**3.4 Mineral admixture in autogenous self-healing**

Supplementary cementitious materials (SCMs) and expansive minerals compatible with cement can improve the self-healing capacity of concrete. Depending on minerals, it can serve either or both functionalities, that is, to *remain considerably un-hydrated* after the initial mixing stage, and to *produce compatible expansive hydrated compounds* that can heal cracks [19]. Both these functionalities contribute to the autogenous healing process. A summary of mineral additives use for selfhealing is illustrated in **Table 1**. SCMs such as fly ash, silica fumes and blast-furnace slag, and expansive minerals such as MgO, calcium sulphoaluminate (CSA), lime, bentonite clay and crystalline additive (CA), have been mostly used for improving the concrete autogenous self-healing performance.

#### **Figure 6.** *(a) Schematic of shape memory PET polymer tendon, and (b) photo of the setup (Both reproduced from [7]).*

HFRCC are costly and maintaining homogeneity of fibres in the matrix for consis-

The shrinkable polymers such as PET can shrink when activated by heating in a specific condition. This shrinkage stress can be used for pre-stressing the concrete thus bringing crack-tip closure for efficient healing. Cardiff University self-healing research team is working with the original crack-closure system for cementitious materials using shrinkable polymer tendons [7]. The system involves the incorpo-

(**Figure 6**). Crack closure is achieved by thermally activating the shrinkage mechanism of the restrained polymer tendons (PTs) after the cement-based material has undergone initial curing. Upon activation, the polymer tendon completely closes the preformed macrocracks and imparts significant stress across the crack faces.

Supplementary cementitious materials (SCMs) and expansive minerals compatible with cement can improve the self-healing capacity of concrete. Depending on minerals, it can serve either or both functionalities, that is, to *remain considerably un-hydrated* after the initial mixing stage, and to *produce compatible expansive hydrated compounds* that can heal cracks [19]. Both these functionalities contribute to the autogenous healing process. A summary of mineral additives use for selfhealing is illustrated in **Table 1**. SCMs such as fly ash, silica fumes and blast-furnace slag, and expansive minerals such as MgO, calcium sulphoaluminate (CSA), lime, bentonite clay and crystalline additive (CA), have been mostly used for improving

*(a) Schematic of shape memory PET polymer tendon, and (b) photo of the setup (Both reproduced from [7]).*

ration of unbonded pre-oriented polymer tendons in cementitious beams

**3.3 Shrinkable polymers action in autogenous self-healing**

This enhances the autogenous self-healing process in concrete.

**3.4 Mineral admixture in autogenous self-healing**

the concrete autogenous self-healing performance.

**Figure 6.**

**196**

tent self-healing is challenging.

*Advanced Functional Materials*



*b A = anhydrite.*

*c L = lime/limestone powder.*

*d CEA = chemical expansive agent.*

*3/4 PB = Three/four-point bending, OPC = ordinary Portland cement, CASH = calcium aluminosilicate hydrate, CSA = calcium sulphoaluminate, CA = crystalline additive, FA = fly ash, SF = silica fume, <sup>e</sup> DME = dynamic modulus of elasticity, LWAs = lightweight aggregates, Na-MFP = sodium mono fluorophosphate (Na2FPO3, Na-MFP)*

#### **Table 1.**

*Advancement in autogenous self-healing of cementitious materials with mineral additives (Adopted from [17]).*

#### *3.4.1 SCMs to enhance autogenous self-healing*

Fly ash (FA) and silica fume (SF) and blast furnace slag (BFS) are mostly used as SCMs in the OPC system to improve concrete self-healing performance [12, 13, 37–43]. A higher amount of healing products of slag-ECC formed due to the higher pH

*Self-healing materials, (a) XRD and (b) SEM image with EDX element detection (Both reproduced*

