The Effect of Synthetic Hydrated Calcium Aluminate Additive on the Hydration Properties of OPC

Abstract

The effect of synthetic CAH (130°C; 8 h; CaO/(SiO2 + Al2O3) = 0.55; Al2O3/(SiO2 + Al2O3) = 0.1, 0.15) with different crystallinity on the hydration kinetics of OPC at early stages of hydration was investigated. Also, the formation mechanism of compounds during OPC hydration was highlighted. It was determined that the synthetic CAH accelerated the initial reaction and shortened the induction period. Also, the second and third exothermic reactions begun earlier, and, during the latter reaction, the higher values of the heat flow were obtained in comparison with pure OPC samples. At later stages of hydration, synthetic CAH affect the OPC hydration as the usual pozzolanic additives; moreover, the larger values of cumulative heat were reached. It should be noted that the nature of synthetic CAH samples accelerated the dissolution of gypsum and stimulates the earliest C3S hydration.

Academic Editor:Cristina Leonelli

Department of Silicate Technology, Kaunas University of Technology, Radvilenu 19, LT-50270 Kaunas, Lithuania

Received 12 May 2016; Revised 22 June 2016; Accepted 17 July 2016; 9 August 2016

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, silica fume, calcined clays, and natural pozzolans, are commonly blended with clinker to make Portland cement or are used as a partial replacement of this component in concrete [1-6]. The practice of using SCMs is increasing, with the world average percent clinker in cement having decreased from 85% in 2003 to 77% in 2010, and it is projected to further decrease to 71% in the future [7]. The use of SCMs is favorable to the industry, generally resulting in concrete with lower cost, lower environmental impact, higher long-term strength, and improved long-term durability [8-10].

It is known that SCMs can be of natural and synthetic origin or used as industrial by-products [11-20]. However, the application of natural minerals or industrial by-products in cement or concrete production is limited by the energy demanding processing of mentioned materials, which includes crushing, grinding, and size separation and, in some cases, it may also involve thermal activation. The latter process is applied for SCMs, which do not have or reach the required pozzolanic activity and also to alter their physical and chemical properties [11-13, 21].

For this reason, due to the high early strength, thermal and chemical stability, and excellent durability with respect to ordinary Portland cement (OPC) the synthetic SCMs have attracted a great scientific attention, when employed in building and ceramic industries [11, 22-29]. The one group of these compounds can be located in the CaO-Al₂ O₃ -SiO₂ -H₂ O system. During hydrothermal synthesis the interreaction between silica, alumina, and calcium ions results in the formation of compounds such as calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH), and calcium aluminium silicate hydrates (CASH). For this reason, many researchers keep their focus on tailoring the final structural and physical properties of mentioned materials mainly by changing the synthesis conditions (using different raw materials and changing C/S ratio of primary mixture or the duration and temperature of isothermal curing) [30-35]. In Baltakys et al. work, when the duration of hydrothermal synthesis (175°C; 0-24 h) was extended, a lower amount of CASH was obtained [35]. Meanwhile, in Meller et al. work [31-34] the stability of obtained phases in CaO-Al₂ O₃ -SiO₂ -H₂ O system was highlighted. It was found that the latter parameter and the formation mechanism of such compounds were significantly influenced by the synthesis temperature (from 200 to 350°C) and even small amounts of Al₂ O₃ additive.

In previous work [35] the influence of Al₂ O₃ on the formation of calcium aluminium silicate hydrates under hydrothermal conditions was also examined. It was determined that the largest amount of calcium aluminium silicate hydrates was obtained after 8 h of hydrothermal treatment at 130°C, in the mixtures with a higher amount of Al₂ O₃ (Al₂ O₃ /(SiO₂ + Al₂ O₃ ) = 0.15). Meanwhile, the formation of mentioned compounds is inhibited, when the duration of isothermal curing is extended from 16 to 72 h.

Aluminium readily enters the calcium silicate hydrate phase (C-S-H) of Portland cement, and this substitution is expected to play a significant role in many aspects of the chemical behavior of cement paste, including the cation and anion exchange behavior, solubility, and the progress of the reactions that occur during delayed ettringite formation [36-42].

