Individual and combined effects of Portland cement-based hydrated mortar components on alkali-activated slag cement

Construction and Building Materials 73 (2014) 515–522

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Individual and combined effects of Portland cement-based hydrated mortar components on alkali-activated slag cement N.R. Rakhimova ⇑, R.Z. Rakhimov Department of Building Materials, Kazan State University of Architecture and Engineering, Zelenaya Str. 1, Kazan 420043, Russian Federation

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 GHM strengthens AASC and allows

Time-series analysis of the influence of ground hydrated cement paste, ground quartz sand and ground hydrated mortar with a cement:sand ratio of 1:1–3 on the properties of fresh and hardened alkali-activated slag pastes.

slag substitution of up to 50%.  GHM consists of a ‘nucleator seed’ and a ‘physically active’ additive.  Blending AASC with GHM increases its two-day strength to 9.9 MPa.  A 1:1.5 cement:sand ratio yields the highest strength after 28–360 days.

a r t i c l e

i n f o

Article history: Received 23 April 2014 Received in revised form 3 September 2014 Accepted 24 September 2014

Keywords: Granulated Blast-Furnace Slag Alkali Activated Cement Cement paste Mortar Waste management

a b s t r a c t A comparative analysis was made of the individual and combined effects of the constituents of ground hydrated mortar (GHM) on the properties of fresh and hardened alkali-activated slag cement (AASC) pastes, which took into consideration properties such as the liquid/solid ratio, setting time, water absorption, density and compressive strength development. From the results obtained, it is evident that GHM is a multifunctional mineral additive that consists of a ‘nucleator seed’ and a ‘physically active additive’. Moreover, these components function both jointly and separately to micro-reinforce and fill the AASC paste, thereby accelerating its curing and providing a prolonged strengthening effect. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Abbreviations: HMW, hydrated mortar waste; AASC, alkali-activated slag cement; PC, Portland cement; GGBFS, ground granulated blast furnace slag; GQS, ground quartz sand; GHCP, ground hydrated cement paste; GHM, ground hydrated mortar; Ssp, specific surface area; l/s, liquid/solid ratio; C:S, cement:sand ratio. ⇑ Corresponding author. Tel./fax: +7 8432362721. E-mail address: [email protected] (N.R. Rakhimova). http://dx.doi.org/10.1016/j.conbuildmat.2014.09.096 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

One of the major challenges facing the cement industry in its bid to achieve a sustainable future is finding a means of producing binders based on a mix of different mineral supplements. This not only requires increased use of known supplementary cementitious

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materials (SCMs), but also the development and application of entirely new SCMs [1]. Mineral wastes are amongst the most promising of the latter; however, further methodical study is still needed to develop an effective approach to their practical application. This is particularly true of multicomponent wastes, which are characterised by a variation in their chemical and mineralogical composition, and therefore require a component-wise approach based on assessing the individual and combined effect of each component on the properties of the binder. Such an approach should assist in determining the utility, feasibility and effectiveness of secondary resources, thereby leading to more effective and efficient recycling of mineral wastes. 1.1. Hydrated mortar waste The highest volume wastes of mixed composition are concrete and demolition wastes, which are currently used as coarse and fine secondary aggregates in concrete production after they first mechanically crushed, sieved, and sorted [2–9]. This crushing of concrete wastes, however, leaves behind residual mortar, which is often referred to as waste concrete powder, concrete fines or hydrated mortar waste (HMW). This HMW typically makes up 25–30% of the crushed-material by volume [7,10], though its composition can vary depending on the composition of the parent concrete, the curing conditions, the time of exploitation and various other factors. The chemical compositions of a number of different concrete wastes are listed in Table 1. As can be seen, HMW typically consists of quartz, calcite, ettringite, non-hydrated Portland cement (PC) minerals, hydrates of calcium silicate and calcium aluminate, feldspars and hydromica. The presence of non-hydrated PC particles makes HMW a particularly promising material for the recycling and production of cement clinker and blended cements. Indeed, with suitable mechanical, mechanochemical, thermal, or hydrothermal activation, HMW can be used as: – a raw mixture to produce cement clinker or as an additive in the raw mixture [16–18], – an additive to ensure a basic or thermally activated state in PC minerals [13,19,20], or – an additive for clinker-free cements [11,14,21]. 1.2. Hydrated mortar waste as an additive in alkali-activated slag cements Alkali-activated slag cements (AASCs) are one of the most promising binders for use in alternative cements [22], producing a C–A–S–H gel on hardening due to the alkali activation of the calcium aluminosilicate that makes up the bulk of ground granulated blast furnace slag (GGBFS) [23–25]. The mineral matrix is therefore characterised by a high binding power, low basicity and solubility of the binder gel, lower porosity and smaller pores in the hardened AASC paste, high adhesive strength due to a high pH and concentration of alkali, and a dense and uniform interfacial transitional