Several types of expansive minerals can enhance autogenous self-healing per-

Magnesium oxide (MgO), bentonite clay and quicklime were used in different

proportions to enhance the autogenous self-healing capacity of concrete and cementitious materials [3, 16–21]. Substitution of PC with up to 12.5–15% by a mix of the three expansive mineral agents, MgO 5–7.5%, bentonite clay 2.5–5%, and quicklime 2.5–5%, results in optimum enhancement of the autogenous self-healing in the cement mix [17, 18]. A typical crack healing image is presented in **Figure 8** that shows how efficiently the expansive mineral containing PC mix sealed 17o-μm crack in 28 days. The flexural strength recovery and crack sealing efficiency of early age (1 day) cracked specimen was enhanced up to 48 and 39%, respectively, in an expansive mineral containing cement mix, compared to the 100% PC cement mix. The permeability (gas permeability coefficient) decreased by about 70% in the

formance of concrete. Calcium sulphoaluminate (CSA) is one of the popular expansive minerals used for improving healing capacity in concrete [8, 9]. A selfhealing agent (SHA) composed of silicon oxide (71.3%) and sodium aluminium silicate hydroxide [Na0.6Al4.70Si7.32O20(OH)4] (15.4%) along with various types of carbonates such as NaHCO3, Na2CO3 and Li2CO3 (etc.), and minerals such as bentonite clay (montmorillonite), feldspar and quartz was also used as an expansive self-healing agent [8]. Cracks of about 150 μm were healed within 33 days in the concrete with SHA, forming alumina silicate and modified gehlenite phases (CASH: calcium aluminosilicate hydrate). The reported healing mechanism was a swelling effect initiated by montmorillonite, and then expansion and re-crystallisation triggered by aluminosilicate with calcium ion. Ferrara et al. [51] used an active silicabased crystalline admixture (CA) as an expansive agent in cement and sand to improve the self-healing potential of raw concrete structures. Crack sealing of over 70–80% was required for reasonable mechanical performance to be recovered, such as stiffness (larger than 20%). The healing compounds formed by the crystalline admixture are similar to cement hydration products such as ettringite and calcium

value of pore solution and CaO content.

*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

**Figure 7.**

*from [50]).*

silicate hydrates.

**199**

*3.4.2 Expansive minerals to improve autogenous self-healing*

The substitution of FA 15–20% in OPC paste system has increased the volume of C-S-H gel and reduced meso-macropores, increasing the autogenous self-healing performance [40]. Watanabe et al. [41] replaced about 5–15% wt. of sand with FA in concrete and found a better dynamic modulus of elasticity recovery at 5% replacement and improving trend at 15% under the non-destructive ultrasonic test method. While freezing and thawing decreased dynamic modulus to 80% of the initial state, curing in water recovered it to over 93–98% after 28 days.

FA and SF, and a crystalline additive (CA) mineral were used for improving the self-healing performance of concrete [39]. CA was composed of 35.58% CaO, 16.81% SiO2, 15.22% Na2O, 1.98% Fe2O3, 1.93% Al2O3 and 1.29% MgO. Four different mixes (OPC, OPC + 30%FA, OPC + 10%SF, and OPC + 1%CA) were compared. Larger cracks (0.05–0.30 mm) healed better with SF additives. Microcracks in the range of 0–0.05 mm in CA additive mixes completely healed within 12 days.

The blast furnace slag (BFS) was used individually and in combination with FA and other minerals for improving self-healing properties. Fibre-reinforced cement composition with a local waste BFS and limestone powder (LP) in a mix proportion of 1:1.2:2 (C:BFS:LP), 0.5 w/b-ratio and 0.018% total mass of superplasticizer demonstrated improved self-healing performance [13]. The specimens cured under water recovered 65–105% deflection capacity compared to virgin specimens, while specimens cured in the air recovered only 40–60%. Small 25-μm cracks were healed efficiently, while larger cracks such as 60 μm were not healed completely. A higher proportions of BSF (50%) substitution in OPC decreases the formation of the healing material at an early age, which alters after 22 days [42]. However, optimum self-healing ability for the mixing content of slag and FA were 30 and 40%, respectively [43].