In previous work [35] it was proved that the synthetic CAH (CaO/(SiO₂ + Al₂ O₃ ) = 0.55; Al₂ O₃ /(SiO₂ + Al₂ O₃ ) = 0.15) as an additive affects the early hydration of OPC. This compound effectively shortens the induction period and accelerates the second and the third exothermic reactions.

For this reason, the main objective of this study was to determine the effect of synthetic CAH with different crystallinity on the hydration kinetics of OPC at early stages of hydration. Also, the compound formation mechanism is highlighted.

2. Materials and Methods

2.1. Materials

In this paper the following reagents were used: SiO₂ ·n H₂ O ("Reaktiv," Russia) was grinded for 3 min in a vibrating cup "Pulverisette 9" mill (speed: 950 rpm) and sieved through a sieve with a mesh size of 80 μ m, _Sa = 1047 m² /kg by CILAS LD 1090 granulometer, the loss of ignition, 7.0 wt.%. Calcium oxide from Ca(OH)₂ ("Reaktiv," Russia) was additionally burned at 500°C temperature for 2 h, grinded for 30 s in a vibrating cup "Pulverisette 9" mill (speed: 650 rpm), and sieved through a sieve with a mesh size of 80 μ m; the quantity of free CaO is equal to 99 wt.%, _Sa = 961 m² /kg. γ -Al₂ O₃ was produced by burning aluminium hydroxide ("Sigma-Aldrich," Germany) at 475°C for 4 h, _Sa = 822 m² /kg.

2.2. Synthetic CAH Samples Preparation

Dry primary mixtures with CaO/(SiO₂ + Al₂ O₃ ) = 0.55 and Al₂ O₃ /(SiO₂ + Al₂ O₃ ) = 0.1; 0.15 were mixed with water to reach the water/solid ratio of the suspension equal to 10.0. The hydrothermal synthesis has been carried out in unstirred suspensions in 25 mL volume PTFE cells, which were placed in "Parr instruments" (Germany) autoclave, under saturated steam pressure at 130°C temperature for 8 h (the temperature was reached within 2 h). After hydrothermal treatments, the autoclave was quenched to room temperature. The suspensions after synthesis were filtered and products were rinsed with ethanol to prevent carbonization of materials, dried at 50 ± 5°C temperature for 24 h, and sieved through a sieve with a width of 80 μ m. These synthesis conditions were chosen according to previously published data [35].

2.3. Cementitious Admixtures with Synthetic CAH Samples

Samples of OPC were prepared in a laboratory grinding mill by grinding cement clinker (JSC "Akmenes cementas," Lithuania) with a 4.5% additive of gypsum ("Sigma-Aldrich," Germany) up to _Sa = 450 m² /kg. The chemical analysis and phase composition of clinker are shown in Table 1. Synthetic CAH sample was added as a partial replacement of the OPC at levels of 10% by weight of the total cementitious material. Therefore the water and cement ratio (W/C) of all OPC samples was equal to 0.5.

Table 1: Chemical and mineralogical composition of clinker.

Oxides	SiO2	Al2 O3	Fe2 O3	CaO	MgO	Na2 O	K2 O	S O 3 2 -	Ignition of losses	Insoluble particles
Amount, %	19.72	5.41	4.21	62.76	3.41	0.16	1.08	2.08	0.93	0.24
Minerals	3CaO·SiO2	2CaO·SiO2	3CaO·Al2 O3	4CaO·Al2 O3 ·Fe2 O3
Amount, %	63.19	8.89	7.21	12.81

In the next stage of experiment, synthesized CAH samples were added as a partial replacement of the OPC at levels of 10% by weight of the total cementitious material. The additives of hydrated calcium aluminate were labeled as OPC-CAH1 ((CaO/(SiO₂ + Al₂ O₃ ) = 0.55, Al₂ O₃ /(SiO₂ + Al₂ O₃ ) = 0.1)) and OPC-CAH2 (CaO/(SiO₂ + Al₂ O₃ ) = 0.55, Al₂ O₃ /(SiO₂ + Al₂ O₃ ) = 0.15) depending on the composition of primary mixtures.

