1. Introduction

Cement being the most important binder of all construction materials has immense demand in civil engineering application. The highly reactive nature of cement with water makes it the best suitable option as a binder. The production process of cement involves chemical conversion of limestone into lime which emits CO2 as byproduct (Gibbs et al., 2000). This CO2 produced in cement industry is considered as major greenhouse gas (GHG), which is responsible for global warming (Zhang et al., 2012). The role of CO2 in global warming is well accounted in latest Intergovernmental Panel on Climate Change (IPCC), special report on global warming of 1.5 °C (Masson-Delmotte et al., 2018). Benhelal et al. (2013) and Andres et al. (2012) reported that 1 ton production of cement contains 900 kg of CO2 which accounts for 5–7% of global carbon emission, which is directly responsible for global warming. Therefore, in order to reduce the carbon emission, the world has come together and signed the Bali agreement and the Kyoto protocol under the United Nation Framework Convention on Climate Change (UNFCCC) (Skutsch and Trines, 2008) and (Heitmann and Khalilian, 2011). In this agreement, member countries are intended to develop substitute mechanism of production in response to counter carbon emission. Therefore, in construction industry, to limit carbon emission, many materials having binding properties are used as a replacement to cement.

The selection of material while replacing cement requires fundamental understanding of cement hydration. Mehta and Monteiro, (2017) and Neville, (1995), discussed the basic hydration chemistry of cement which states that cement compounds which reacts with water to produce strength contributing compounds are tri Calcium silicate (C3S), di Calcium silicate (C2S), tri Calcium aluminate (C3A), and tetra Calcium alumino-ferrite (C4AF). The initial hydration occurs in C3A and C4AF to produce calcium trisulphoaluminate hydrate (ettringite) in presence of sulphate (primarily in gypsum), to provide initial setting to mixture; however, once the sulphate content depletes, ettringite converts into calcium monosulphoaluminate hydrate. On later C3S and C2S hydrates to form calcium silicate hydrate (C–S–H) gel, which provides later stage strength. Therefore, the key ingredients in cement are oxides of calcium, silica, and alumina with gypsum. Hence, in same literatures, authors suggested the use of natural pozzolans and cementitious materials as replacement to cement. These pozzolans are natural occurring materials that have binding properties, and cementitious materials are generally by-products of different industries, having rich content of aluminosilicate.

The most widely recommended industrial by-product having cementitious materials property is blast furnace slag due to its high content of aluminosilicate (Ozbay et al., 2016) and (Shi and Qian, 2000). Ogawa et al. (2012) and Ulubeyli and Artir, (2015), performed several experiments to investigate the optimum replacement percentage of furnace slag with cement which vary from 20% to 40%. The optimum replacement percentage is that at which considerable amount of strength is obtained. Meanwhile, Ofuyatan et al. (2020) and Dinakar et al. (2013) preferred using furnace slag with other mineral admixtures like they developed self-compacting concrete and high-performance concrete using blast furnace slag in conjunction with different mineral admixtures like eggshell and metakaolin. They observed that utilization of blast furnace slag can be incorporated in the production concrete with improved durability properties like lesser shrinkage, cracking resistance, less water penetration depth, and freezing resistance. Nicula et al. (2020) investigated the resistance of concrete prepared using slag powder as a replacement to cement and artificial crushed blast furnace slag replacement to natural aggregate against freezing and thawing action. They observed that mixture containing 15% slag powder and 20% artificial crushed blast furnace slag performed better compared to specimens prepared using other conventional materials. Further, Sadawy and Nooman, (2020) used nano blast furnace slag as replacement to cement in different percentages and noticed that very dense microstructure was observed and significant amount of resistance was offered against corrosion. Palod et al. (2020) used blast furnace slag with steel slag to produce ternary cement in which comparable compressive strength was observed in later days. Even blast furnace slag has been used as a replacement to cement in the production of sprayed concrete/shotcrete, leading to improved mechanical strength (Salvador et al., 2019).