zone between the aggregate and AASC paste [26–31]. Furthermore, alkali activation ensures effective interaction between the AASC paste and any fillers or modifiers, as well as ensuring compatibility with blended-mineral materials of various compositions and structures. Indeed, a much wider range of mineral materials can be successfully blended with AASCs than with ordinary Portland cements (OPCs) [32,33]. The use of mineral additives as means of manipulating the GGBFS content, physical properties and structural performance of AASC therefore appears to be quite a promising option for the sustainable development of mineral binders, as it should allow the range of useable mineral wastes to be greatly expanded. The interaction between AASC pastes and mineral additives can, however, vary significantly in relation to differences in raw material composition, the structure formation process, and the composition and properties of the PC and AASC binder gel. These factors therefore determine the feasibility of replacement and modification, as well as the way in which such additives are classified. Taking into consideration the individual effects of the raw material and binder gel, as well as the structural and physical properties of AASC, a new classification system was proposed that defines supplementary materials for AASCs as either: (1) nucleator seeds, (2) chemically active, (3) physically active, or (4) physically active and reactive [32]. Chemically active supplementary materials have an amorphous structure that forms hydration products with cementitious properties and modifies the composition of the binder gel. Physically active supplementary materials, on the other hand, have a crystalline and/or chemically inert structure that does not modify the composition of the binder gel, but does affect the physical structure of the mixed binder. Physically active and reactive supplementary materials combine both of these effects. The activity of HMW, as depicted in Fig. 1, is based on a ‘nucleator seed’ of hydrated and non-hydrated PC and a ‘physically active’ quartz sand (QS) additive. The influence of hydrated mortar on fresh and hardened AASC pastes will therefore depend greatly on its ratio of hardened PC paste to QS. 1.2.1. Hydrated mortar and its individual components as additives in AASC Calcium silicate hydrate (C–S–H), Portland clinker/cement and concrete wastes have been the most widely studied in relation to their effect on the early strength of AASC pastes. Hubler et al. [34] reported that the addition of C–S–H seeds causes the hydration rate of AASC to peak earlier and higher, resulting in a compressive strength after one day of curing that is at least four-times greater than a control sample. This increase in strength was, however, strongly dependent on the curing method used, with sealed curing more favourable in this regard than underwater curing. The addition of a small percentage of Portland clinker (1–7%

Hydrated cement paste (crystallizaon seed)

Нydrated mortar

Quartz sand

(crystallizaon seed+ ”physically” acve admixture

(“physically acve admixture”)

Fig. 1. Activity of hydrated mortar and its constituents.

Table 1 The chemical composition of concrete wastes. Waste type

Waste concrete Waste concrete powder Hydrated mortar waste Ground waste concrete Construction and demolition waste materials

Chemical composition (wt.%)

Reference

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

LOI

41.15 51.4 27.98 40.1 30.99–60.2

7.80 5.01 3.87 9.6 4.5–11.27

12.57 3.72 2.10 3.5 1.43–4.52

20.64 35.23 36.01 20.6 11.6–23.96

1.50 1.25 1.48 2.1 2.78–5.49

2.07 1.5 1.29 2.3 1.05–2.04

1.98 0.51 0.26 1.7 0.55–1.53

13.07 0.093 26.3 – 11.24–26.77

[11] [12] [13] [14] [15]