A considerable proportion (up to 70% of total weight) of slag and two classes fly ash (FA) were used as SCMs in ECC for improving autogenous self-healing performance [50]. Microscopic observation showed that slag-ECC healed up to 100-μm width crack. On the other hand, both F- and C-Class FA containing ECC sealed up to 50- and 30-μm width cracks, respectively. A microstructural investigation on the self-healed materials revealed that it was mostly composed of calcite and C-S-H gels and that composition varied with the supplementary minerals used (**Figure 7**).

*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

**Figure 7.** *Self-healing materials, (a) XRD and (b) SEM image with EDX element detection (Both reproduced from [50]).*

A higher amount of healing products of slag-ECC formed due to the higher pH value of pore solution and CaO content.

### *3.4.2 Expansive minerals to improve autogenous self-healing*

Several types of expansive minerals can enhance autogenous self-healing performance of concrete. Calcium sulphoaluminate (CSA) is one of the popular expansive minerals used for improving healing capacity in concrete [8, 9]. A selfhealing agent (SHA) composed of silicon oxide (71.3%) and sodium aluminium silicate hydroxide [Na0.6Al4.70Si7.32O20(OH)4] (15.4%) along with various types of carbonates such as NaHCO3, Na2CO3 and Li2CO3 (etc.), and minerals such as bentonite clay (montmorillonite), feldspar and quartz was also used as an expansive self-healing agent [8]. Cracks of about 150 μm were healed within 33 days in the concrete with SHA, forming alumina silicate and modified gehlenite phases (CASH: calcium aluminosilicate hydrate). The reported healing mechanism was a swelling effect initiated by montmorillonite, and then expansion and re-crystallisation triggered by aluminosilicate with calcium ion. Ferrara et al. [51] used an active silicabased crystalline admixture (CA) as an expansive agent in cement and sand to improve the self-healing potential of raw concrete structures. Crack sealing of over 70–80% was required for reasonable mechanical performance to be recovered, such as stiffness (larger than 20%). The healing compounds formed by the crystalline admixture are similar to cement hydration products such as ettringite and calcium silicate hydrates.

Magnesium oxide (MgO), bentonite clay and quicklime were used in different proportions to enhance the autogenous self-healing capacity of concrete and cementitious materials [3, 16–21]. Substitution of PC with up to 12.5–15% by a mix of the three expansive mineral agents, MgO 5–7.5%, bentonite clay 2.5–5%, and quicklime 2.5–5%, results in optimum enhancement of the autogenous self-healing in the cement mix [17, 18]. A typical crack healing image is presented in **Figure 8** that shows how efficiently the expansive mineral containing PC mix sealed 17o-μm crack in 28 days. The flexural strength recovery and crack sealing efficiency of early age (1 day) cracked specimen was enhanced up to 48 and 39%, respectively, in an expansive mineral containing cement mix, compared to the 100% PC cement mix. The permeability (gas permeability coefficient) decreased by about 70% in the

*3.4.1 SCMs to enhance autogenous self-healing*

**Minerals Composition Damage type Curing**

shrinkage, 3 PB

*CSA = calcium sulphoaluminate, CA = crystalline additive, FA = fly ash, SF = silica fume, <sup>e</sup>*

MgO 4–12% of cement Drying

*a*

*b*

*c*

*d*

**Table 1.**

*H = hauyne.*

*A = anhydrite.*

*L = lime/limestone powder.*

*CEA = chemical expansive agent.*

*Advanced Functional Materials*

respectively [43].

**198**

Fly ash (FA) and silica fume (SF) and blast furnace slag (BFS) are mostly used as SCMs in the OPC system to improve concrete self-healing performance [12, 13, 37–43]. The substitution of FA 15–20% in OPC paste system has increased the volume of C-S-H gel and reduced meso-macropores, increasing the autogenous self-healing performance [40]. Watanabe et al. [41] replaced about 5–15% wt. of sand with FA in concrete and found a better dynamic modulus of elasticity recovery at 5% replacement and improving trend at 15% under the non-destructive ultrasonic test method. While freezing and thawing decreased dynamic modulus to 80% of the

*3/4 PB = Three/four-point bending, OPC = ordinary Portland cement, CASH = calcium aluminosilicate hydrate,*

