for each combination, both stabilized blocks as well as mortar blocks and the average compressive strengths were reported. Figure 2 shows the methodology of the experimental investigation.

Figure 2.

Cement and lime are the most commonly adopted soil stabilizers. However, the utilization of cement increases the carbon footprint of the product. This investigation focuses on the performance of lime-stabilized solid waste blocks and mortars and attempts to compare it with the performance of lime-stabilized soil blocks from earlier investigations. Table 2 shows the proportions of the various solid wastes in the mixes adopted along with their compressive strengths. The proportions have been worked as a percentage of the total blend. The “M” in the combinations refer to mortar mixed with sand. The sum total of the mortar mixes shown in the table adds up to 25% as the remaining 75% is sand used to make the mortar.

4.1.

Compressive Strength of Stabilized Solid Waste Blocks

The compressive strength test on the stabilized solid blocks was done based on the procedure recommended by the BIS code. According to the BIS specifications [29], there are two classes of stabilized soil blocks (SSB), viz. class 20 with a minimum permissible strength of 1.96 MPa and class 30 with a minimum permissible strength of 2.94 MPa. BIS code [30] also recommends two classes of solid concrete blocks (SCB), viz. C4 and C5 with a minimum permissible strength of 4 and 5 MPa, respectively. Figure 3 shows the compressive strengths of the group LFSPX12. The strengths of the two classes of SSB and SCB have also been included for the purpose of comparison.

Figure 3.

From Figure 3, it can be seen that the strength of the LFSPX12 combination gives the maximum strength of 2.54 MPa for 2% lime stabilization. A further increase in lime content has resulted in lower strengths of 2.12 MPa and 1.05 MPa, respectively. Thus, 2% lime may be considered optimum for this blend. Comparing the strength of the stabilized blocks with standard blocks, it can be seen that two of the block types viz. LFSP212 and LFSP412 develop strength higher than a class 20 SSB. However, they do not attain the recommended strengths of the other categories of standard blocks. Figure 4 shows the compressive strengths of the group LFSPX11. It can be seen that the combination LFSP411 gives the maximum strength of 2.64 MPa when compared to the other two combinations. LFSP211 developed a strength of only 0.84 MPa whereas LFSP611 developed a strength of 2.18 MPa. Thus, it can be seen that 4% lime was the optimal dosage for this blend for the development of maximum strength. Similar to the earlier group, two of the three combinations have resulted in comparatively good strength. Compared to the standard blocks, LFSP411 and LFSP611 developed strength greater than the class 20 stabilized blocks. However, this combination is still inadequate when compared to the other standard blocks.

Figure 4.

Figure 5 shows the compressive strengths of blocks in the group LFSPX21. At the outset, it is clear that the block with the maximum strength is the one stabilized by 6% lime. This combination LFSP621 develops a strength of 4.06 MPa when compared to the strengths of 3.63 MPa and 3.94 MPa developed by LFSP211 and LFSP411, respectively. Compared to the earlier groups, this blend develops the maximum strength of the stabilized blocks. This may be due to the reduced proportion of SS in the mix. Sabapathy et al. [22] reported 25% SS as the optimum replacement dosage for coarse aggregate in concrete. In the present study, the blocks belonging to the group LFSPX21 have SS proportions close to 25%. When compared to the standard blocks, all three combinations within the group develop strength greater than both classes of SSBs. Thus, it can be stated that this combination of lime-stabilized solid blocks can act as an effective replacement for SSBs. Comparing the blocks with CSBs, it can be seen two of the three combinations, develop strength similar to C4 CSB. However, none of them develop-strength similar to that of the C5 CSB.

Figure 5.

