Influence of delayed ettringite formation on the durability of mortars: the importance of thermal control

Original article

Зомахун К.В., Хоуану К.А., Содомон М., Самченко С.В. Влияние позднего образования эттрингита на долговечность растворов: важность термического контроля. Нанотехнологии в строительстве. 2026;18(3):363–378. – EDN: NYYMFH.

In African countries, particularly in Benin, climatic conditions are characterized by high temperatures, which frequently exceed 30 °C and may rise even higher depending on the season. These thermal conditions constitute an important environmental factor influencing the behavior of construction materials, especially concrete and mortar.

High temperatures during the early stages of concrete hardening or during mortar application may alter the me- chanical and physicochemical properties of these materials. Numerous studies have shown that elevated temperatures (above 25 °C) accelerate cement hydration reactions [1–3]. An increase in temperature during concrete curing disrupts the internal microstructure and promotes the development of defects in concrete, including internal sulfate reaction (ISR).

Delayed ettringite formation (DEF) is a concrete anomaly first identified in real structures at the end of the 1980s. Some researchers reported cases in which precast elements subjected to heat curing subsequently

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experienced deterioration, leading to a reduction in their mechanical properties [4].

Delayed ettringite formation (DEF) was first identified in precast reinforced concrete elements in the late 1980s [4]. Subsequently, numerous case studies and expert reports were published in the literature, including [5] and [6], describing cases involving the simultaneous occurrence of alkali–silica reaction and delayed ettringite formation [7–9]. In France, internal sulfate reaction was identified in several dozen massive concrete structures [10–13], as well as in precast reinforced concrete elements [14].

Sulfate attacks represent a major durability problem for concrete structures. These attacks damage concrete by degrading its mechanical properties. In particular, internal sulfate reaction leads to delayed ettringite formation, which causes expansion within the concrete and the development of network cracking [15].

When a structure is affected, it is essential to accurately diagnose the development of defects, assess its serviceability, predict the mechanical evolution of the structure, and apply appropriate repair methods. Achieving these objectives requires a thorough understanding of the consequences of DEF at both the microstructural and macrostructural levels. Numerous experimental and theoretical studies have been carried out to identify the physicochemical mechanisms and parameters influencing delayed ettringite formation (DEF). However, the complexity of the underlying processes makes it difficult to directly extrapolate this knowledge in order to explain structural-scale effects. Therefore, macroscopic approaches may provide additional insights based on the knowledge already available in the literature [2].

The development of defects also affects mortar coatings applied to concrete substrates, thereby degrading the visible surface quality, whereas the actual cause of cracking and other observed damage lies in the DEF occurring within the concrete itself. Such deterioration generally appears after several years or even decades, making both the study of this anomaly and the selection of appropriate repair strategies aimed at extending the service life of damaged structures particularly difficult [3].

The objective of this work is to evaluate, under real environmental conditions, the risk of delayed ettringite formation (DEF) in concrete, and then to analyze the behavior of coatings applied to this substrate in order to assess the long-term evolution and surface condition of the mortar.

To achieve this objective, the adopted methodology was based on the following main aspects:

• •    Evaluation of temperature rise in accordance with the IFSTTAR guidelines (October 2017) [16];

• •    Conducting a test on a 1 m cubic specimen equipped with thermally insulated sensors (semi-adiabatic conditions), using the concrete mixture exhibiting the highest temperature rise;

• •    Conducting a test on a 1.6 m cubic specimen equipped with non-insulated sensors, using the concrete mixture exhibiting the highest temperature rise;

• •    Application of a plaster coating onto the corresponding concrete using mortars reinforced with jute fibers, in order to monitor the surface condition of the mortar over a period of 90 days.

MATERIALS AND METHODS

Research Equipment

During the experimental program, laboratory equipment complying with current standards for the analysis of the mechanical and physical properties, as well as the durability of materials, was used. All the equipment employed was certified and recognized within standardized protocols, thereby ensuring the reliability and reproducibility of the obtained results.

Materials

The materials used in this work are as follows:

• •    0/4 sand from Cocotomey in southern Benin.

• •    Crushed stone (0/5, 5/15, and 15/25) from Setto in central Benin.

