Effect of chemical admixtures on borogypsum containing cement mortar

Abstract

In this study, the influences of air-entraining, set accelerating and superplasticizer admixtures on the properties of borogypsum containing cement mortars were investigated. According to results, by using 3% borogypsum and water/binder ratio of 0.45, the compressive and flexural strength values were increased up to 196.38%. The usage of various ratios of air-entraining and set accelerating admixtures showed reverse effects on the compressive and flexural strength. However, 0.4% superplasticizer containing mortars had 65.39 MPa compressive strength and 9.08 MPa flexural strength which were 33.12% and 26.82% higher than the reference. Durability test results showed addition 3% borogypsum provided endurance to sulfate attacks up to 30 days.

Keywords

Borogypsum chemical admixtures durability mortar

1 Introduction

Different chemical admixtures are used to develop physical, mechanical and durability properties of cement containing composites. Choosing suitable chemical admixtures, which can be air-entraining, set retarding, set accelerating, water-reducing, and plasticizer, brings desired properties to the construction materials and ensures to overcome the problems about preparation and long-term features of these materials [1 –3].

Superplasticizers are water reducers which affect the rheological properties of cement paste and concrete. Decreasing the water content of the concrete is important in terms of strength, sulfate durability and permeability of concrete. The concrete that has been prepared with superplasticizer has flowability even for lower water/cement ratio [4 –6]. When air-entraining additives are used, microscopic and stable air bubbles form in the concrete and this improves the freeze-thaw durability of the concrete. Moreover, the addition of air-entraining additives reduces segregation and bleeding in concrete and provides higher workability and lower water/cement ratio [7, 8]. Set accelerating additives promote the hydration reactions and diminish the setting time [9].

The usage of boron containing wastes obtained from boric acid (H₃BO₃) production or the enrichment process of tincal (Na₂B₄O₇·10H₂O) as supplementary materials in cement composites enhances the properties of the cement composites. The gypsum-like composition and pozzolanic features of borogypsum, which is obtained as a by-product of colemanite -a type of calcium borate ore- and sulfuric acid reaction, provide an improvement in physical and mechanical properties [10 –12]. Also, borate minerals and their wastes are used in the composition of construction materials because of their neutron shielding properties [12, 13].

In recent years, many researchers have focused on utilization of borogypsum in mortar or concrete composition because of the advantages in terms of economic and environmental issues [14 –22]. Studies showed that the compressive and flexural strength of mortars can be increased by using appropriate amounts of borogypsum. On the other hand, the studies pointed out that increasing amounts of borogypsum prolonged the setting time and decrease early age strength [15, 16].

Boncukoglu et al. [18] found adding borogypsum instead of natural gypsum increases the compressive strength while prolongs setting time which is corroborated by Sevim and Tümen [19]. Furthermore, Elbeyli [20] pointed out that the addition 5% borogypsum in cement mortar provides an increase in compressive strength when according to Topçu and Boga [16] 10% borogypsum can be used in place of cement. Various amounts of clay waste containing boron were used by Özdemir and Öztürk [11] as the cement additive and the results showed that the ratios up to 5% and 10% can be used according to Turkish standard (TS) values. Also, in a few studies boron containing compounds were used with chemical admixtures to improve properties of cement composites. Davraz et al. [21] added calcium chloride and sodium aluminate as set accelerating additives to boric acid included mortars and they indicated that sodium aluminate addition eliminated the negative effect of boron compounds on the early strength of the mortars. In the study of Erdogan et al. [22] in which colemanite waste and acidic pumice were used, they added superplasticizer to concrete mixture to keep the slump in a specific range.

The objective of this study is using an industrial waste of borogypsum with chemical admixture in cement mortar, and thus improve physical and mechanical properties of cement mortar. For this purpose, borogypsum containing cement mortars were prepared with various water/binder ratios and the impacts of different chemical admixtures (superplasticizer, air-entraining and set accelerating) on the mortars containing optimum borogypsum and water/binder ratios were investigated. Also, external magnesium sulfate (MgSO₄) durability of optimum mixtures were observed.