Figure 6 shows the comparison of the optimal blend in each of the groups in comparison with the standard blocks. It can be seen that all three optimal blends can effectively replaced by class 20 SSB. However, LFSP621 can replace class 30 SSB and C4 CSB in terms of compressive strength requirement. But, it should be noted that as per BIS specification [30] for CSBs, the standard strength of the CSBs required by the code is after a curing period of 28 days whereas the strengths of the LFSP blocks reported in this investigation were after a curing period of 7 days. Thus, the provision of higher curing can reveal whether LFSP621 is capable of developing enough strength to replace C5 CSB. The BIS code [30] also reports the average minimum strength of the blocks in the specification based on the average of eight blocks. It reports the individual minimum strength of the blocks to be not less than 4.0 MPa for C5 CSB and 3.2 MPa for C4 CSB. Considering the individual strengths as well, the combinations LFSP621 comfortably meets the requirements. LFSP621 also meet the minimum strength requirement of 3.5 MPa of lime-based solid blocks and pulverized fuel ash-lime bricks as recommended by BIS codes [31, 32].

Figure 6.

4.1.1.

Influence of Mix Composition on Compressive Strength

The influence of the mix was studied by evaluating the variation in strength with respect to lime content for different FA/SS ratio. Figure 7 shows the variation of strength with lime content for different FA/SS ratios adopted in the investigation. It can be seen that for FA/SS ratio of 0.5, the maximum strength was attained when the lime content was 2%. Further increase in lime content resulted in a reduction in the strength of the block. The strength reduced from 2.54 MPa to 1.05 MPa when lime content increased from 2% to 6%. The reduction in strength beyond a particular lime dosage has been reported earlier by researchers in the case of lime-stabilized soil [33,34,35]. Jambor [36] states that excessive lime may increase the porosity of the lime-stabilized composite leading to a reduction in strength. Kumar et al. [37] state that lime content higher than optimum can result in unreacted platy lime particles being present which may also be responsible for lower strength. For FA/SS ratio of 1, the strength of the block increased with an increase in lime content to 4% and reduced thereafter. The strength increased from 0.84 MPa to 2.64 MPa for lime content increased from 2% to 4%. Further increase in lime content to 6% reduced the strength to 2.18 MPa. Compared to the FA/SS ratio of 0.5, the maximum strength of the stabilized block is obtained at 4% lime stabilization when the ratio increases to 1. This may be because of the increase in content of FA and reduction in SS content. FA used is also rich in pozzolanic materials and hence, an increase in FA content would have necessitated higher lime content for gain in strength. However, the strength of the block stabilized with 2% lime was unusually low. This may be due to the lime content being insufficient to meet the pozzolanic requirements of increased FA content. When FA/SS ratio increases to 2, the strength of the block increased with an increase in lime content and the maximum strength of 4.06 MPa was obtained when the lime content was 6%. The trend of higher lime requirement with increasing FA content is maintained in this ratio as well. This group has the maximum proportion of FA in its blending and hence, the lime requirement for achieving higher strength increases to 6%. A similar increase in lime requirement with the increase in FA content was earlier reported by James et al. [25] but in contrast to the work reported by Shen et al. [38]. They reported the maximum strength was obtained when the FA/SS ratio was close to 1 whereas in the present study, the maximum strength was obtained when the ratio was 2. However, it needs to be noted that in the study conducted by Shen et al. [38] there was no lime involved in the mix.

Figure 7.

One additional point to be noted is the influence of the specific gravity of the components on the weight of the blocks. The increase in the weight of the blocks will influence the weight of the masonry resulting in greater seismic load in earthquake-prone regions. The specific gravity of lime is in the range of 2.3 to 2.4 [39]. The ranges of specific gravity for other components PG, FA and SS are 2.3–2.6 [20], 2.2–2.8 [40] and 3.2–3.6 [41], respectively. It is clear that SS has the highest range of specific gravity of all materials and definitely contributes to the increase in weight of the block which can cause an increase in the seismic load on masonry walls constructed using such blocks. However, the results of the present investigation indicate that blocks with FA/SS ratio of 2 alone generate a strength of more than 3.5 MPa; it is the minimum strength recommended for use in earthquake-prone regions according to BIS Code [42]. Thus, in combinations with an FA/SS ratio of 2, the quantity of SS is restricted to less than 30% and it also meets the minimum strength requirements for earthquake-resistant construction material.