*Advancement in autogenous self-healing of cementitious materials with mineral additives (Adopted from [17]).*

*of elasticity, LWAs = lightweight aggregates, Na-MFP = sodium mono fluorophosphate (Na2FPO3, Na-MFP)*

**condition**

**Performance (healed crack width in time**

durability improved

**Source**

[3]

*DME = dynamic modulus*

**etc.)**

Water <500 μm in 28d

FA and SF, and a crystalline additive (CA) mineral were used for improving the

The blast furnace slag (BFS) was used individually and in combination with FA and other minerals for improving self-healing properties. Fibre-reinforced cement composition with a local waste BFS and limestone powder (LP) in a mix proportion of 1:1.2:2 (C:BFS:LP), 0.5 w/b-ratio and 0.018% total mass of superplasticizer demonstrated improved self-healing performance [13]. The specimens cured under water recovered 65–105% deflection capacity compared to virgin specimens, while specimens cured in the air recovered only 40–60%. Small 25-μm cracks were healed efficiently, while larger cracks such as 60 μm were not healed completely. A higher proportions of BSF (50%) substitution in OPC decreases the formation of the healing material at an early age, which alters after 22 days [42]. However, optimum

initial state, curing in water recovered it to over 93–98% after 28 days.

self-healing performance of concrete [39]. CA was composed of 35.58% CaO, 16.81% SiO2, 15.22% Na2O, 1.98% Fe2O3, 1.93% Al2O3 and 1.29% MgO. Four different mixes (OPC, OPC + 30%FA, OPC + 10%SF, and OPC + 1%CA) were compared. Larger cracks (0.05–0.30 mm) healed better with SF additives. Microcracks in the range of 0–0.05 mm in CA additive mixes completely healed within 12 days.

self-healing ability for the mixing content of slag and FA were 30 and 40%,

A considerable proportion (up to 70% of total weight) of slag and two classes fly ash (FA) were used as SCMs in ECC for improving autogenous self-healing performance [50]. Microscopic observation showed that slag-ECC healed up to 100-μm width crack. On the other hand, both F- and C-Class FA containing ECC sealed up to 50- and 30-μm width cracks, respectively. A microstructural investigation on the self-healed materials revealed that it was mostly composed of calcite and C-S-H gels and that composition varied with the supplementary minerals used (**Figure 7**).

100% PC and PC-expansive minerals mixes were up to 160 and 400–500 μm, respectively, after 28 days healing in water [3, 19]. Contained expansive minerals, such as reactive MgO can enhance healing compounds within the crack (**Figure 9**)

Expansive minerals can also improve the self-healing capacity of ECCs [46, 52]. Bentonite (Na-Montmorillonite) as a nanoclay was mixed with slag and limestone powder and used in ECC to improve its self-healing performance [46]. An ECC-MgO system resulted in higher flexural strength recovery of pre-cracked prismatic specimens cured under accelerated autoclaved conditions compared to their precracked ECC without MgO [52]. The combined effect of fibre to restrict crack and

In the autonomic self-healing system, different kinds of active healing agents are encapsulated into the concrete or composites. Popular encapsulation systems are microvascular glass tube network [23, 24] and microcapsules [1, 25, 26]. **Table 2** presents an overall conception of encapsulation materials and technical developments for the autonomic self-healing process. Typically a mobile liquid healing agent is always required. Less viscosity of healing agents is expected so that it can enrich a longer crack path in the damage zone, including microcracks [54]. Healing agents also should possess the ability to make a strong bond between the crack