Siddique, (2011) studied the use of industrial by-product silica fume as replacement to cement due to its higher content of silica. Suda and Rao, (2020) recommended GGBS, Madani et al. (2018) recommended pumice, Meddah et al. (2018) recommended metakaolin and Mustapha et al. (2020) recommended fly ash as a mineral admixture with silica fume to enhance the strength. Tripathi et al. (2020) assessed the durability of concrete prepared using silica fume in acidic environment of nitric acid. They observed that resistance against aggressive environment was significantly improved. However, Tam and Tam, (2008) adopted the strategic use of silica fumes in strengthening the properties of recycled coarse aggregate concrete by coating the gaps and pores of adhered mortar on surface of recycled aggregate. Okoye et al. (2017) and Daniel et al. (2017) observed significant improve in engineering properties of geopolymer concrete prepared in conjunction with silica fume against sulphuric acid, chloride solution and torsional load. Some researchers assessed the durability properties of mortar prepared using silica fumes like water absorption (Nayana and Rakesh, 2018), sorptivity test (Zhang et al., 2016) and sulphate attack test (Chakraborty and Dutta, 2001), where it performed remarkably well.

Scrivener et al. (2018) proposed a revolutionary method in cement production field in which they recommended the use of clinker, limestone, clacined clay, and gypsum in specified proportion to produce new cement referred as Limestone calcined clay cement (LC3). The basic ingredients of cements like calcium are obtained from limestone and aluminasilicates from calcined clay. Clay chosen for calcination is natural mineral kaolinite, since it requires lesser temperature for calcination compared to Portland cement hence emitting lesser carbon. Dhandapani et al. (2018) observed the improved mechanical and durability properties of concrete prepared using LC3 cement attributing to its dense and strong microstructure. Even, Pillai et al. (2019) performed the life cycle assessment of reinforced concrete using LC3 and its equivalent reduced carbon footprint compared to specimen prepared using OPC.

Jia et al. (2019) and Li et al. (2014), recommended the use of chemically bonded phosphate magnesium phosphate cement (MPC) for rapid construction and repair work which got its tremendous demand in cold surroundings temperature. It requires Magnesium oxide which can be obtained by calcination of natural mineral magnesite (Arora et al., 2019). Researchers observed that because of its high chemical reactivity with silica fume, it delays the initial setting of magnesium phosphate cement and improves the strength as well (Ahmad and Chen, 2018) and (Ahmad and Chen, 2020). Ma and Chen, (2017) and Li et al. (2019), recommended the production of foamed concrete using magnesium phosphate cement. They utilized the unique advantage of rapid hardening potential of MPC for quick setting and early high strength by using foaming agent as sodium bicarbonate and hydrogen peroxide. Feng et al. (2020) and Fang et al. (2018), also assessed the novel use of MPC by using different fibers like steel fiber and glass fiber. Feng et al. (2020) observed that using 10% of silica fume in MPC greatly improved the bond and interfacial transition zone (ITZ) between steel fiber and MPC resulting into improved strength. Fang et al. (2018) established the use of glass fiber in MPC mortar which significantly improved the resistance against water penetration. Similarly, Ahmad and Chen, (2020) proposed the use of basalt fiber in MPC mortar which established that using basalt fiber in conjunction with silica fumes in MPC could be used against water resistance and high temperature resistance in mortar.

Similarly in reducing the carbon emission from cement production industry, researchers also recommended Calcium sulphoaluminate (CSA) cement which is also adopted in many researches because of its early strength attainment quality (Pera and Ambroise, 2004). It has explicit use in cold environment against frost attack because of its rapid initial strength (Li et al., 2020). Huang et al. (2019) cured cement mortar prepared using CSA in frozen sand to mimic the influence of permafrost. They observed that specimen responded well in frozen environment and attained rapid strength in between −5 °C to −10 °C. Ingaglio et al. (2019) and Chen et al. (2020) used CSA cement with sand as a binder in producing 3D printed samples having adequate mechanical strength with desired geometries. Winnefeld et al. (2017) and Wu et al. (2020), investigated the role of gypsum obtained from different sources in CSA cement and observed that by increasing the content of gypsum in a specific proportion, the initial strength of specimen gained significantly.