• •    Admixtures from SIKA Chemical Corporation (Sika ViscoFlow 800 POWER, Sika ViscoFlow 200 TIMER, and SikaTard 300 SLS).

• •    CEM III/B 42.5N cement from NOCIBE Benin (New Cement Plant of Benin).

• •    The mixing water comes from SONEB (Benin National Water Corporation).

• •    The jute fibers come from jute bags from the Dantopka market (Benin).

Methods

The tests performed on the concrete and mortar were carried out in accordance with the applicable regulations regarding the constituents and performance of concrete. They are primarily based on harmonized European standards (NF EN) adopted by AFNOR, as well as on French standards from the NF P series and national normative documents such as fascicule de documentation.

Tests related to delayed ettringite formation (DEF) were conducted following standardized and widely recognized protocols for evaluating this type of concrete pathology. In particular, the experimental procedures were carried out in accordance with the IFSTTAR (French Institute of Science and Technology for Transport, Development and Networks) recommendations aimed at preventing and identifying ISR-related disorders.

To ensure the reliability and statistical robustness of the results, each test was repeated six (6) times. The re-

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Fig. 1. a – Sand equivalent test apparatus; b – Free bleeding test apparatus; c – Flow table; d – Slump test (Abrams cone test); e – Thermocouple; f – Methylene blue value test apparatus

Fig. 2. a – Hydrometer ; b – Balances ; c – Tensile testing machine

sults presented correspond to the average of the measurements obtained, which helps minimize the influence of experimental uncertainties and improves the scientific significance of the conclusions.

RESULTS

Material Characterization

Sand

The measured characteristics are summarized in the table above and are compared with the standard speci- fications. According to the results, the sand gradation complies with the requirements.

The measured values show that the 0/4 sand from Cocotomey (Benin) can be classified under Code A, according to Article 10 of standard NF P 18-545 (2021). It therefore meets the requirements and is considered suitable for use in concrete.

Crushed aggregate

Crushed aggregate 15/25

The measured values show that the 15/25 fraction can be classified under code A according to Article 10

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Fig. 3. a – Series of sieves; b – Oven; c – Compression testing machine

Table 1. Results of the particle size analysis of the sand

Sieve (mm)		5.6	4 (2*D)	2.8 (1.4*D)	2	1	0.50	0.250	0.125	0.063
Unit – Method		Cumulative passer (%) – NF EN 933-1 (2012)
Values found		100	100	100	99	96	72	30	6	1.0
Limit values	Max	–	100	100	100	100	–	50	-	10
	Min	–	100	95	90	60	–	0	-	0

Table 2. Summary of the test results on sand

Parameter	Method	Unit	Average Value	Specification
Real density	NF EN 1097-6 (2022)	Mg/m3	2,62	Declared value
Water absorption coefficient		%	0,5	≤ 2,5 AbA ≤ 5,0 AbB
Fineness modulus (FM)	NF EN 12620 + A1 (2008)	–	1,97	1,5 ≤ FM ≤ 4
Fines content	NF EN 933-1 (2012)	%	1,0	≤ 10 fA ≤ 16 fB
Sand equivalent (SE)	NF EN 933-8 (2015)	%	77	≥ 65 PA ≥ 60 P B
Friability coefficient (FS)	NF P 18-576 (2013)	%	18	–
Methylene blue value (MB)	NF EN 933-9+A1 (2022)	g/kg	1,0	≤ 1,5 (PA et PB)
Prohibited impurities	XP P 18-546 (2020)		<0,01	≤0,1
Water-soluble chlorides	NF EN 1744-1+A1 (2014)	%	0,004	To declare
Total sulfur	NF EN 1744-1+A1 (2014)	%	0,12	≤ 1 (S A et S B )
Acid-soluble sulfates	NF EN 1744-1+A1 (2014)	%	0,04	≤ 0,2 (SA et SB)
Humic substances	NF EN 1744-1+A1 (2014)	–	Absent	Absent
Soluble active alkalis	XP P 18-544 (2015)	%Na2Oeq	0,0300	Declared value
Alkali-reaction	FD P 18-542 (2023)	–	NR	To declare

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of standard NF P 18-545 (2021). Therefore, this material is considered suitable for use in concrete mixes with a 14/22.4 grading, corresponding to particle size classes: series 1 and series 2 (the 15/25 fraction is not recognized in standard NF EN 12620 (2008)).