N.R. Rakhimova, R.Z. Rakhimov / Construction and Building Materials 73 (2014) 515–522

depending on slag basicity) has also been shown to improve the strength of hardened AASC pastes, particularly early on [35–38]. Indeed, these Portland clinker-modified AASCs demonstrated very similar properties to rapid-hardening and super-rapid-hardening cements, but without the need for steam curing [36]. The effect of PC on the properties of fresh and hardened AASC pastes has been quite widely studied in terms of the type and quantity of alkali activator, the concentration of PC and the water/binder ratio. The addition of 30% Na2SiO3 was demonstrated by Fu Sheng to produce rapid setting [39], with Douglas et al. identifying that adding 2–8% PC to these sodium-silicate-activated AASCs increased their strength at 2, 7, and 28 days of age [40]. Later, it was found that using this same activator with 10% PC addition increases the 1-day, 28-day [39], and 90-day [41] strengths. This is concordant with the studies by Zivica, which showed that 10–30% PC addition increases the 1-day, 28-day, and 90-day strength of AASC-based mortars with a silica-fume alkali activator [42,43]. Singh et al. [44] also demonstrated that the addition of 4% Na2SO4 improves the strength of slag mixed with 50% PC. There are, however, conflicting reports [47–49] that the addition of PC to AASC pastes and mortars is in fact not effective at all in improving their strength. The issue of strength aside, the addition of 10% PC to AASC has been shown to produce a lower drying shrinkage than PC alone [39], resulting in a reduction in total porosity and median pore size [42,45], as well as an increased resistance to attack by sodium sulphate and acids [45,46]. Ground hardened-cement paste (GHCP) has been also been found accelerate the setting and hardening of AASC pastes, considerably improving their early strength as well as their 28-day and long-term strength to a point that allows for a slag replacement of up to 45% [50]. However, such improvement was very much dependent on the properties of the fresh and hardened AASC pastes, with GHCP being more effective in terms of improvement the early strength when sodium carbonate is used as an alkali activator rather than sodium silicate and when it is allowed to harden naturally instead of being steam-cured. The density of the hardened AASC paste was also reduced, and its water adsorption increased, with a greater quantity or fineness of GHCP. With regards to the QS component of HMW, Rashad et al. [51] found that a 30% addition of fine QS (80–85% < 32 lm particles)

increases the compressive strength of sodium silicate activated AASC by as much as two times within 28 days. Indeed, the fineness of ground quartz sand (GQS) is considered the most important factor [52], with up to 30% and 50% of a slag with a fineness of 300 m2/kg able to be replaced in a sodium carbonate activated AASC by GQS with a fineness of 500 and 800 m2/kg, respectively. This introduction of GQS reduces the early strength, but improves the 1-year strength by up to 50%. Ground hydrated mortar (GHM) can also have a positive effect on the strength characteristics of hardened AASC pastes, allowing for a slag replacement of up to 50% [53]. Factors affecting the strength of these GHM-modified AASCs are, in order of influence: alkali activator type > curing conditions > cement:sand ratio (C:S) (1:1–1:3) > GHM fineness. In light of these previous investigations into the properties and modification of AASC pastes, the aims of this paper are to: – provide a comparative analysis based on new and previously published results [51–53] of the individual and combined effects of the constituents of GHM on the properties of fresh and hardened AASC pastes, taking into consideration the dispersity, quantity and composition of each additives, – define the role of each individual GHM component in relation to the final properties of AASC pastes. 2. Experimental details An overview of the experimental steps and the influencing factors considered as variables in this investigation is shown schematically in Fig. 2.

2.1. Materials Ground granulated blast furnace slag (GGBFS) was obtained from the Chelyabinsky factory. Mixed cements were alkali-activated using commercial sodium carbonate (NaCO3) solutions, their addition being adjusted to 5% (by weight) with Na2O. Portland cement (CEMI 42,5 N) was sourced from ‘Ul’yanovskcement’ and had a mineral composition of: 57.2% C3S, 17.5% C2S, 6.9% C3A, and 13.3% C4AF. Quartz river sand with a fineness modulus of 2.5, bulk density of 2540 kg/m3 and water absorption of 2% was used for the preparation of mortars. Laboratory-produced GHCP and GQS were used as simulated additives to mimic the individual components of GHM and GHM of different compositions. Table 2 shows the chemical composition of starting materials.