2.4. Test Methods

The X-ray powder analysis (XRD) was performed on the D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) operating at the tube voltage of 40 kV and tube current of 40 mA. The X-ray beam was filtered with Ni 0.02 mm filter to select the CuK_α wavelength. Diffraction patterns were recorded in a Bragg-Brentano geometry using a fast counting detector Bruker LynxEye based on silicon strip technology. The specimens were scanned over the range 2θ = 3-70° at a scanning speed of 6°C min^-1 using a coupled two-theta/theta scan type.

Simultaneous thermal analysis (STA: differential scanning calorimetry, DSC, and thermogravimetry, TG) was also employed for measuring the thermal stability and phase transformation of samples at a heating rate of 15°C min^-1 ; the temperature ranged from 30°C up to 900°C under air atmosphere. The test was carried out on a Linseis instrument STA PT1000. The ceramic sample handlers and crucibles of Pt were used.

Differential scanning calorimetry (DSC) analysis was performed by a Netzsch DSC 214 Polyma instrument. This method was employed for measuring the thermal stability and phase transformation of samples at a heating rate of 10°C min^-1 , and the temperature ranged from 30°C up to 600°C under air atmosphere. Ceramic sample handlers and Al crucibles were used.

An eight-channel TAM Air III isothermal calorimeter was used to investigate the heat evolution rate of the samples. Glass ampoules (20 mL) each containing 3 g dry cementitious material were placed in the calorimeter and the injection units for each ampoule filled with amounts of water equivalent to a W/(OPC and additive) ratio of 0.5. After a steady temperature of 25°C had been reached, the water was injected into the ampoules and mixed inside the calorimeter with the dry material for 20 s (frequency 2-3 s^-1 ). The heat evolution rate was then measured over a period of 72 h. Repetition of the measurements showed deviations in total heat below 3% for samples of similar type. Apart from the first minutes of water additive and mixing, the heat evolution rates were essentially identical. The rate of heat evolution was calculated on the basis of a unit weight of OPC.

In order to investigate the minerology and chemical composition of compounds formed during hydration, the heat evolution experiments were repeated at 25°C for different time periods (1.8, 3, 9.5, 13, and 24 h), which corresponded to the onset/peak/end duration of different early hydration periods. Hydration of samples was stopped by using acetone. Later on, the samples were crushed to powder, dried at the temperature of 50±5°C, and put through a sieve with an 80 μ m mesh.

3. Results and Discussion

3.1. Heat Evolution in Hydrated Cementitious Admixtures with Synthetic CAH Samples

The data of cumulative heat of hydration as well as the rate of heat evolution of the binary blended pastes are presented in Figure 1.

Figure 1: The heat evolution rate (a-c) and cumulative heat (d) of OPC (1), OPC-CAH1 (2), and OPC-CAH2 (3) samples during the early stage of hydration.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

(d) [figure omitted; refer to PDF]

The heat of hydration curves for pure OPC and OPC with additives shows the typical five stages of the hydration reaction (the initial reaction, the induction period, the acceleratory period, the deceleratory period, and the period of slow continued reaction) as described in the literature [43-45].

It was determined that the additives of synthetic CAH samples in OPC samples accelerated the initial reaction (1-2 min) because an increase in the maximum heat evolution rate was observed from 0.005 Wg^-1 to 0.011 Wg^-1 (OPC-CAH2) and 0.018 Wg^-1 (OPC-CAH1) (Figure 1(a)). And also, the induction period, which is assigned to the growing C-S-H and CH on the surface of the particles of primary compounds, was effectively shortened. In pure OPC samples, the mentioned process continued after 3 h, while in a case of OPC-CAH1 and OPC-CAH1 only after ~1.8 h (Figure 1(b)). However, the accelerating effect begun earlier: from 1.9 h in the samples with an additive and from 3.2 h in pure OPC samples (Figure 1(c)). Moreover, the lower values of the heat flow of the second exothermic reaction, typical to the dissolution of C₃ S, were reached in OPC mixtures with additives: 0.0023 Wg^-1 at 5.02 h for OPC-CAH1 and 0.0023 Wg^-1 at 6.02 h for OPC-CAH2. Meanwhile, during the third exothermic reaction, which is characteristic to the dissolution of C₃ A and the formation of ettringite, the highest value of the heat flow (0.0032 Wg^-1 ) at 13.1 h was obtained in OPC-CAH2 sample (Figure 1(c)).