Crushed aggregate 5/15

The measured values show that the 0/5 aggregate can be classified under Code A in accordance with Article 10 of standard NF P 18-545 (2021). Consequently, this material is considered suitable for use in concrete mix designs with a 0/5.6 grading, corresponding to a granular

class: base series + series 1 (the 0/5 size fraction is not recognized in NF EN 12620 (2008)).

Crushed aggregate 0/5

The measured values show that the 0/5 aggregate can be classified under Code A in accordance with Article 10 of standard NF P 18-545 (2021). Consequently, this material is considered suitable for use in concrete mix designs with a 0/5.6 grading corresponding to a particle size class: basic series + series 1 (the 0/5 size fraction is not recognized in standard NF EN 12620 (2008))

Table 3. Results of the particle size distribution analysis of the 15/25 crushed aggregate

Sieve (mm)		31.5 (1.4*D	25	22.4 (D)	20	16	14 (d)	12.5	6.3 (d/2)	0.063
Unit – Method		Cumulative passer (%) – NF EN 933-1 (2012)
Values found		100	99	94	84	39	11	4	2	0.8
Limit values	Max	100	–	99	–	–	20	–	5	1.5
	Min	98	–	90	–	–	0	–	0	0

Table 4. Summary of test results on the 15/25 crushed aggregate

Parameter	Method	Unit	Average Value	Specification
Real density	NF EN 1097-6 (2022) et 9 (2014)	Mg/m3	2.76	Declared value
Water absorption coefficient		%	0.4	≤ 2,5 AbA ≤ 5,0 AbB
Flakiness Index (FI)	NF EN 933-3 (2012)	%	11	≤ 20 FIA ≤ 35 FIB
Alkali-reaction	FD P 18-542 (2023)	%	NR	To declare

Table 5. Results of the particle size distribution analysis of the 15/25 crushed aggregate

Sieve (mm)		20 (1.4*D)	16	14 (D)	11.2	10 (D/1.4)	8	5.6 (d)	4	3.15 (d/2)	0.063
Unit – Method		Cumulative passer (%) – NF EN 933-1 (2012)
Values found		100	100	93	69	54	30	8	3	3	1.2
Limit values	Max	100	–	99	–	70	–	20	–	5	1.5
	Min	98	–	80	–	25	–	0	–	0	0

Table 6. Summary of test results on the 5/15 crushed aggregate

Parameter	Method	Unit	Average Value	Specification
Real density	NF EN 1097-6 (2022) et 9 (2014)	Mg/m3	2.77	Declared value
Water absorption coefficient		%	0.4	≤ 2,5 AbA ≤ 5,0 AbB
Flakiness Index (FI)	NF EN 933-3 (2012)	%	14	≤ 20 LI A ≤ 35 FIB
Fragmentation resistance	NF EN 1097-2 (2020) et 5 (2008)	%	27	≤ 20 LA A ≤ 40 LA B
Micro-Deval coefficient	NF EN 1097-1 (2023)	%	16	Declared value
Total sulfur content	NF EN 1744-1 + A1 – 11.1 (2014)	%	0.08	≤1
Water-soluble sulfates	NF EN 1744-1 + A1 – 12 (2014)	%	0.03	≤ 0.2

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Table 7. Results of the particle size distribution analysis of the 0/5 crushed aggregate

Sieve (mm)		12.5	11.2 (2D)	7.1 (1.4D)	5.6 (D)	4	2	1	0.5	0.25	0.125	0.063
Unit - Method		Cumulative passer (%) – NF EN 933-1 (2012)
Values found		100	100	100	100	85	57	40	30	24	16	9.4
Limit values	Max	100	100	100	100	90	–	60	–	–	–	11
	Min	100	100	95	90	50	–	20	–	–	–	1