2 Experimental studies

2.1 Materials

Clinker with Blaine surface area of 3000±20 cm²/g was supplied from Akçansa Cement Factory, (Akçansa, Istanbul, Turkey) and used without pretreatment during experiments. Borogypsum was provided from Bandirma Boron Works (EtiMaden, Balikesir, Turkey). Since the obtained borogypsum was in the form of filter cake, it was placed in an Ecocell LSIS-B2 V/EC55 model incubator (MMM Medcenter Einrichtungen GmbH, Planegg, Germany) maintained at 105°C to evaporate its water content. Dried borogypsum was ground by Retsch RM 100 (Retsch GmbH & Co KG, Haan, Germany) and sieved by Fritsch analysette 3 Spartan pulverisette 0 vibratory sieve-shaker (Fritsch, Idar-Oberstein, Germany) to access a Blaine specific surface area of 3200±20 cm²/g. The crystallographic analysis of borogypsum was carried out by PANalytical Xpert Pro (PANalytical B.V., Almelo, The Netherlands) X-ray diffractometer (XRD) with Cu-Kα radiation (λ= 1.53 Å) at 45 kV and 40 mA. According to XRD analysis borogypsum identified as a mixture of gypsum (CaSO₄·2H₂O, powder diffraction file (pdf) no: 00-006-0046) and calcium borate hydrate (Ca₂B₁₀O₁₇·5H₂O, pdf no: 00-022-0146). The composition of borogypsum was determined by PANalytical Minipal 4 brand X-ray fluorescence (XRF) with the parameters between 4 kV-30 kV. Rilem Cembureau sand (according to EN 196-1 standard [23]) was supplied from Limak Trakya Cement Plant (Limak Trakya, Kirklareli, Turkey) and used without pretreatment. The chemical composition of clinker, borogypsum and sand as determined by XRF is given in Table 1.

Table 1
Chemical composition of clinker, borogypsum and sand

Composition (%) Clinker Borogypsum Sand

SiO₂ 14 4.1 90.8

Al₂O₃ 3 – 5.7

Fe₂O₃ 4.7 0.99 0.86

CaO 76.5 45.1 0.41

SO₃ – 48.7 –

TiO₂ – – 0.87

K₂O 1.3 – 1.3

B₂O₃ – 1.1 –

LOI^* 2.4 14.5 2.5

Composition (%)	Clinker	Borogypsum	Sand
SiO₂	14	4.1	90.8
Al₂O₃	3	–	5.7
Fe₂O₃	4.7	0.99	0.86
CaO	76.5	45.1	0.41
SO₃	–	48.7	–
TiO₂	–	–	0.87
K₂O	1.3	–	1.3
B₂O₃	–	1.1	–
LOI^*	2.4	14.5	2.5

^*Loss of ignition.

MasterGlenium^®51, polycarboxylic ether based superplasticizer admixture, MicroAir 200, air-entraining admixture, and MasterSet^®AC 326B, calcium nitrate salt based set accelerating admixture, were obtained from BASF (BASF Construction Chemicals, Turkey). In Table 2, the technical properties of chemical admixtures are given.

Table 2

Technical properties of chemical admixtures

Chemical admixture	Composition	Density (kg/L)	Alkaline Content (%) (EN 480-12)	Chloride Content (%) (EN 480-10)
MasterGlenium^®51	Polycarboxylic ether	1.082–1.142	<3	<0.1
MicroAir 200	Oil alcohol and ammonium salt	0.98–1.02	<10	<0.1
MasterSet^®AC 326B	Calcium nitrate salt	1.316–1.376	<5	<0.1

MgSO₄, which was used in the durability studies, was provided from Merck Chemicals with the purity of 99.5% and 0.5 M MgSO₄ solution was prepared with distilled water.