With the aim of reducing carbon emission during cement production, many agricultural wastes like baggase ash, rice husk ash, palm oil fuel ash serve as prime constituent in replacing cement because of its rich silica content (Loganayagan et al., 2020) and (Hu et al., 2020). Utilization of sugar cane bagasse ash in production of sustainable concrete is most appreciated technique in country like India and Brazil, due to its largest production of sugarcane (Yogitha et al., 2020). Bagasse ash use in construction industry is not only limited to cement replacement but also to sand replacement (Almeida et al., 2015). The silica content in bagasse ash is obtained by higher degree calcination of sugarcane waste which is done during its extraction process. Though initially, it is not cementitious in nature but once it comes in contact with alkaline solution like calcium hydroxide and water, it can be used as cementitious compounds (Yadav et al., 2020). Rajasekar et al. (2018) and Gar et al. (2017) investigated the durability properties of concrete prepared using bagasse ash and it was observed that its performance in resistance against chloride penetration, water absorption, and elevated atmosphere temperature improved significantly. Le et al. (2018) and Pereira et al. (2015) stated that potential of bagasse ash as binder is increased when it is used with furnace slag in the mixture. Their studies ensured the improved mechanical strength in longer days of curing and also better resistance against sulphate attack. Larissa et al. (2020) recommended the use of bagasse ash in conjunction with metakaolin as replacement to cement. They observed that using metakaolin with bagasse ash improved the deterioration of concrete at high temperature curing without affecting its workability.

This paper attempts to summarize and review the available literature on the sustainable application of different industrial by-products like blast furnace slag and silica fume. Moreover, the detailed study and micro-analysis is done for the natural minerals like kaolinite, magnesite, ye'elimite, gehlenite and belite for the production of sustainable cements like Calcined clay limestone cements, Magnesium phosphate cement and Calcium sulphoaluminate cement. Further, the production of sustainable cement from agricultural waste sugarcane ash is discussed and reviewed. Also, the detailed comparative analyses of their engineering properties are performed. Besides, this to understand better the micro-structure development of sustainable concrete, a detailed explanation of cement hydration established from published results is presented.

2. Cement hydration reaction at microstructural level

2.1. Ettringite formation

Quennoz and Scrivener, (2012) showed the initial stage of hydration of OPC in presence of sulphate (present in cement in the form of gypsum (CaSO4·2H2O) or in the locally available soil or in water) reacts with C3A to form a needle shaped prismatic crystals of calcium trisulfoaluminate hydrate (Aft phase), called Ettringite shown in Fig. 1(a).${\mathrm{C}}_{\mathrm{3}}\mathrm{A}\mathrm{C}\bar{S}{\mathrm{H}}_{\mathrm{18}}+2\mathrm{C}\mathrm{H}+\mathrm{2}\bar{S}+12\mathrm{H}\to {\mathrm{C}}_{\mathrm{3}}\mathrm{A}3\mathrm{C}\bar{S}{\mathrm{H}}_{\mathrm{32}}(\mathrm{A}\mathrm{f}\mathrm{t}\mathrm{p}\mathrm{h}\mathrm{a}\mathrm{s}\mathrm{e})$

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Fig. 1

In normal OPC, the gypsum content is 5–6%, which helps in formation of Ettringite which leads to stiffening, setting, and early strength development in concrete. Later, depending on the alumina/sulphate ratio of the OPC, once when the sulphate content starts depleting, the aluminate ion in the solution increases due to further hydration of C3A and C4AF, which makes ettringite unstable and it decompose to form calcium monosulfoaluminate hydrate (AFm phase) C4A$\bar{S}$H18 (Mehta and Monteiro, 2017).$2{\mathrm{C}}_3\mathrm{A}+{\mathrm{C}}_{\mathrm{6}}\mathrm{A}\bar{S}3{\mathrm{H}}_{\mathrm{32}}\to {\mathrm{C}}_{\mathrm{4}}\mathrm{A}\bar{S}{\mathrm{H}}_{\mathrm{18}}(\mathrm{A}\mathrm{f}\mathrm{m}\mathrm{p}\mathrm{h}\mathrm{a}\mathrm{s}\mathrm{e})$

Calcium monosulfoaluminate hydrate is having the hexagonal plate morphology which is the characteristics of strength imparting compound Calcium aluminosilicate hydrate (C-A-S-H) (Mehta and Monteiro, 2017). Scrivener, (2004) noticed that formation of Ettringite is possible in old paste also but its individual crystals are difficult to identify since it is densely intermixed with C–S–H gel. This similar property is observed in elevated temperature curing leading to delayed formation of ettringite.