Table 8. Summary of test results on the 0/5 crushed aggregate

Parameter	Method	Unit	Average Value	Specification
Real density	NF EN 1097-6 (2022) et 9 (2014)	Mg/m3	2.76	Declared value
Water absorption coefficient		%	0.4	≤ 2,5 AbA ≤ 5,0 AbB
Fine content	NF EN 933-1 (2012)	%	9.4	≤ 11
Methylene blue value	NF EN 933-9 + A1 (2022)	g/kg	0.2	≤ 1.5 (PAet PB)
Water-soluble chloride	NF EN 1744-1+A1 (2014)	%	< 0.002	To declare
Humic substances	NF EN 1744-1+A1 – 15.1 (2014)	–	Absent	Absent
Water-soluble active alkalis	XP P 18-544 (2015)	%Na2Oeq	0.0174	Declared value
Alkali-reaction	FD P 18-542 (2023)	–	NR	To declare

Le cement

Table 9. Chemical and Physical Characteristics of CEM III 42.5 Cement

Analyses	Unit	Result
Chemical Properties of Hydraulic Binders
Siliceous constituents (P)	%	0.00
Calcareous constituents (C)	%	0.00
Setting regulator (R)	%	3.70
Clinker content (K)	%	30.80
Slag content (L)	%	65.51
Al₂O₃ (XRF)	% m/m	10.38
CaO (XRF)	% m/m	48.04
Fe₂O₃ (XRF)	% m/m	1.31
K₂O (XRF)	% m/m	0.35
MgO (XRF)	% m/m	5.17
Mn₂O₃ (XRF)	% m/m	0.18
Na₂O (XRF)	% m/m	0.29
SiO₂ (XRF)	% m/m	28.26
TiO₂ (XRF)	% m/m	0.54
Na₂O equivalent (XRF)	% m/m	0.52
SiO₂ + Al₂O₃ + Fe₂O₃ (XRF)	% m/m	39.95

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Jute fibers

The jute fibers used in the cement mortar are short jute fibers with a maximum length of 5 mm. They were obtained through the preparation and treatment process described in [17] . This procedure, which has already been experimentally validated, ensured good fiber dispersion within the cementitious matrix, thereby providing a homogeneous mixture, as demonstrated in the referenced study.

– Prevention level

For our study, the structural category is Category II. The exposure class is XH3, as the concrete structure is regularly exposed to water splashing.

According to the matrix above, the corresponding exposure class is Cs.

– Precautions related to the prevention level

For this level of prevention, the risk of distress with regard to delayed ettringite formation (DEF) shall be taken into account through one of the following two precautionary measures, designated 1 and 2:

• 1.    The maximum temperature reached within the concrete shall remain below 70 °C.

• 2.    If the maximum temperature reached within the concrete cannot be maintained below 70 °C, it shall re-

main below 80 °C, and at least one of the following six conditions shall be satisfied:

• •    Thermal curing shall be properly controlled (Note 1); the duration during which the concrete temperature exceeds 70 °C shall not exceed 4 hours, and the active equivalent alkali content of the concrete shall be less than 3 kg/m³;

• •    Use of a cement complying with standard NF P 15-319 (ES), or a sulfate-resisting (SR) cement in accordance with standard NF EN 197-1 (except CEM I SR3 and SR5 cements), for concretes subjected to temperatures above 70 °C for periods exceeding 10 hours. CEM I SR3 and SR5 cements are permitted where the duration above 70 °C is less than 10 hours, provided that the active equivalent alkali content of the concrete is lower than 3 kg/m³;

• •    Use of a CEM I SR3 or SR5 cement bearing the NF Hydraulic Binders certification mark and qualified in accordance with the methodology specified in Annex 5, where the duration above 70 °C exceeds 10 hours;

• •    Use of cements not complying with NF P 15-319 (ES), of types CEM II/B-V, CEM II/B-Q, CEM II/B-M (S-V) with more than 20% fly ash, CEM III/A or CEM V, provided that all these cements have an SO₃

Сontinuation of the Table 9

Analyses	Unit	Result
MnO (XRF)	% m/m	0.16
Loss on ignition at 950 °C	% m/m	2.45
Mechanical Properties of Hydraulic Binders
Analyses	Unit	Result
Average compressive strength (2 days)	MPa	14.2
Average compressive strength (7 days)	MPa	35.1
Average compressive strength (28 days)	MPa	49.8
Propriétés physiques des liants hydrauliques
Analyses	Unit	Result
Soundness	mm	0
Water content for standard consistency	%	35.0
Specific surface (Blaine)	cm²/g	5350
Specific surface (Blaine)	m²/kg	535
Initial setting time	min	250
Average density	g/cm³	2.80