2.2 Preparation of mortars

In this study, the experimental procedure contained two steps. Firstly, various borogypsum ratios (1%, 3%, 5%, 7% and 10% by weight of binder) were attempted with different water-to-binder (w/b) ratios of 0.50, 0.45 and 0.40 to obtain optimum mortar composition, which serves maximum compressive and flexural strength. In the second step, the determined waste and w/b ratio were used with different chemical admixtures to find out their influence on the properties of cement mortar. The detailed mixing proportions of mortars with borogypsum are given in Table 3.

Table 3
Mixing proportions of mortar

Mix Clinker (%) BJ^* (%) w/b

BJ-1 100 0 0.50

BJ-2 99 1 0.50

BJ-3 97 3 0.50

BJ-4 95 5 0.50

BJ-5 93 7 0.50

BJ-6 90 10 0.50

BJ-7 100 0 0.45

BJ-8 99 1 0.45

BJ-9 97 3 0.45

BJ-10 95 5 0.45

BJ-11 93 7 0.45

BJ-12 90 10 0.45

BJ-13 100 0 0.40

BJ-14 99 1 0.40

BJ-15 97 3 0.40

BJ-16 95 5 0.40

BJ-17 93 7 0.40

BJ-18 90 10 0.40

Mix	Clinker (%)	BJ^* (%)	w/b
BJ-1	100	0	0.50
BJ-2	99	1	0.50
BJ-3	97	3	0.50
BJ-4	95	5	0.50
BJ-5	93	7	0.50
BJ-6	90	10	0.50
BJ-7	100	0	0.45
BJ-8	99	1	0.45
BJ-9	97	3	0.45
BJ-10	95	5	0.45
BJ-11	93	7	0.45
BJ-12	90	10	0.45
BJ-13	100	0	0.40
BJ-14	99	1	0.40
BJ-15	97	3	0.40
BJ-16	95	5	0.40
BJ-17	93	7	0.40
BJ-18	90	10	0.40

^*BJ: Borogypsum.

The specimens were prepared according to EN 196-1 standard [23]. For reference specimen (BJ-1), 450 g of clinker were mixed with 225 g of distilled water at low speed for 30 seconds and then within 30 seconds sand (1350 g) was added to this mixture. Following stirring for 4 minutes at high speed, the mortar was placed in the three-cell prismatic molds of 40×40×160 mm for compressive and flexural strength tests. After filling the molds, a vibration table was used to have a compact matrix by decreasing the amount of air bubbles in the structure. In order to avoid loss of water because of hydration heat, the top of specimens was covered with glass plates and then mortars were stored in test cabinet (Nuve TK 120, Turkey) at 20±2°C with 90% relative humidity for 24 hours prior to demolding. After demolding, specimens were placed in curing tank 20±2°C until flexural strength and compressive strength tests of 3, 7 and 28 curing days.

For borogypsum containing mixtures, the aforementioned preparation process was applied by providing homogenization of the desired amount of borogypsum and clinker by a laboratory mill before preparation mortars. For the mortars containing chemical admixtures, firstly, water and admixtures were mixed in the mixer. The ratios of chemical admixtures were chosen between the ranges that producing company advised. The air-entraining admixture was added between 0.1-0.2% of mortar when set accelerating and superplasticizer admixture containing experiments were conducted at the ranges of 1-2% and 0.4–0.6%, respectively. Following preparation blends, fresh and hardened mortar tests were carried out. The mortar designs which were constituted from chemical admixtures are given in Table 4.