2.2. C–S–H

(Mehta and Monteiro, 2017) highlighted that Calcium Silicate Hydrate (C–S–H) is principal binding compound formed during hydration of tri Calcium Silicate (C3S) and di Calcium Silicate (C2S) by giving Calcium Hydroxide (CH) as by product. It consists of 50–60% of the total volume of hydrated cement paste responsible for long term strength of concrete. Sometimes it is also referred as tobermorite gel because of its resemblance with natural mineral tobermorite. Richardson, (2004) showed that C–S–H product is classified into two types: the outer product (Op) and inner product (Ip) shown in Fig. 2. The unhydrated cement clinkers form Ip, whereas original water filled space form Op. The Op formed in paste is fibrillar in nature whose density depends on no. of curing days. The pore space is also responsible for finer and coarser formation of fibrils. Minimal pore space forms finer fibrils, whereas larger pore space forms coarse fibrils.2C3S + 6H → C3S2H3 + 3CH2C2S + 4H → C3S2H3 + CH (Mehta and Monteiro, 2017).

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Fig. 2

3. Blast furnace slag blended cement

In Cho et al. (2017) research, blast furnace slag blended cement is prepared with OPC and GGBFS in 1:1 ratio. Further, GGBFS content is varied by different percentages of Aluminum rich slag (ARS) and gypsum to investigate its microstructural change. Aluminum rich slag is obtained as a byproduct from steel manufacturing industry which primarily contains oxides of Calcium and Aluminum. The compressive strength of the mortar obtained is highest in case of GGBFS blended OPC (Plain O50B50) i.e., blast furnace slag blended cement, next mortar containing 67.5% ARS and 22.5% gypsum (O50B30AS15G5), and after that 45% ARS and 45% gypsum (O50B30AS10G10). However, they conducted Scattered Electron Microscopic (SEM) test for two samples only, containing 90% gypsum (O50B3020), and 45% of ARS and gypsum each (O50B30AS10G10). In Fig. 3 (a), sample O50B3020 is observed under SEM. The blending of OPC with GGBFS provides sufficient amount of aluminosilicates in mixture, and an additional amount of gypsum present reacts to produce ettringite in hydration process which is responsible for initial setting and strength. But once the content of alumina depletes, ettringite becomes weak and small. It can be observed in picture that ettringites are small and rare. But in Fig. 3 (b), sample O50B30AS10G10, ARS, and gypsum are present in equal proportion; hence, the formation of needle shaped prismatic crystal of calcium trisulfoaluminate hydrate (Aft phase) are dense and it fills the gap between C–S–H and C-A-H hydrates. However, slower decomposition of sulphate in gypsum results into formation of calcium monosulphoaluminate (Afm phase) which provides strength to the mortar.

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Fig. 3

4. Silica fume blended cement

Rossen et al. (2015) adopted 10, 20, 25, and 45 percentage of silica fume in OPC to observe change in C–S–H growth. CH formed during hydration of C3S and C2S precipitated around the cement clinker as CH rim and gets consumed during pozzolanic reaction with silica fume shown in Fig. 4. They also observed that cement hydration kinetics was accelerated at elevated temperature resulting into speedy formation of C–S–H. In Fig. 4 (a) and (b) samples shows hydrated product of OPC in the form of C–S–H and CH. Since there wasn't enough silica present, to react with CH, to produce more C–S–H, hence unreacted CH presence is observed in 4 (a) and a rim of CH over unhydrated clinker is seen in 4 (b). In Fig. 4 (c), 25% of OPC is replaced by silica fume, which provided extra silica content to mix. But the replaced amount of silica is not sufficient enough to completely dissolve the obtained CH, hence in SEM images, very clearly CH and a rim of CH around unhydrated clinker are visible. Lastly, in Fig. 4 (d), 45% replaced sample have enough silica for complete reaction of CH, hence a strong and dense C–S–H is observed in the sample with no clear presence of CH.