Table 10. Chemical characterization of jute fibers [16]

Designation	Alpha Cellulose (%)	Hemicellulose (%)	Lignin (%)	Ash (%)	Density (g/cm³)	pH
Average value	45.50	23.0	3.9	0.58	1.46	5.63

Table 11. Constituent components of the sprayed concrete mix design

Sprayed Concrete Constituent	Dosage (kg/m³)	Specification (kg/m³)
CEM III/B 42.5 N – NOCIBE	420	≥380
Lagoon sand 0/4	633	–
Crushed aggregate 0/5	293	–
Crushed aggregate 5/15	480	–
Crushed aggregate 15/25	339	–
Sika ViscoFlow 800 Power	3.36 (0.80% of C)	–
Sika ViscoFlow 200 Timer	1.47 (0.35% of C)	–
Sika SikaTard 300 SLS	1.26 (0.30% of C)	–
Effective water	180.0	–
Theoretical density	2317	–

Table 12. Calculation performed in accordance with the IFSTTAR recommendations

Constituent	Sprayed concrete
Concrete thickness	1.02 m
Maximum temperature for Cs level	80 °C
Concrete density (kg/m³)	2317
Cement content CEM III/42.5N – NOCIBE	420
Cement compressive strength at 2 days (MPa)	13
Cement compressive strength at 28 days (MPa)	47
Q41h cement (J/g)	178
Q120h cement (J/g)	194
Qm/Q41	1.39
Effective water (kg)	180
Effective water / Equivalent binder	0.43
Impact E/C-α	0.98
Adiabatic temperature rise ΔT (without heat dissipation)	39.5 °C
Thermal loss R	0.492
Temperature rise with heat dissipation ΔT	19.4 °C
Maximum initial temperature of the concrete	60.2 °C

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Table 13. Mechanical and physical properties of fresh and hardened concrete

Parameter		Specification	Test Results on Fresh Concrete
			BP 1	BP 2	BP 3
Cube DEF			1 x 1 x1m	1.6 x 1.6 x 1.6 m	1.6 x 1.6 x 1.6 m
Fresh concrete density (kg/m3)			2418	2381
Entrapped air (%)			1.0	1.3	1.1
Bleeding (ml/min)			0	0	0
Ambient temperature (°C)			32.0	32.5	32.0
Concrete temperature (°C)		≤ 35	34.2	33.6	38
Slump (mm)		210±30	220	223	230
Inverted cone (s)		≤ 4	2.8	2.8	2.0
Impact mark (mm)		600±50	600	600	620
			Test Results on Hardened Concrete
Compression MPA	7 days		41.5	45.9	35.3
	28 days	≥ 49	51.9	55.2	42.6
Tensile strength MPA	28 days	≥ 2.7	2.9	3.4	2.34

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content not exceeding 3% and are manufactured from clinker with a C3A content not exceeding 8%;

Use, in combination with CEM I or CEM II/A cement, of fly ash complying with NF EN 450-1, ground granulated blast furnace slag complying with NF EN 15167-1, silica fume complying with NF EN 132631, or metakaolin complying with NF P 18-513. Their proportion in the binder (cement + additions) shall be at least 20% for fly ash, 35% for ground granulated blast furnace slag, 10% for silica fume (Note 2), and 20% for metakaolin. The binder used shall meet the following requirements: C3A (relative to clinker) < 8% and SO₃ (relative to binder) < 3%;

Verification of concrete durability with regard to DEF by means of a performance test and compliance with the decision criteria.

Due to the use of an SR cement in accordance with NF EN 197-1, the maximum permissible temperature during concrete setting is 80 °C.

– Evaluated mix design

In order to assess the risk of delayed ettringite formation (DEF), a mix design leading to the highest temperature rise, with a high cement content, will be adopted.

The composition of the investigated mix, designated “BPS XA2-XA3 C35/45 Cl 0.65 210 ± 30 mm”, hereafter referred to as “Sprayed Concrete”, is presented in the Table 11.

– Estimation of temperature rise

The calculation carried out in accordance with the provisions of the DEF guidelines is presented in the Table 12.