**4.1 Autonomic microvascular and tabular capsules for self-healing**

Capillary glass tubes are a popular choice for the microvascular network or tabular system to carry the healing agent into the concrete matrix [23, 24, 27, 28]. Diameters of the glass tubes typically range from 0.8 mm [23] to 4 mm [55]. A cyanoacrylate (<5 cP viscosity) enclosed in capillary tubes (0.8 mm inner diameter and 100 mm length), with 50 μl capacity and sealed the end with silicon considerably recovered flexural stiffness in beams [23]. Mihashi et al. [28] used embedded glass pipes with two types of healing agent, alkali-silica based and two-part epoxy resin. Considerable strength recovery performance was noted with both types of the healing agent within the crack range between 300 and 500 μm. Nevertheless, efficient mixing of two-component resin inside the crack was a challenging issue. Cardiff University researchers have investigated the type of healing agent, delivery technique, mortar mix design and the quantity of steel reinforcement used [27]. They used three popular healing agents, (i) epoxy resins following [28], (ii) cyanoacrylates following [23] and (iii) alkali-silica solutions following [28]. During the first and second loading cycles under a three-point bend test, both primary and secondary healing occurs. Low-viscosity (typically 5 cP) single-agent cyanoacrylate adhesive resulted in optimum self-healing due to its efficient infiltration into microcracks. However, healing agents carried into the cracks are limited due to the capillary action [27]. This limitation can be eliminated with the use of an open-

The most recent advancement of a vascular network system in concrete was used in a filed trail of a road improvement scheme by Materials for Life (M4L) project [56]. The vascular network systems with shape memory polymer tendons (PET) were combined in large-scale structural elements (**Figure 10**). The self-

healing performances were promising in this field trial.

to effectively heal the crack.

*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

faces.

ended system.

**201**

the expansive minerals to heal the crack is promising.

**4. Autonomic self-healing system in concrete**

#### **Figure 8.**

*The typical crack sealing pattern in 28 days: (a) 100% PC cement mix and (b) cement with expansive minerals (Reproduced from [17]).*

expansive mineral containing mix compared to the 100% PC cement mix. Besides common healing compounds, calcite, portlandite, ettringite and C-S-H, MgO formed brucite, other magnesium hydro-carbonate products. Although, the healing capacity of cementitious materials decreases with the increase in the age of cement paste mix at crack formation, expansive minerals improved the autogenous selfhealing capacity of PC mixes at all ages compared to the 100% PC paste [18].

Expansive minerals combination, that is, MgO, bentonite clay and quicklime can improve the autogenous self-healing capacity of drying shrinkage cracks in the cementitious materials. The maximum healable drying shrinkage cracks width in

#### **Figure 9.**

*Ternary diagrams of healing compounds EDX computed atomic mass percentage formed in PC-MgO cement mixes (Reproduced from [2]).*

*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

100% PC and PC-expansive minerals mixes were up to 160 and 400–500 μm, respectively, after 28 days healing in water [3, 19]. Contained expansive minerals, such as reactive MgO can enhance healing compounds within the crack (**Figure 9**) to effectively heal the crack.

Expansive minerals can also improve the self-healing capacity of ECCs [46, 52]. Bentonite (Na-Montmorillonite) as a nanoclay was mixed with slag and limestone powder and used in ECC to improve its self-healing performance [46]. An ECC-MgO system resulted in higher flexural strength recovery of pre-cracked prismatic specimens cured under accelerated autoclaved conditions compared to their precracked ECC without MgO [52]. The combined effect of fibre to restrict crack and the expansive minerals to heal the crack is promising.

## **4. Autonomic self-healing system in concrete**

In the autonomic self-healing system, different kinds of active healing agents are encapsulated into the concrete or composites. Popular encapsulation systems are microvascular glass tube network [23, 24] and microcapsules [1, 25, 26]. **Table 2** presents an overall conception of encapsulation materials and technical developments for the autonomic self-healing process. Typically a mobile liquid healing agent is always required. Less viscosity of healing agents is expected so that it can enrich a longer crack path in the damage zone, including microcracks [54]. Healing agents also should possess the ability to make a strong bond between the crack faces.

#### **4.1 Autonomic microvascular and tabular capsules for self-healing**

Capillary glass tubes are a popular choice for the microvascular network or tabular system to carry the healing agent into the concrete matrix [23, 24, 27, 28]. Diameters of the glass tubes typically range from 0.8 mm [23] to 4 mm [55]. A cyanoacrylate (<5 cP viscosity) enclosed in capillary tubes (0.8 mm inner diameter and 100 mm length), with 50 μl capacity and sealed the end with silicon considerably recovered flexural stiffness in beams [23]. Mihashi et al. [28] used embedded glass pipes with two types of healing agent, alkali-silica based and two-part epoxy resin. Considerable strength recovery performance was noted with both types of the healing agent within the crack range between 300 and 500 μm. Nevertheless, efficient mixing of two-component resin inside the crack was a challenging issue.