(a) Preparaon of simulated admixtures Starng materials Addives

Preparaon

OPC

GHCP

Mixing, hydraon, grinding

QS

GНM

Mixing, hydraon, grinding

GQS

Grinding

(b) Preparaon and tesng of AASC pastes combined with simulated admixtures Starng materials GGBFS Alkali acvator

Addives GQS GHCP + GНM1 GНM1,5 GНM2 GНM3

Preparaon of AASC paste Mixing, hydraon

517

Tesng Water requirement, seng me, compressive strength up to 360 days, density, water absorpon

Influencing factors

*Dispersity: - 200 m2/kg - 400 m2/kg - 600 m2/kg

* Cement:sand rao: - 1:1 - 1:1.5 - 1:2 - 1:3 *Percentage of admixtures up to 50%

Fig. 2. Schematic diagram of the experimental steps and influencing factors.

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Table 2 Chemical composition of starting materials. Starting material

Component (mass % as oxide)

GGBFS PC QS GHCP GHM 1:1 GHM 1:1.5 GHM 1:2 GHM 1:3

SiO2

CaO

Al2O3

MgO

MnO

Fe2O3

TiO2

Na2O

K2O

P2O5

SO3

LOI

37.49 21.14 97.39 18.53 45.42 58.64 70.22 86.43

36.22 65.3 1.02 52.13 33.93 23.19 12.56 5.99

11.58 5.97 0.58 4.18 3.59 3.25 2.54 1.01

8.61 0.91 0.69 1.16 1.18 0.54 1.22 1.08

0.50 – – 0.04 – 0.03 – –

0.16 4.38 0.32 3.30 3.00 2.21 2.26 1.50

1.80 – – 0.21 0.20 0.15 0.15 0.18

0.68 1.00 – 0.25 0.26 0.38 0.23 0.31

0.95 – – 0.57 0.4 0.43 0.5 0.3

0.01 – – 0.28 – 0.13 0.25 0.16

2.00 1.3 – 2.96 2.01 1.38 2.52 1.87

– –

2.2. Methods Simulated samples of GHCP and GHM were produced by grinding hardened cement paste and hardened mortar specimens that were prepared by mixing their respective components to homogeneity in a mixer, then curing for 28 days at 20 °C and 95% RH for 3 months in air. The mix proportions and its abbreviations for GHM are listed in Table 3. The blast furnace slag was ground in a laboratory-scale planetary mill to a specific surface area (Ssp) of 300 m2/kg, whilst the GQS, GHCP, and GHM were ground to Ssp values of 200, 400 and 600 m2/kg (Blaine). The particle size distribution and the Ssp area of the materials were tested using a Fritsch particle sizer ANALYSETTE 22 and a Blaine air-permeability apparatus. Samples of AASC paste were prepared by homogenously mixing GGBHS with 2.5–60% simulated additive to a standard consistency. The setting time of each blended paste was determined by measuring the change in their liquid/solid (l/s) ratio using a Vicat apparatus. To enable testing of the compressive strength of each paste, samples were cast in 2  2  2 cm cubic moulds and demoulded after 1 day. These cubes were stored in sealed plastic bags at 20 °C temperature and 95% RH until required. Each strength measurement quoted represents an average of six values from the same casting. Surfaces exposed by cutting 28-day old samples were gold-coated, and then imaged using electron microscopy (Phenom G2 Pure).