Table 4

Mixing proportions of mortars with chemical additives (wt. %)

Mix	Clinker	BJ	Sand (g)	Air-entraining admixture	Set accelerating admixture	Superplasticizer admixture	w/b
BJ-19	97	3	1350	0.10	–	–	0.35
BJ-20	97	3	1350	0.15	–	–	0.35
BJ-21	97	3	1350	0.20	–	–	0.35
BJ-22	97	3	1350	–	1.0	–	0.35
BJ-23	97	3	1350	–	1.5	–	0.35
BJ-24	97	3	1350	–	2.0	–	0.35
BJ-25	97	3	1350	–	–	0.4	0.35
BJ-26	97	3	1350	–	–	0.5	0.35
BJ-27	97	3	1350	–	–	0.6	0.35

2.3 Testing samples

Setting time and consistence of fresh mortar was determined according to TS EN 480-2 and TS EN 1015-3, respectively. The initial and final set time of mortar was determined by using Vicat apparatus with regard to penetration of vicat needles [24]. Flow table was used to find out effects of different w/b ratios, borogypsum amounts and chemical admixtures on consistency [25].

Mechanical properties of compressive and flexural strength of hardened mortars were measured by UTEST brand automatic cement compression and flexure testing machine after 3, 7 and 28 days of curing. The flexural strength test was applied to specimens initially and the obtained two halves were tested for compressive strength.

XRD analysis was carried out to identify crystalline phases of cement mortars in the pattern range of 5–90° with the scanning rate of 0.006°/s. The characteristic bonding of hydration reactions was determined by infrared spectroscopy, which was conducted with Fourier transform infrared spectroscopy (FT-IR) in the range of 4000-450 cm^–1 (Perkin Elmer Spectrum One, MA, USA). Before IR spectroscopy, the samples were grounded below 90μm and mixed with KBr powder at 1 : 100 ratio and pressed to obtain pellets.

In order to investigate the durability of borogypsum containing cement mortars, which have maximum compressive and flexural strength, the samples cured for 28 days were immersed aqueous solution of 0.50 M MgSO₄. The sulfate ions (SO₄^2–) in the environment which mortar or concrete structure are exposed cause a decrease in the endurance by deterioration [16]. These experiments were performed by curing samples both in 0.50 M MgSO₄ solution and tap water for 15, 30 and 45 days and measuring the weight and strength changes. MgSO₄ solution and tap water were renewed every two weeks.

3 Results and discussion

3.1 Fresh properties of mortars

The flow diameters of fresh mortars for different w/b ratios and chemical additive contents are presented in Figs. 1 and 2, respectively. The flow table tests demonstrated that all mortars which included borogypsum showed flexibility and workability even though the control samples showed harder compaction. The replacement of clinker with borogypsum leads more workable mortars due to water absorption capacity of borogypsum. Furthermore, the chemical admixtures increased the flowability of mortars despite w/b of 0.35. The flow diameters were changed between 10.25–16.00 cm for mortars with chemical admixtures. Air-entraining additive creates numerous finer air bubbles and decrease the superficial tension at the air–water interface. Thus, the workability of mortars is increased. On the other hand, superplasticizer increases the flowability of mortars by separating the cement particles by opposing their attractive forces with steric and/or electrostatic forces [26 –28].

Fig.1

The flow diameters of fresh mortars for different w/b ratios.

Fig.2

The flow diameters of fresh mortars for different chemical admixtures.

The relative change in initial and final setting time for various w/b ratios is given in Table 5. As in recent studies, the increasing amount of borogypsum increased the initial and final setting time for all the w/b ratios, which indicated a slower hydration process compared to the reference sample [26]. For instance, using 10% borogypsum in the content for mortars prepared with a w/b ratio of 0.50 delayed the initial set time from 200 min to 450 min. On the contrary, using air-entraining additive fastened the setting times when hardening process was slower in the presence of superplasticizer set accelerating admixture, unexpectedly.