At later stages of hydration, synthetic CAH samples affect the OPC hydration as the usual pozzolanic additive; moreover, the largest value of cumulative heat was reached in the OPC-CAH2 sample (Figure 1(d)).

3.2. Quantity of Compounds in the Cement Paste

It was determined that, during early OPC hydration, the stability and reactivity of OPC-CAH1 and OPC-CAH2 significantly depend on the primary mixture composition used for hydrothermal synthesis. OPC-CAH1 additive fully reacted already after 1.8 h of hydration; meanwhile, in a case of OPC-CAH2, only 33% (14.65 J/g) of calcium aluminium hydrate reacted (Figure 2(a): endothermic effect at ~271°C; Table 2).

Table 2: The main characteristics of thermal effects typical of CAH.

Composition	Hydration time (h)	Onset, °C	Peak, °C	Heat of process, J/g	Unreacted CAH, wt.%
OPC-CAH2	0	250.2	268.6	21.81	100
1.8	253.8	271.0	14.65	67.17
3	254.5	273.3	14.51	66.53
5.5	256.7	276.3	13.67	62.68
9.5	261.6	280.5	12.24	56.12
13	265.2	282.5	11.94	54.75
16.5	264.1	286.0	11.71	53.69
24	267.7	286.8	10.39	47.64

Figure 2: DSC curves OPC with 10 wt.% additive after 1.8 h (a) and 16.5 h (b): 1, OPC; 2, OPC-CAH1; 3, OPC-CAH2.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

However, when the duration of OPC hydration was extended to 24 h, the amount of unreactive compounds was decreased (Figure 2; Table 2).

It should be noted that the nature of CAH accelerated the dissolution of gypsum. In the pure system, the latter compound fully reacted only after 16.5 hours of hydration, whereas in the samples with additives it fully reacted already after 13 h. It is clearly visible in DSC curve: the endothermic effect in a 105-135°C temperature range, which corresponded to the dehydration of gypsum, disappeared (Figure 2; Table 3). Moreover, in comparison with pure OPC samples, only a smaller amount of semicrystalline C-S-H (Figure 2: endothermic effect at 67°C; Table 3) as well as portlandite (Figure 2: endothermic effect at 440°C; Table 3) were obtained in the mixtures with additives by prolonging the duration of OPC hydration.

Table 3: The main characteristics of thermal effects typical of CSH and gypsum by DSC method.

Composition	Hydration time (h)	CSH	Gypsum
Onset, °C	Peak, °C	Heat of process, J/g	Onset, °C	Peak, °C	Heat of process, J/g	Unreacted gypsum, wt.%
OPC	1.8	45.8	66.7	14.89	112.5	122.4	16.23	69.01
3	48.0	64.4	17.22	108.1	118.3	16.69	70.96
5.5	52.4	73.6	28.12	111.1	119.4	10.97	46.64
9.5	54.7	76.9	40.88	113.3	119.8	6.86	29.15
13	57.5	82.0	53.01	116.2	121.3	0.80	3.36
16.5	61.0	87.8	71.84	--	--	--	--
24	58.8	86.3	77.42	--	--	--	--
OPC-CAH1	1.8	49.0	66.8	18.61	113.6	121.0	13.63	74.93
3	48.4	68.0	22.53	112.2	119.4	10.33	43.92
5.5	54.5	75.7	37.60	115.1	121.0	6.02	25.57
9.5	58.7	81.5	51.10	116.6	122.7	0.29	1.24
13	62.7	87.6	63.13	--	--	--	--
16.5	59.5	84.8	66.49	--	--	--	--
24	58.5	84.5	71.74	--	--	--	--
OPC-CAH2	1.8	44.4	63.0	17.20	110.5	118.3	13.54	67.60
3	49.4	69.3	21.10	111.7	119.6	11.15	55.67
5.5	55.2	76.9	37.32	114.7	120.8	4.74	23.68
9.5	64.7	85.1	54.86	116	121.8	0.51	2.54
13	61.9	85.8	64.14	--	--	--	--
16.5	61.4	85.1	58.51	--	--	--	--
24	60.2	86.6	66.50	--	--	--	--