3. Results and discussion 3.1. Influence of GHM on the properties of fresh AASC paste The change in l/s of fresh AASC pastes containing various simulated additives shown in Fig. 3 reveals that in the case of

Table 3 The mix proportions and its abbreviations for GHM. GHM

(%) Portland cement

(%) River sand

GHM1 GHM1.5 GHM2 GHM3

50 40 33.3 25

50 60 66.6 75

0.89 16.39 10.01 9.67 7.55 1.17

GHM, it is the Ssp value that determines whether the rate of change is increased or decreased. Thus, when introducing additives with a Ssp of 200 m2/kg, the l/s ratio is increased only in the case of a cement to sand ratio of 1 (i.e. GHM1); any further increase in the relative amount of sand only reduces the amount of alkaline solution required. With a higher Ssp of 400–600 m2/kg, increasing the amount of additive by up to 50% can increase the l/s ratio from 0.25 to 0.32–0.51 depending on the composition and Ssp of the particular additive. Meanwhile, the increase in l/s depends on the GHCP content and occurs in the following order: GHM1 > GHM1.5–2 > GHM3 > GHCP > GQS. As shown in Fig. 4, the time required for fresh AASC paste to set is reduced by the addition of up to 15% GQS, but increased with concentrations greater than this. It is known that when mixed with OPC, GQS promotes the hydration of cement particles in blended cements through heterogeneous nucleation and dilution [54,55]. Presumably, introduction of GQS up to 15% and increase of its fineness intensifies the first stage of structure formation process, but more percentage of GQS slows down the setting time of fresh AASC paste. With the other additives, even very low concentrations and/or Ssp values intensify the rate of setting. The incorporation and the increase in the Ssp of all of the GHCP-containing simulating additives shorten the setting time in the following order: GHCP > GHM1–1.5 > GHM2 > GHM3. 3.2. Influence of simulated additives on the properties of hardened AASC pastes The influence of the quantity and fineness of simulated additives on the 28-day compressive strength of hardened mixed-AASC pastes is depicted in Fig. 5, from which it is clear that GHM

100

200 m2/kg

600 m2/kg

400 m2/kg

90

Liquid/solid rao

0.55

200 m2/kg

400 m2/kg

Inial seng me, min

80

600 m2/kg

0.5 0.45 0.4 0.35

70 60 50 40 30

0.3

20

0.25

10

0.2

0 0 10 20 30 40 50

0 10 20 30 40 50

0 10 20 30 40 50

0

15 30 50

Amount of addive, % GHCP

GНM1

GНM1,5

GНM2

0

15 30 50

0

15 30 50

Amount of addive, % GНM3

GQS

Fig. 3. Liquid/solid ratio of fresh AASC paste vs. the Ssp and relative amount of simulated additive.

GHCP

GHM1

GHM1,5

GHM2

GHM3

GQS

Fig. 4. Initial setting time of fresh AASC paste as a function of the amount and Ssp of additives.

N.R. Rakhimova, R.Z. Rakhimov / Construction and Building Materials 73 (2014) 515–522

Compressive strength, MPa

120

200 m2/kg

400 m2/kg

519

600 m2/kg

100 80

GHCP GHM1

60

GHM1,5 GHM2

40

GHM3 GQS

20 0 0 5 10 15 20 25 30 40 50

0 5 10 15 20 25 30 40 50

0 5 10 15 20 25 30 40 50

Amount of addive, % Fig. 5. 28-day strength of hardened AASC pastes as a function of the amount and Ssp of additives.

functions as both a reinforcing and filling material. This means that not only can GHM with a C:S ratio of 1–1.5 and Ssp of 400 m2/kg be added to increase the strength of an AASC paste by up to 97%, but can also occupy up to 50% of the paste as a filler without any significant loss in strength relative to the unmodified paste. Those AASC pastes that returned the highest compressive strengths after 28 days were also tested for strength after 2, 7, 14, 180, and 360 days. The experimental compositions of these are given in Table 4. As seen in Fig. 6, the addition of hydrated and non-hydrated PC particles to AASC paste considerably improves its 2-day compressive strength from 2 to 27.7 MPa. According to Krivenko [31,35], the most likely mechanism for the Fig. 7. Electron micrographs of hardened AASC-1 paste.