Table 5

Relative change in initial and final setting times

Mix	Initial setting time change (%)	Final setting time change (%)
BJ-1	0.00	0.00
BJ-2	30.00	18.33
BJ-3	45.00	30.00
BJ-4	47.50	33.33
BJ-5	55.00	48.33
BJ-6	125.00	73.33
BJ-7	0.00	0.00
BJ-8	3.00	–5.00
BJ-9	30.30	–10.00
BJ-10	42.42	15.00
BJ-11	60.61	20.00
BJ-12	o.s^*	o.s
BJ-13	0.00	0.00
BJ-14	17.65	9.52
BJ-15	29.41	14.29
BJ-16	64.71	23.81
BJ-17	100.00	42.86
BJ-18	o.s	o.s
BJ-19	0	0
BJ-20	–14.06	–11.70
BJ-21	–23.13	–19.15
BJ-22	0	–7.66
BJ-23	9.38	–2.13
BJ-24	15.63	0
BJ-25	100.00	60.00
BJ-26	o.s	o.s
BJ-27	o.s	o.s

^*Over the standard.

3.2 Compressive and flexural strength of mortars

The progress of compressive strength of cement mortars with different w/b ratios, borogypsum ratios and hydration periods is given in Fig. 3. As anticipated, the increase in hydration time causes enhancement of compressive strength because of continuing hydration reactions and formation of calcium silicate hydrate (C-S-H) gels which create a dense and compact matrix.

Fig.3

Compressive strength of cement mortars with different w/b ratios, borogypsum ratios and hydration periods.

For the w/b ratio of 0.50, the best compressive strength was obtained by adding 3% borogypsum which caused 51.43% gain in compressive strength compared to the reference sample. The compressive strengths of 3% borogypsum containing sample were 23.45, 36.27 and 45.70 MPa for 3rd, 7th and 28th curing days, respectively. Despite having 16.68% higher strength than reference, more than 7% borogypsum replacement with clinker reduces compressive strength.

When the w/b was changed to 0.45, the highest compressive strength was reached among all non-admixture containing samples with a gain in compressive strength between 196.38-74.21%. Likewise 0.50, the optimum borogypsum addition percentage was 3% for w/b ratio of 0.45 which developed 27.99, 38.88 and 49.13 MPa compressive strength at 3, 7 and 28 curing days, respectively.

On the other hand, when w/b ratio was reduced to 0.40, lower compressive strengths were observed for all borogypsum amounts according to 0.45 and 0.50. Up to 7% borogypsum replacement, the compressive strengths were increased with increasing borogypsum. The compressive strengths were determined as 14.35, 22.33, 37.07 MPa for curing days of 3, 7, 28 for 7% borogypsum ratio which showed the highest strength.

Flexural strength values of borogypsum containing cement blends are shown in Fig. 4. According to the results, the reference sample showed 2.71 MPa, 2.99 MPa and 3.84 MPa of flexural strength for 3, 7 and 28 days when w/b ratio was chosen as 0.50. However, addition borogypsum with various amounts promoted the hydration reactions and increased the flexural strength up to 98.70% of the reference sample. The best results were obtained for the percentages of 3% and 5%, which had flexural strengths of 7.02 and 7.63 MPa for 28 days, respectively.

Fig.4

Flexural strength of cement mortars with different w/b ratios, borogypsum ratios and hydration periods.

When all w/b ratios were compared, the results showed that there was an inverse relation between mechanical properties and w/b ratio for 0.50 and 0.45. However, the mortars prepared with a w/b ratio of 0.40 have lower strength values which can be explained by the lack of water for further hydration.