Cardiff University researchers have investigated the type of healing agent, delivery technique, mortar mix design and the quantity of steel reinforcement used [27]. They used three popular healing agents, (i) epoxy resins following [28], (ii) cyanoacrylates following [23] and (iii) alkali-silica solutions following [28]. During the first and second loading cycles under a three-point bend test, both primary and secondary healing occurs. Low-viscosity (typically 5 cP) single-agent cyanoacrylate adhesive resulted in optimum self-healing due to its efficient infiltration into microcracks. However, healing agents carried into the cracks are limited due to the capillary action [27]. This limitation can be eliminated with the use of an openended system.

The most recent advancement of a vascular network system in concrete was used in a filed trail of a road improvement scheme by Materials for Life (M4L) project [56]. The vascular network systems with shape memory polymer tendons (PET) were combined in large-scale structural elements (**Figure 10**). The selfhealing performances were promising in this field trial.

expansive mineral containing mix compared to the 100% PC cement mix. Besides common healing compounds, calcite, portlandite, ettringite and C-S-H, MgO formed brucite, other magnesium hydro-carbonate products. Although, the healing capacity of cementitious materials decreases with the increase in the age of cement paste mix at crack formation, expansive minerals improved the autogenous selfhealing capacity of PC mixes at all ages compared to the 100% PC paste [18].

*The typical crack sealing pattern in 28 days: (a) 100% PC cement mix and (b) cement with expansive*

**Figure 8.**

**Figure 9.**

**200**

*mixes (Reproduced from [2]).*

*minerals (Reproduced from [17]).*

*Advanced Functional Materials*

Expansive minerals combination, that is, MgO, bentonite clay and quicklime can

improve the autogenous self-healing capacity of drying shrinkage cracks in the cementitious materials. The maximum healable drying shrinkage cracks width in

*Ternary diagrams of healing compounds EDX computed atomic mass percentage formed in PC-MgO cement*


**Shell material**

**203**

Glass Glass Glass Ceramics

Perspex Plant fibre PP with wax concentric glass capsule

> Pellets

Cement

PVA

PVA

Vascular network for self-

Tubular

Glass Glass Glass Glass Glass Spiral twisted wire with

EVA

Porous concrete

> **Table 2.**

*Autonomic*

 *self-healing:*

*Encapsulation*

 *materials and techniques used ('–' means 'not reported', 'x' means 'not applicable', '*√*' means 'yes' and '/' means 'no'). (upgraded from [53]).*

Epoxy

—

25,000–

—

x

∕

35,000

Foam, epoxy, silicon, CA

Epoxy

Alkali silica, epoxy

CA Epoxy

CA

healing

CA, epoxy,

polyacrylate,

bacteria

PU Epoxy

—

MMA

MgO, bentonite, lime

Na2FPO3, Na-MFP

MgO

CSA

 —

–

–

–

–

∕

√

–

450

11,400

6150

–

4000

–

x

√

x

√

500

√

10–50

12–73

600 500 600 400

x

∕

x

∕

x

∕

x

∕

600–4000

500

–

–

800

3000

4800

3200

 1500 2000

 3400

—

 —

700

x

∕

x

∕

 4000

 6000

 4000

 2000

40–188

—

——

2500–3500

 3000–4000

——

 250

 —

 —

∕

15–50

∕

*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

> PU,

2000–3000

 2200–3350

 100

CA CA

3200

—

100

\_

63.5 20–80

∕

√

 4000

**Core material**

**Øi (μm)**

 **Øo (μm)**

 **Wall thickness**

**Length**

**Mixed**

**(mm)**

**in**

**(μm)**

400

200

∕

#### *Advanced Functional Materials*