Table 4 Experimental compositions of blended AASCs. Type of AASC

Reference AASC-1 AASC-2 AASC-3 AASC-4 AASC-5 AASC-6

% GGBHS

100 85 95 92.5 90 85 92.5

Additive Type of additive

% Additive

Ssp of additive (m2/kg)

– GHCP GHM 1:1 GHM 1:1.5 GHM 1:2 GHM 1:3 GQS

– 15 5 7.5 10 15 7.5

– 400 400 400 400 400 400

120

Compressive strength (MPa)

100

80

60

40

rapid development of strength is: (i) the basicity of the AASC is increased by the Portland clinker and results in faster formation of reaction products, and (ii) the fast hydration of Portland clinker minerals in an alkaline environment produces crystal seeds (lowbasicity calcium silicate hydrates) that only grow in one direction and with the reinforcing binder gel. This notion agrees well with the microstructure of the hardened AASC-1-based paste shown in Fig. 7, in which the gel-like AASC matrix clearly contains fibrous 1–2 lm (Fig. 7a) and lamellar reaction products (Fig. 7b). Both combining GHCP with GQS and increase the content of GQS in GHM gradually reduces the effectiveness of this additive as an accelerator, and leads to a reduction in the early strength improvement seen in hardened blended-AASC. For instance, with a 1:1 ratio of GHM (AASC-2) the 2-day compressive strength of the hardened AASC paste is increased by 4.7 times to 9.4 MPa, but this is drastically reduced when the ratio is increased to 1:1.5 (AASC-3). With further hardening for 7 and 14 days the difference between these AASC pastes diminishes because the hydration activity of Portland clinker is mostly exhausted in the early stages of AASC paste hardening. With further curing for 28, 180 and 360 days it becomes apparent that of all the simulated additives, GHM1.5 has the greatest influence, with the effect of other additives weakening in order of:

20

0

2

7

14

28

28

180

360

– GHM1.5 > GHM1 > GHCP > GHM2 > GHM3 > GQS (28 and 180 days). – GHM1.5 > GHM1 > GHCP > GHM2 > GHM3 > GQS (360 days).

Age (days) reference

AASC-1

AASC-5

AASC-6

AASC-2

AASC-3

AASC-4

Fig. 6. Development of strength in hardened AASC paste with varying C:S ratios and GHM content.

It can be concluded from this that the amount of GQS added needs to be equal to or greater than 1.5 times the amount of PC in order to ensure the maximum reactivity and improvement in strength after 28–360 days, regardless of the Ssp. This can be explained by the fact that grinding GHCP not only reduces its

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particle size, but also mechanically activates non-hydrated PC grains. It can also GHCP at the interfacial transitional zones between hydrated cement paste and QS or on grains of QS to disintegrate, with the reduction in the size of the QS grains contributing to their effectiveness and surface activity through partial amorphisation of the surface. Mechanochemical transformations can also occur in highly loaded grinding machines, such as the planetary ball mill used in this study. Given that GHCP contains portlandite Ca(OH)2, it is quite plausible for meta-calcium silicate hydrate and calcium silicate hydrate to form as a result of crushing according to the reaction proposed by Hinte and Kuzminov [56]:

CaðOHÞ2 þ SiO2 ! CaO  SiO2  H2 O: This calcium silicate then absorbs and reacts with carbon dioxide from the air Fig. 9. Electron micrograph of hardened AASC-6 paste.

CaO  SiO2  H2 O þ CO2 ! CaCO3  SiO2 þ H2 O: There is another possibility related to the evolution and strengthening over time of the interfacial zone between the binder gel of the AASC paste and particles of GQS. That is, since GQS exhibits ‘physical activity’ during interfacial zone formation, then according to Brough et al. [57], this interface region is more porous and fragile after 1–7 days than the bulk paste, but after 14 days the amount of hydrates in the interfacial zone is increased. Shi et al. [28] identified that the interface between an alkali-activated slag cement paste and sand appears to be very dense and uniform after 28 days of hydration, whereas a 27-year-old alkali-activated slag has been found to be not only a dense and uniform interface, but also show evidence of chemical reactions between the cement paste and aggregate [29]. Presumably, it is therefore a strengthening of the interfacial zone between the AASC paste and GQS of GHM that contributes to its 28-day strength and continued increase in strength by up to 57.3–60.2% after 1 year. However, this