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Fig. 4

4.1. Engineering properties

Koksal et al. (2008) used different percentage of steel fibers in conjunction with silica fumes by replacing cement in the mixture. Moreover, before adding steel fibers, they investigated the effect of silica fumes by varying its percentage from 0, 5, 10, and 15%. They noticed that specimen with 15% of silica fumes achieved highest strength. Verma et al. (2020) replaced the fine aggregate with stone dust, and cement with silica fume. The fractionally replaced cement with silica fume was 0, 5, 10, and 15%. They observed that sample containing 10% silica fumes and 30% stone dust, achieved highest compressive strength after 28 days of curing. The strength of specimen containing 30% stone dust gained compressive strength by 2.32%, but when silica fume with 10% replacement was used in conjunction with stone dust, the gain in compressive strength recorded was 6.83%. The gain in strength is attributed to stone dust which provided a well graded denser matrix paste and high content of silica provided denser formation of C–S–H gel in the paste. Wong et al. (2005) investigated the strength of concrete prepared using calcined kaolin (metakaolin) and silica fume at different water binder ratio individually. The replacement percentage was 0, 5, 10, and 15% for both the admixtures. In the mixture, water binder ratio adopted was 0.27, 0.30, and 0.33. Initially it was observed that early stage strength of silica fume induced concrete was slow as compared to metakaolin specimen. This is because of lesser content of aluminate in silica fumes. But after 28 days of curing, it was observed that strength of silica fumes specimen was higher than metakaolin specimen, and it was highest at 15% replacement level among total fraction of silica fume. This strength development is in agreement with the fact that silica fume contains higher silica content which produces C–S–H gel, which provides later strength to the concrete. Roastami and Behfarnia, (2017) examined the response of silica fumes in alkali activated slag with different percentages like 0, 5, 10, and 15%. The slag used in the investigation was activated using sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). It was observed that with the increasing percentage replacement with silica fume, the compressive strength of the concrete was increased after 28 days of curing. Moreover, the use of silica fume also prevented the flow of charge in rapid chloride penetration test, hence ensuring its better durability properties. The total development in strength was 28% higher in 15% replaced sample. This enhanced mechanical and durability property is in agreement with the formation of denser C–S–H gel, which induced strength in the mixture. The comparative compressive strength from different literature is shown in Fig. 5.

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Fig. 5

5. Calcined clay limestone cements (LC3)

Moreover, its Alumina content further reacts with limestone in presence of gypsum, to form calcium trisulfoaulminate hydrate/ettringite (Aft phase) and later converts into monosulphoaluminate hydrate (Afm phase), to strengthen the mechanical and durability properties. In Avet et al. (2019) C-A-S-H morphology is compared at microstructural level between Portland cement (PC) and LC3. In LC3 sample, content of kaolinite in calcined clay is varied at 17.0%, 50.3%, and 95.0%. In Fig. 6, fibrillar C-A-S-H morphology is shown in Bright Field (BF) mode and in High Angle Annular Dark Field (HAADF) mode. It is observed that C-A-S-H is clearly visible in PC as well as in LC3-50 (17%).

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Fig. 6

This clearly highlights that formation of strength compound like C-A-S-H is not diminished using calcined clay containing lesser content of kaolinite at 17%. Moreover, needle shaped crystal of calcium trisulfoaluminate hydrate/ettringite (Aft) is also visible in LC3 sample. Presence of Aft phase at 28 days of curing denotes that alumina and gypsum content is still not depleted. Similarly, in Fig. 7, both the samples of LC3-50 (50.3%) and LC3 -50 (95.0%) are shown in which clear fibrillar C-A-S-H morphology is visible, which validates the reaction of calcined clay and the different content of kaolinite in calcined clay doesn't affect the C-A-S-H morphology, and at high content of kaolinite in calcined clay, also show unreacted metakaolin. This shows the presence of aluminosilicates in mixture which is responsible for the further production of C-A-S-H and C-A-H. Hence, the presences of strength compounds are ensured in LC3 cement.