In this report, the amounts of C₃ S that have reacted after heat evolution experiments at different time period under normal conditions were determined by the quantitative analysis (QXRD). The quantity of C₃ S was calculated from the intensity change of the basic reflection (d-spacing, 0.2604 nm). Each calculation was done five times, and it was determined that their data declined no more than ±2% from the mean (Figure 3). C₃ S quantity in the mixtures of pure OPC and OPC with additives (before hydration) was equal to 100%.

Figure 3: The quantity of unreactive C₃ S in pure OPC, OPC-CAH1, and OPC-CAH2 samples after different duration of hydration.

[figure omitted; refer to PDF]

It was determined that after 5.5 h of hydration, only 9% of C₃ S reacts in pure OPC samples and the further reduction of its quantity depends on duration of hydration (Figure 3). Meanwhile, within the same duration of hydration, in the samples with CAH1 and CAH2 additives, 21% and 18% of this compound reacted, respectively (Figure 3). It was noticed that mentioned additives stimulate the earliest C₃ S hydration in all samples (5.5-16.5 h) (Figure 3).

Presumably, the synthetic CAH sample also induced the formation mechanism of ettringite.

In order to prove this fact, the intensity change of the basic reflection of ettringite (d-spacing, 0.972 nm) in cement samples was evaluated by X-ray diffraction analysis (XRD) according to the area of mentioned diffraction maximum per shift at different hydration time.

The obtained results showed that after 3 hours of hydration, the area of the main diffraction peak typical to ettringite increased in two times in comparison with the pure OPC samples (Figure 4). The same tendency was also observed, when the duration of hydration was extended to 9.5 hours (Table 4).

Table 4: The values of the main diffraction peak area of ettringite in OPC, OPC-CAH1, and OPC-CAH2 samples.

Composition	Hydration time (h)
0	1.8	3	5.5	9.5	13	16.5	24
OPC	--	0.14	0.15	0.26	0.40	0.60	1.00	1.45
OPC-CAH1	--	0.15	0.37	0.42	0.89	0.90	1.18	1.39
OPC-CAH2	--	0.21	0.22	0.54	0.51	1.03	1.18	1.30

Figure 4: The change of the main diffraction peak of ettringite in samples after 3 h of hydration: 1, OPC; 2, OPC-CAH1; 3, OPC-CAH2.

[figure omitted; refer to PDF]

In addition, due to the accelerated dissolution of gypsum, a larger amount of formed ettringite was observed in OPC-CAH1 and OPC-CAH2 samples. Meanwhile, in an excess of gypsum in the pure OPC samples, the area of the main diffraction maximum of ettringite was slightly higher after 24 h of hydration (Table 4).

4. Conclusions

It was examined that the additives accelerated the initial reaction and shortened the induction period. Also, the second and third exothermic reactions begun earlier, and, during the latter reaction, the higher values of the heat flow were obtained in comparison with pure OPC samples. At later stages of hydration, synthetic CAH affect the OPC hydration as the usual pozzolanic additives; moreover, the larger values of cumulative heat were reached.

It was noticed that additives stimulate the earliest C₃ S hydration (5.5-16.5 h): after 5.5 h of hydration, only 9% of C₃ S reacts in pure OPC samples and the further reduction of its quantity depends on duration of hydration. Meanwhile, in a case of CAH1 and CAH2 additives, 21% and 18% of this compound reacted.

It should be noted that the nature of synthetic CAH samples accelerated the dissolution of gypsum. In the pure system, the latter compound fully reacted only after 16.5 hours of hydration, whereas in the samples with additives it reacted already after 13 h. For this reason, a larger amount of formed ettringite was observed in OPC-CAH1 and OPC-CAH2 samples till 16.5 h of hydration.

Acknowledgments

This research was funded by a grant (no. MIP-025/2014) from the Research Council of Lithuania.