strengthening effect is reduced to 14.7–32.3% if the C:S ratio is increased to 1:2 or 1:3 (AASBC-3,4), as the additive then acts more as a filler than a reinforcing element. Nevertheless, replacing GGBFS with up to 40% GHM does not notably reduce the strength at either the 28-day or 1-year interval. Figs. 8 and 9 show the microstructure of hardened blended AASC-2 and AASC-6 pastes, respectively. As can be seen in Fig. 9, the microstructure of the AASC-6-based paste consisted mainly of gel-like reaction products, whereas the microstructure of AASC-2 (Fig. 8) contained both gel-like and lamellar reaction products that were very similar to those observed in AASC-1. In Fig. 10, we see the influence that the quantity and Ssp of GHCP, GHM1.5 and GQS additives has on the density and water absorption of hardened AASC pastes after 28 days. It is clear from this that the density decreases and water absorption increases as the percentage of additive is increased, with these changes most likely being interdependent with the increase in l/s ratio and decrease in particle size. For example, with 15% additive the density of hardened AASC pastes with GHCP, GHM1.5 or GQS decreases by up to 8%, 3.2% and 1.6%, respectively, whilst the water absorption increases up to 41%, 16.6% and 2.1%. These results show that the hardened AASC paste becomes stronger after GHM addition not because of densification, but rather as a result of the solid phase of the hardened paste being strengthened. These findings are summarised in Fig. 11, which shows the change in the properties of

25

2 1.8 1.6 1.4

15

1.2 1

10

0.8

Density, g/sm 3

Water absorpon (%)

20

0.6 5

0.4 0.2

0

0 0

10

20

30

40

50

Content of admixture (%)

Fig. 8. Electron micrographs of hardened AASC-2 paste.

W GHCP

W GHМ1,5

ρ GHМ1,5

ρ GQS

W GQS

ρ GHCP

Fig. 10. Water absorption and density of blended AASC pastes vs. GHM content (C:S 1:1.5, fineness 400 m2/kg).

N.R. Rakhimova, R.Z. Rakhimov / Construction and Building Materials 73 (2014) 515–522

1

2 3

4 GHCP

GHM1

GHM1,5

GHM2

GHM3

GQS

“physically acve admixture” “crystallizaon seed” 1 – 2-day strength, 2 – 28-, 180-, 360-day strength, 3 – seng mes, 4 – water requirement Fig. 11. Change in the properties of fresh and hardened AASC pastes vs. the type and composition of simulated additives.

fresh and hardened AASC pastes with respect to the simulated additive used. 4. Conclusions Based on the results of a comparative analysis of the individual and combined effect of the constituents of GHM on the properties of fresh and hardened AASC pastes, the following major conclusions can be drawn: 1. The addition of GHM and GQS with an Ssp of 200 m2/kg reduces the l/s ratio, but if the Ssp is increased to more than 200 m2/kg then l/s ratio is increased regardless of type or quantity of additive, or the alkali activator type. Furthermore, the addition of GHCP, GHM and up to 15% GQS improves the setting time of fresh AASC proportionally to any increase in the fineness and the content of both activators. 2. At a C:S ratio of 1:1–1:3, GHM is a multifunctional mineral additive that consists of both a ‘nucleator seed’ (GHCP) and ‘physically active additive’ (GQS). The combined and separate effects of these components produce parallel and sequential processes of micro-reinforcing and filling of the AASBC paste, which results in an accelerated and prolonged strengthening effect. 3. The presence of GHCP accelerates strengthening, with this effect becoming more pronounced with an increase in the amount added or its Ssp. In this way, a hardened AASC paste can exhibit a compressive strength of up to 9.4 MPa after just two days, depending on C:S ratio and dispersion of GHM. 4. A combination of GHCP and GQS at a ratio 1:1–1.5 provides the greatest strengthening effect with hardened AASC pastes after 28 days and up to 360 days, with an increase in compressive strength of up to 97.1% more being possible depending on the concentration and Ssp of the additive. 5. The addition of up to 50% GHM1-3 allows GGBFS to be replaced without loss of strength or a significant increase in the l/s ratio.

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