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Fig. 7

5.1. Engineering properties

Bishnoi et al. (2014) investigated the mechanical property of LC3 by comparing the compressive strength test results of different samples. LC3 specimens were prepared from two different types of clay in which Clay1 was containing 70–80% kaolinite content, and Clay2 was containing 20–30% kaolinite content. The overall composition of LC3 was 50% clinker, 5% gypsum, 15% limestone, and 30% calcined clay. Further, the limestone used in LC3 was of two types, having slight higher content of silica and carbonate in the later one. Finally the samples were prepared, designated as LC3A, LC3B, LC3C, and LC3D in which clay 1 was mixed with both the types of limestone to produce LC3A and LC3B, and clay 2 was mixed with both the types of limestone to produce LC3C and LC3D. For control mix, Portland cement (OPC) was used, prepared using the same clinker used in production of LC3 cement. Moreover, one more sample was prepared using same clinker with locally available fly ash cement as Portland pozzolanic cement (PPC). In Fig. 8, the compressive strength results of all the samples are shown together.

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Fig. 8

It is observed in Fig. 8 (a), that higher strength is achieved by LC3B mortar sample after 28 days of curing compared to OPC specimen and in, Fig. 8 (b), LC3A concrete sample attained highest strength after 28 days of curing. The point to be noted here is that both the samples contain Clay 1 which contains higher content of kaolinite. Hence, the kaolinite content in clay plays major role in gaining strength. The developments of LC3 cement is also getting attention in developing countries like Cuba and India where it is being running as pilot project (Berriel et al., 2016) and (Emmanuel et al., 2016).

6. Magnesium phosphate cement (MPC)

MPC is used very frequently for its niche applications like cold weather repair material because of its rapid hardening property (Le Rouzic et al., 2017). It composes of Dead Burnt Magnesite (DBM) MgCO3 which is calcined at 1500 °C to obtain Magnesia (MgO), commercially available crystalline Mono-Ammonium Phosphate (NH4H2PO4), and retarders like Sodium Tri-Polyphosphate (Na5P3OI0), and Di-Sodium-Tetraborate (Borax) (Na2B407.10H20) (Seehra et al., 1993). The following reaction occurs during its hydration (Miyaji et al., 1982)MgO + 2NH4H2PO4 + 3H2O → Mg(NH4)2(HPO4)2. 4H20Mg(NH4)2(HPO4)2. 4H2O + MgO + 7H20 → 2MgNH4PO4. 6H20

The product obtained in above reaction 2MgNH4PO4. 6H20 is referred as mineral Struvite which is main binding material in MPC (Allan and Asgar, 1966). In order to address the issue of chemical stability of Struvite (2MgNH4PO4. 6H20), Rouzic et al. (2017) used Potassium di hydrogen phosphate in place of Ammonium di hydrogen phosphate leading to formation of k-struvite (MgKPO4. 6H2O) which acts as main binding material in cement.MgO + KH2PO4 + 5H2O → MgKPO4. 6H2O

Boisettle et al. (1983) paid emphasis on pH of solution, at lower pH, struvite precipitate to form newberyite (MgHPO4·3H2O) which may tend to reduce the strength. Hence, Rouzic et al. (2017) recommended magnesia to phosphate molar ratio (Mg/P) of 1 (lesser than 5) and water to binder weight ratio of 0.2 to avoid the precipitation from struvite into newberyite shown in Fig. 9.

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Fig. 9

Liu et al. (2020) fractionally replaced the Iron oxide (Fe2O3) with 0%, 5%, 10%, 20%, and 30% of MgO and named it magnesium-iron phosphate cement. A detailed microstructure analysis was conducted in SEM which is shown in Fig. 10. Specimen containing 20% of iron oxide showed the highest strength among other specimens because of lesser micro-cracks and micropores. Moreover, presence of iron oxide powder accelerated the formation of strength imparting compound struvite, leading to development of denser microstructure.