References

[1] C. H. Un, J. G. Sanjayan, R. San Nicolas, J. S. J. van Deventer, "Predictions of long-term deflection of geopolymer concrete beams," Construction and Building Materials , vol. 94, pp. 10-19, 2015.

[2] E. Gartner, H. Hirao, "A review of alternative approaches to the reduction of CO₂ emissions associated with the manufacture of the binder phase in concrete," Cement and Concrete Research , vol. 78, pp. 126-142, 2015.

[3] K.-H. Yang, Y.-B. Jung, M.-S. Cho, S.-H. Tae, "Effect of supplementary cementitious materials on reduction of CO₂ emissions from concrete," Journal of Cleaner Production , vol. 103, pp. 774-783, 2015.

[4] E. Aprianti, P. Shafigh, S. Bahri, J. N. Farahani, "Supplementary cementitious materials origin from agricultural wastes--a review," Construction and Building Materials , vol. 74, pp. 176-187, 2015.

[5] S. H. Kosmatka, B. Kerkhoff, W. C. Panarese, "Fly ash, slag, silica fume, and natural pozzolans,", chapter 3 Design and Control of Concrete Mixtures, EB001 , Portland Cement Association, 2008., 14th.

[6] M. H. Cornejo, J. Elsen, C. Paredes, H. Baykara, "Thermomechanical treatment of two Ecuadorian zeolite-rich tuffs and their potential usage as supplementary cementitious materials," Journal of Thermal Analysis and Calorimetry , vol. 115, no. 1, pp. 309-321, 2014.

[7] M. C. G. Juenger, R. Siddique, "Recent advances in understanding the role of supplementary cementitious materials in concrete," Cement and Concrete Research , vol. 78, pp. 71-80, 2015.

[8] A. Korpa, T. Kowald, R. Trettin, "Hydration behaviour, structure and morphology of hydration phases in advanced cement-based systems containing micro and nanoscale pozzolanic additives," Cement and Concrete Research , vol. 38, no. 7, pp. 955-962, 2008.

[9] N. M. Khalil, E. M. Hassan, M. M. E. Shakdofa, M. Farahat, "Beneficiation of the huge waste quantities of barley and rice husks as well as coal fly ashes as additives for Portland cement," Journal of Industrial and Engineering Chemistry , vol. 20, no. 5, pp. 2998-3008, 2014.

[10] M. Hamidi, L. Kacimi, M. Cyr, P. Clastres, "Evaluation and improvement of pozzolanic activity of andesite for its use in eco-efficient cement," Construction and Building Materials , vol. 47, pp. 1268-1277, 2013.

[11] M. S. Amin, S. A. Abo-El-Enein, A. A. Rahman, K. A. Alfalous, "Artificial pozzolanic cement pastes containing burnt clay with and without silica fume: physicochemical, microstructural and thermal characteristics," Journal of Thermal Analysis and Calorimetry , vol. 107, no. 3, pp. 1105-1115, 2012.

[12] B. Lothenbach, K. Scrivener, R. D. Hooton, "Supplementary cementitious materials," Cement and Concrete Research , vol. 41, no. 12, pp. 1244-1256, 2011.

[13] C. A. Utton, M. Hayes, J. Hill, N. B. Milestone, J. H. Sharp, "Effect of temperatures up to 90°C on the early hydration of Portland-Blastfurnace slag cements," Journal of the American Ceramic Society , vol. 91, pp. 948-954, 2008.

[14] L. Krishnan, S. Karthikeyan, S. Nathiya, K. Suganya, "Geopolymer concrete an eco-friendly construction material," International Journal of Research in Engineering and Technology , vol. 3, pp. 164-167, 2014.

[15] M. E. Natali, S. Manzi, C. Chiavari, M. C. Bignozzi, S. A. Bernal, J. L. Provis, "Steel reinforced geopolymer mortar: corrosion behaviour in chloride-rich environment," in Proceedings of the 34th Cement and Concrete Science Conference, pp. 41-44, University of Sheffield, September 2014.

[16] A. Rafeet, R. Vinai, W. Sha, M. Soutsos, S. A. Bernal, J. L. Provis, "Alkali activated fuel ash and slag mixes: optimization study from mortars to concrete building blocks," in Proceedings of the 34th Cement and Concrete Science Conference, pp. 349-354, University of Sheffield, September 2014.