The maximum concrete temperature at the time of placement must be lower than 60.2 °C in order to prevent the risk of internal sulfate reaction. Since the maximum permitted concrete temperature at the time of casting is 35 °C, this requirement will be met.

BP1: cubic specimen with dimensions of 1×1×1 m.

BP2: cubic specimen with dimensions of 1.6×1.6×1.6 m.

BP3: cubic specimen with dimensions of 1.6×1.6×1.6 m.

– Thermal test

A 1 m³ cube insulated with 10 cm of polystyrene on all six faces and two cubes measuring 1.6 m on each side were cast. The specimens were instrumented with three thermocouples (one at the top, one at the center, and one at the bottom). A fourth thermocouple was used to record the ambient temperature near the element. The cube was monitored over seven consecutive days.

– Thermal test on the 1 × 1 × 1 m cube (BP1)

The curves recorded by the three probes embedded in the concrete are similar, and the maximum temperature is maintained for several hours.

The maximum temperature rise recorded during the test was 31.2 °C (65.2 - 34.0). Given that the maximum concrete temperature at placement was 34.2 °C, the maximum temperature that could be reached during setting is therefore 66.2 °C. Since this temperature is below the 80 °C threshold associated with prevention level Cs, the concrete is considered compliant.

– Thermal test on the 1.6 × 1.6 × 1.6 m cube (BP2)

The curves recorded by the three probes embedded in the concrete are similar, and the maximum temperature is maintained for several hours.

The maximum temperature rise recorded during the test was 31.4 °C (63.4-32.0). Given that the maximum temperature of the concrete during placement was 35 °C, the maximum temperature that could have been reached during the setting process is 66.4 °C. Since this temperature is below the threshold value of 80 °C associated with the Cs prevention level, the concrete meets the requirements.

– Essai thermique au cube 1.6 x 1.6 x 1.6 m (BP3)

As shown in Figure 6, the temperature exhibits unstable behavior due to the high temperature of the fresh concrete. This instability is particularly evident in the temperature curve of the lower part of the concrete, where, after the concrete reaches its peak temperature—corresponding to the maximum recorded value—successive phases of temperature rise and fall can be clearly observed.

Applying Mortar to Concrete

The mortar was applied on the concrete to test the effect of the concrete on the mortar. To test whether the compliant concrete would have any effect on the mortar. No cracks or sudden changes were seen in the mortar after 90 days.

No cracks were observed on the mortar after its application on the compliant concrete up to 90 days.

DISCUSSION

As in the case of mixtures BP1 and BP2, the temperature of fresh concrete remained below 35 °C, which is considered a favorable condition for the proper progress

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Fig. 4. Temperature evolution in the BP1 cubic specimen

Fig. 5. Temperature evolution in the BP2 cubic specimen

of hydration reactions and the optimal development of mechanical properties. During the delayed ettringite formation (DEF) test on the cubic specimens, the thermal behavior remained normal, characterized by a gradual increase in temperature followed by a cooling phase, while the maximum temperature did not exceed 80 °C.

Such control of the thermal regime ensured mechanical properties that complied with standard requirements in both compression and tension. The results clearly show that controlling the temperature of fresh concrete is essential for obtaining good mechanical properties after hardening. Without temperature control, the mechani-

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Fig. 6. Temperature evolution in the BP3 cubic specimen

Fig. 7. On the left: DEF cube 1×1×1 m³, and on the right: DEF cube 1.6×1.6×1.6 m³

cal performance becomes poorer. Hydration reactions themselves generate heat, and massive structures tend to heat up more rapidly and cool down more slowly [18–22]. The observations made in this study are fully consistent with the conclusions reported by the authors in the literature.

For specimen BP3, the temperature of fresh concrete exceeded 35 °C, and this immediately affected the behav- ior of the material. During the internal sulfate reaction (ISR) test, the thermal behavior became unstable, with repeated phases of temperature increase and decrease, although the maximum temperature still remained below 80 °C. This thermal instability led to poor mechanical properties. The material exhibited weaker performance in both compression and tension. In our opinion, this case clearly demonstrates that the higher the temperature

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Table 14. Summary of mortar application on concrete

Days