To gauge the performance of the lime-stabilized solid waste blocks with respect to similar stabilized blocks, a comparison of the strengths of the present study and a few previous studies was analysed. The criteria for the selection of the previous studies were 1. the use of lime as the stabilizer, 2. the use of secondary additives like FA, SS and PG in the mix 3. blocks made of soil or blocks made of solid wastes and 4. investigations made in the past 5 years. Based on the above criteria, the exact combination of the lime-FA-SS-PG composite was virtually not present in the literature. Investigations that used combinations of one or more solid wastes were hence considered for comparison. This led to the following investigations being considered for the comparison. The investigation was done by Mashifana et al. [43], James et al. [25], Pai et al. [44]. Table 3 shows the proportions of the various materials in the blend adopted by different investigators.

Table 3.

Proportions of Various Solid Wastes in the Present and Three Earlier Studies (%)

Study	Notation	Lime	FA	SS	PG	MgO Content (SS)
Present Study	LFSP411	3.85	43.27	43.27	9.62	7.46
	LFSP621	5.66	56.60	28.30	9.43
Mashifana et al. [43]	LFSP1212*	23.33	46.67	10	20	3.53
	LFSP1213*	20	40	10	30
James et al. [25]	LFP11P	3.85	48.08	-	48.08	-
	LFP31P	4.76	71.43	-	23.81
Pai et al. [44]	LFS1314*	5.5	16.5	78	-	0.23
	LFS1316*	5.0	15.0	80	-

*

Named for the purpose of this comparison

Mashifana et al. [43] attempted to develop the composite for use in buildings and roads whereas Pai et al. [44] investigated the use of the composite as road base material. James et al. [25] on the other hand, attempted the use of the composite as a mortar for masonry construction. However, there were some differences in the three studies selected for the comparison despite their similarities to the present study. Only one of the studies viz. the work done by Mashifana et al. [43] used all the materials that were also a part of the blend in the present study. James et al. [25] did not use SS whereas Pai et al. [44] did not use PG in their investigations. Figure 8 shows the compressive strengths of the blocks tested in the studies chosen in comparison with the present work. The two best combinations from all the studies have been chosen when a number of combinations tested was more than two. For the purpose of the discussion, the studies carried out by Mashifana et al. [43], James et al. [25] and Pai et al. [44] have been designated as studies 1, 2 and 3, respectively.

Figure 8.

From the figure, it is evident that the strength results from the present study were lower when compared to the results from earlier studies. The common material in all the studies is lime and FA. Study 2 has PG as an additional material whereas study 3 has SS as additional material. The lime contents adopted in studies 2 and 3 were comparable to that of the present study. The strength of the stabilized composites in study 2 were 7.11 MPa and 6.64 MPa for LFP11P and LFP31P, respectively whereas the composite mixes in study 3 developed strengths of 5.26 MPa and 5.15 MPa for LFS1314 and LFS1316, respectively. Thus, it is clear that the addition of PG to the lime-FA mix has developed good strength of the stabilized composite mix. Addition of PG to lime stabilized soils hastens the pozzolanic reactions resulting in quicker strength gain [45, 46]. The addition of PG to FA-lime systems results in the acceleration of the pozzolanic reactions [24, 38, 47]. Similarly, in study 3, the introduction of SS in the lime-FA mix also developed good strength. Despite using significantly higher proportions of SS in their mix, the composites in study 3 were capable of developing reasonably good strengths. This may be explained based on the chemical composition of the SS used in study 3. It had a good proportion of silica and alumina and a very low proportion of magnesia (MgO). This magnesia has a slow reaction with water [48], thus leading to a delayed hydration. The very low magnesia content in the SS used in study 3 may have been very beneficial to their mix and hence, was capable of developing good strengths. Clearly, both studies reported good strengths in the absence of one of the materials. In the present study as well as study 1, both PG and SS are introduced in the mix along with lime and FA.