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Fig. 10

Qin et al. (2020) used silica fumes in magnesium phosphate cement referred as Magnesium silicon phosphate cement (MSPC). They used silica fumes in different percentages to investigate the mechanical strength of the MSPC. They observed that specimen with 15% replacement achieved highest strength as compared to other specimens. Xu et al. (2017) proposed the use of fly ash with Magnesium Potassium Phosphate Cement (MKPC) and deeply analyzed its microstructural changes. In Fig. 11, two specimen's SEM images are shown and discussed. In Fig. 11 (a), control specimen is shown referred as M8 is containing M/P molar ratio as 8. The formation of strength imparting compound k-struvite crystals is clearly visible with unreacted magnesia and sand. Meanwhile, lots of cracks and pore are visible in the image

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Fig. 11

with heterogeneously distributed constituents; whereas, in Fig. 11 (b), use of fly ash in the mixture referred as M8 FA-C is clearly noticeable because very dense microstructure is observed and the traces of unreacted fly ash is also seen in between sand and unreacted magnesia. Overall homogenously distributed constituents are observed which attribute to its better strength.

7. Calcium sulphoaluminate cement

The construction issues in subzero temperatures like delay in setting time and lesser workability is successfully overcomed by using CSA cement (Li et al., 2018) and (Zhang et al., 2020). Li et al. (2018) recommended the use of admixture likes lithium carbonatealuminum sulphate (LC-AS), which significantly improved the formation of ettringite and accelerated the process of belite hydration. Moreover, Zhang et al. (2020) used CAS as mineral accelerator in Portland cement and observed that freezing of water and inorganic salts are successfully avoided. The hydration process of CAS cement is discussed in detail (Kasselouri et al., 1995) and (Pelletier et al., 2010). Winnefeld and Lothenbach, (2010) performed thermodynamic modeling for two different cement pastes specimens which were prepared by varying the water/cement ratio. In the initial hour hydration of CSA's main element ye'elimite occurs in presence of gypsum to produces ettringite (Aft phase) and Al(OH)3${\mathrm{C}}_{\mathrm{4}}{\mathrm{A}}_{\mathrm{3}}\bar{S}+2\mathrm{C}\bar{S}+38\mathrm{H}+\to {\mathrm{C}}_{\mathrm{3}}\mathrm{A}3\mathrm{C}\bar{S}{\mathrm{H}}_{\mathrm{32}}+2\mathrm{A}{\mathrm{H}}_3$

Once the sulphate presence is exhausted, Aft phase converts into Afm phase, means formation of calcium monosulphoaluminate hydrate. Further, the hydration of C3S starts and C–S–H and CH is formed. Now AH3 produced in reaction (8), reacts with C3S and CH also to produce C-A-S-H and C-A-H.C3S + AH3 + 6H → C2ASH8 + CHC3S + 3:7H → CSH1.7 + 2CHAH3 + 3CH + CC + 5H → C4ACH11 (Pelletier et al., 2010).

Now the formation of CAH is continuous through reaction of CH with C3A and AH3 till it gets exhausted.(12)AH3 + 4CH + 6H → C4AH13C3A + CH + 12H → C4AH13 (Pelletier et al., 2010).

Finally the remaining Aft, reacts with newly formed C-A-H to produce monosulphoaluminate hydrate. In Fig. 12, thermodynamic modeling development of individual phase volume is shown (Winnefeld and Lothnbach, 2010). They observed that with the passage of time, the volume of liquid phase decreased and volume of solid phase got increased. Overall, in the solid phase it is observed that there is a major contribution of ettringite. Zhao et al. (2014) prepared two samples with same mix proportions, one with portland cement, another with CAS cement. Both the samples were tested against the chloride ion penetration resistance in which CAS sample performed better in resisting the penetration; hence, ensuring the lower permeability. In Fig. 13, SEM images of OPC and CAS specimen is shown. The end products of CAS cement are Aft, C–S-A-H, and Fe/Al gel. Aft is responsible for early strength attainment and also acts as filler in microcracks and pores. It also seals the gap between aggregate and mortar, and restricts the transport of chloride ions. This makes it less permeable which is clearly seen in Fig. 13(b). Meanwhile, the morphology of monosulphoaluminate obtained has the characteristics of C–S-A-H which is important element in determining the transport properties of concrete. Hence, altogether it resulted into denser and stronger microstructure of hydrated CAS.

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Fig. 12

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Fig. 13

8. Bagasse ash blended cement