[17] C. Villa, E. T. Pecina, R. Torres, L. Gómez, "Geopolymer synthesis using alkaline activation of natural zeolite," Construction and Building Materials , vol. 24, no. 11, pp. 2084-2090, 2010.

[18] M. Babaee, A. Castel, A. Akbarnezhad, S. A. Bernal, J. L. Provis, "Active steel corrosion in blended slag and fly ash geopolymer concrete," in Proceedings of 34th Cement and Concrete Science Conference, pp. 55-58, University of Sheffield, Sheffield, UK, 2014.

[19] T. Bakharev, "Geopolymeric materials prepared using Class F fly ash and elevated temperature curing," Cement and Concrete Research , vol. 35, no. 6, pp. 1224-1232, 2005.

[20] T. Bakharev, "Resistance of geopolymer materials to acid attack," Cement and Concrete Research , vol. 35, no. 4, pp. 658-670, 2005.

[21] P. K. Mehta, P. J. M. Monteiro Concrete: Microstructure, Properties, and Materials , The McGraw-Hill Companies, 2005., 3rd.

[22] G. Roviello, L. Ricciotti, C. Ferone, F. Colangelo, R. Cioffi, S. A. Bernal, J. L. Provis, "Hybrid organic-inorganic materials: novel perspectives for the application of geopolymer based materials," in Proceedings of the 34th Cement and Concrete Science Conference, pp. 199-202, University of Sheffield, 2014.

[23] K. Baltakys, A. Eisinas, T. Dizhbite, L. Jasina, R. Siauciunas, S. Kitrys, "The influence of hydrothermal synthesis conditions on gyrolite texture and specific surface area," Materials and Structures , vol. 44, no. 9, pp. 1687-1701, 2011.

[24] K. Baltakys, R. Siauciunas, R. Gendvilas, A. Eisinas, T. Dambrauskas, S. Kitrys, "Physically and chemically bound H₂ O in the α -C₂ S hydrate structure," Journal of Thermal Analysis and Calorimetry , vol. 118, no. 2, pp. 807-816, 2014.

[25] H. C. Yong, H. Kamarudin, M. M. Al Bakri Abdullah, L. Y. Ming, "Effect of alkali concentration on mechanical properties of kaolin geopolymers," Romanian Journal of Materials , vol. 42, no. 2, pp. 179-186, 2012.

[26] M. A. Longhi, F. Gaedke, E. D. Rodríguez, A. C. Passuello, A. P. Kirchheim, S. A. Bernal, J. L. Provis, "Geopolymers based on calcined kaolin sludge/bottom ash blends and an alternative sodium silicate activator," in Proceedings of 34th Cement and Concrete Science Conference, pp. 359-362, University of Sheffield, 2014.

[27] B. Walkley, J. S. J. van Deventer, S. A. Bernal, J. L. Provis, "Stoichiometrically controlled C-A-S-H / N-A-S-H gel blends via alkali-activation of synthetic precursors," in Proceedings of the 34th Cement and Concrete Science Conference, pp. 27-32, University of Sheffield, Sheffield, UK, 2014.

[28] K. Baltakys, T. Dambrauskas, R. Siauciunas, A. Eisinas, "The formation of α -C₂ S hydrate in the mixtures with CaO/SiO₂ = 1.75 by hydrothermal treatment at 200°C," Revista Romana de Materiale--Romanian Journal of Materials , vol. 44, pp. 109-155, 2014.

[29] G. Mucsi, Z. Molnár, Á. Rácz, R. Szabó, B. Csoke, S. A. Bernal, J. L. Provis, "Geopolymer from mechanically activated low and high calcium fly ash," in Proceedings of the 34th Cement and Concrete Science Conference, pp. 1391-1396, University of Sheffield, Sheffield, UK, 2014.

[30] R. Siauciunas, A. Baltusnikas, "Influence of SiO₂ modification on hydrogarnets formation during hydrothermal synthesis," Cement and Concrete Research , vol. 33, no. 11, pp. 1789-1793, 2003.