The introduction of both PG and SS in the blend has resulted in the modification of the development of the strength of the stabilized composite. LFSP1212 combination as reported in study 1 developed the highest strength of 7.4 MPa. However, it should be noted that they used a higher lime content of 23.33%. Moreover, their SS content was limited to just 10%. Thus, the stabilized composite in study 1 developed higher strength comparable to those of studies 2 and 3 but it required a higher lime content to achieve that. Thus, it is evident that the utilization of the PG and SS together in the blend can alter the strength development of the lime-FA composite. The development of lower strength of the stabilized blocks in the present study can also be attributed to the higher proportion of SS in the present study with a comparatively higher presence of magnesia in its chemical composition. Lower compressive strengths of cementitious mortars [49] and concretes [48] have been reported in the literature. However, the slowdown or reduction in strength development can be countered by increasing the lime content in the mix to develop higher strength of the stabilized composite. Clearly, more future investigations are necessary to optimize the content of lime-FA-PG-SS composite from the point of view of valorization of solid wastes in the development of alternative stabilized composites for use in civil engineering applications.

4.2.

Compressive Strength of Stabilized Solid Waste Mortar

An attempt was also made to determine the performance of lime-stabilized solid waste as a mortar for use in masonry construction. To achieve this, the part of each blend was mixed with three parts of well graded sand to prepare the mortar for the investigation. Figure 9 shows the strength of lime-stabilized solid waste mortar blocks. It is very obvious that the trend in the strength of the mortar blocks is unlike the case of stabilized solid blocks. Additionally, the strengths of the mortars are significantly lesser than those developed without sand in the mix. The introduction of sand in the mix has significantly modified both the physical as well as chemical interaction between the constituents of the mix. The mortar combinations LFSP411M and LFSP621M developed strength significantly higher than the other combinations. Both the mortars developed strength greater than 1 MPa with the former developing 1.05 MPa and the latter developing 1.18 MPa. This meets the specification of mortar mix (MM) 0.7 as per BIS code with strengths in the range of 0.7 MPa to 1.5 MPa [50]. However, they could not achieve the minimum strength of 2.5 MPa recommended for cement mortar as per BIS [51]. James et al. [25] reported a very good strength of nearly 14 MPa for a solid waste mortar developed using a combination of lime-FA-PG composite. Clearly, the introduction of sand in the mix has resulted in reduced strength of the mortar mix. This was in contrast to the results obtained by James et al. [25]. In their study, the introduction of sand was beneficial for further improvement in the strength of the stabilized binder composite. As discussed in the previous section, the presence of SS with high magnesia content in the present study was responsible for the reduced strength of the mix. Moreover, the pozzolanic acceleration achieved by PG is also reduced due to PG content being limited to 10%. The introduction of sand in the mix lowers the SS content in the mortar mix but at the same time, it also reduces the lime content required for the stabilization of the mix and the beneficial acceleration effect of PG. Moreover, due to the introduction of sand with larger particle sizes, the water content for achieving the required consistency reduces. However, the water content may not have been enough for the hydration and formation of hardening compounds in the mortar mix. Thus, the reduced lime content coupled with the delayed hydration effect of magnesia from the SS, the limited acceleration of pozzolanic reactions due to lower PG content and the reduced water content of the mix can be plausible reasons for the mortar mix not developing good strength. However, the veracity of the plausible mechanism for lower strength needs to be further investigated in future investigations coupled with mineralogical and microstructural evaluations.

Figure 9.

The experimental investigation revolved around the development of alternative building blocks made up of stabilized solid wastes. A combination of three solid wastes viz. FA, SS and PG were stabilized using lime to form the blocks and their compressive strengths were evaluated. The potential of the mix to function as a mortar in place of lime and cement mortars was also evaluated by blending the mix with sand to prepare mortar cubes. The results of the tests conducted on lime-stabilized solid waste blocks and mortars revealed the following conclusions.

The present study limited the investigation to only the strength of the stabilized solid waste blocks. Future investigations can focus on the mineralogy and microstructure of the blocks to reveal the mechanism responsible for strength gain. Moreover, the durability aspect of the stabilized solid waste blocks can be investigated to reveal their performance over extended durations under varying conditions.