[31] N. Meller, C. Hall, K. Kyritsis, G. Giriat, "Synthesis of cement based CaO-Al₂ O₃ -SiO₂ -H₂ O (CASH) hydroceramics at 200 and 250 °C: ex-situ and in-situ diffraction," Cement and Concrete Research , vol. 37, no. 6, pp. 823-833, 2007.

[32] N. Meller, K. Kyritsis, C. Hall, "The mineralogy of the CaO-Al₂ O₃ -SiO₂ -H₂ O (CASH) hydroceramic system from 200 to 350 °C," Cement and Concrete Research , vol. 39, no. 1, pp. 45-53, 2009.

[33] N. Meller, C. Hall, J. S. Phipps, "A new phase diagram for the CaO-Al₂ O₃ -SiO₂ -H₂ O hydroceramic system at 200 °C," Materials Research Bulletin , vol. 40, no. 5, pp. 715-723, 2005.

[34] N. Meller, K. Kyritsis, C. Hall, "The hydrothermal decomposition of calcium monosulfoaluminate 14-hydrate to katoite hydrogarnet and β -anhydrite: an in-situ synchrotron X-ray diffraction study," Journal of Solid State Chemistry , vol. 182, no. 10, pp. 2743-2747, 2009.

[35] K. Baltakys, A. Eisinas, J. Doneliene, D. Monstvilaite, A. Bankauskaite, A. Urbutis, "The hydrothermal synthesis of calcium aluminium silicate hydrates and its application on Portland cement hydration," Romanian Journal of Materials , vol. 45, no. 3, pp. 218-225, 2015.

[36] C. A. Ríos, C. D. Williams, M. A. Fullen, "Hydrothermal synthesis of hydrogarnet and tobermorite at 175 °C from kaolinite and metakaolinite in the CaO-Al₂ O₃ -SiO₂ -H₂ O system: a comparative study," Applied Clay Science , vol. 43, no. 2, pp. 228-237, 2009.

[37] I. G. Richardson, "The nature of C-S-H in hardened cements," Cement and Concrete Research , vol. 29, no. 8, pp. 1131-1147, 1999.

[38] I. G. Richardson, J. G. Cabrera, "Nature of C-S-H in model slag-cements," Cement and Concrete Composites , vol. 22, no. 4, pp. 259-266, 2000.

[39] A. R. Brough, A. Katz, G.-K. Sun, L. J. Struble, R. J. Kirkpatrick, J. F. Young, "Adiabatically cured alkali-activated cement-based wasteforms containing high levels of fly ash, formation of zeolites and Al-substituted C-S-H," Cement and Concrete Research , vol. 31, pp. 1437-1447, 2001.

[40] J. Schneider, M. A. Cincotto, H. Panepucci, "29Si and 27Al high-resolution NMR characterization of calcium silicate hydrate phases in activated blast-furnace slag pastes," Cement and Concrete Research , vol. 31, no. 7, pp. 993-1001, 2001.

[41] O. P. Shrivastava, R. Shrivastava, "Sr²⁺ uptake and leachability study on cured aluminum-substituted tobermorite-OPC admixtures," Cement and Concrete Research , vol. 31, no. 9, pp. 1251-1255, 2001.

[42] S.-Y. Hong, F. P. Glasser, "Alkali sorption by C-S-H and C-A-S-H gels: part II. Role of alumina," Cement and Concrete Research , vol. 32, no. 7, pp. 1101-1111, 2002.

[43] D. Jansen, F. Goetz-Neunhoeffer, B. Lothenbach, J. Neubauer, "The early hydration of Ordinary Portland Cement (OPC): an approach comparing measured heat flow with calculated heat flow from QXRD," Cement and Concrete Research , vol. 42, no. 1, pp. 134-138, 2012.

[44] P. Juilland, E. Gallucci, R. Flatt, K. L. Scrivener, "Dissolution theory applied to the induction period in alite hydration," Cement and Concrete Research , vol. 40, no. 6, pp. 831-844, 2010.

[45] A. Kumar, S. Bishnoi, K. L. Scrivener, "Modelling early age hydration kinetics of alite," Cement and Concrete Research , vol. 42, no. 7, pp. 903-918, 2012.