For determination impacts of chemical admixtures on cement mortars, the optimum borogypsum ratio of 3% was used in mortars. Both air-entraining and set accelerating admixtures decreased the compressive and flexural strength of mortar in compression to non-additive containing blends. The main desired effect of air-entraining admixture is to improve the workability by increasing the volume of air in the mortar and as a result, the strength values are reduced. Although it was expected set accelerating admixture to develop early age strength of mortars, addition set accelerating admixture showed reverse influence on the mechanical properties of mortars [27]. On the other hand, superplasticizer addition with different ratios showed a significant increase in both compressive and flexural strengths. The introduction of superplasticizer caused a more compact matrix because superplasticizer increased the dispersion on the cement grains and water interface. At the end of the 28 days of curing, the compressive strengths for superplasticizer ratios of 0.4%, 0.5%, 0.6% were 65.39 MPa, 60.52 MPa and 59.03 MPa, respectively. Moreover, the same superplasticizer amounts brought the mortars flexural strengths of 9.08 MPa, 8.63 MPa and 8.19 MPa. From results, the optimum superplasticizer ratio was determined as 0.4% which improved the compressive and flexural strengths up to 33.12% and 26.82%, respectively. As seen in the results, the dosage of the superplasticizer affected the strength of mortars and the lower addition of superplasticizer enhanced the mechanical endurance. The efficiency of superplasticizer on the mortar properties is related to the adsorbed amount of superplasticizer by the cement particles. When the adsorbed superplasticizer amount reaches its saturation point, the increased amount of superplasticizer affects the strength slightly [28].

The strength test results of the chemical additive containing mortars are given in Fig. 5.

Fig.5

Strength test results of chemical additive containing mortars.

3.3 XRD results of mortars

XRD patterns of selected mortars of 28 days with borogypsum which showed highest compressive strength for each water/binder ratio are shown in Fig. 6. XRD analysis gives information about crystalline compounds in the studied matrix.

Fig.6

XRD patterns of selected mortars for different w/b ratios (Q: Quartz, D: Dolomite, CH: Portlandite, CSH: Calcium Silicate Hydroxide).

For the w/b ratio of 0.50 and 0.45, the products after hydration progress, which were quartz (SiO₂) and portlandite (Ca(OH)₂), showed similarity. On the contrary, despite showing low intensity, the sample of BJ-17 (w/b = 0.40) has dolomite (Ca(Mg, Fe)(CO₃)₂) peaks with powder diffraction file number (pdf no) of 00-034-0517 which show lack of hydration progress.

XRD patterns of the mortars containing different chemical admixtures are given in Fig. 7. Generally, the components of air-entraining admixture and set accelerating containing mortars shows low crystallinity when obtainment of crystal phases was succeeded with the addition of superplasticizer. According to XRD results, the BJ-25 and BJ-27 mortars are consisting of SiO₂ (pdf no: 01-087-2096), Ca(OH)₂ (pdf no: 00-044-1481) and Gismondine (CaAlSi₂O₈·4H₂O, pdf no: 00-020-0452). In the XRD pattern of BJ-26 mortar, the characteristic peaks of CaAlSi₂O₈·4H₂O are not observed which can be because of formation of amorphous structures. The main hydration products of calcium silicates had crystalline structure, which caused small peaks.

Fig.7

XRD patterns of chemical additive containing mortars (Q: Quartz, G: Gismondine, CH: Portlandite, CSH: Calcium Silicate Hydroxide).

3.4 FT-IR results of mortars

The IR spectra of the mortars which showed high strength values for each experimental set are shown in Fig. 8. The spectral range of 4000-2800 cm^–1 represents vibrations of calcium hydroxide and water [29, 30]. The peaks at 3639 cm^–1 are attributed to non-hydrogen bonded O-H stretching vibration which is because of Ca(OH)₂ content. The peaks between 3447–3432 cm^–1 and around 1631 cm^–1 are assigned to stretching and bending modes of water, respectively. The limestone (CaCO₃) peaks, which is formed as a result of carbonization of Ca(OH) with atmospheric CO₂, can be observed around 1713 cm^–1, between 1425-1384 cm^–1 and around 872 cm^–1, respectively. The peaks between 1097-1003 cm^–1 comprise asymmetric stretching vibrations of Si-O-Si when the IR absorption bands around 778 cm^–1 [31, 32] are observed because of the vibration of tetrahedral units of AlO₄ [28]. In-plane Si-O bending causes peaks around 464 cm^–1 when out-plane Si-O is observed at 522 cm^–1.

Fig.8

FT-IR spectra of selected mortars.

3.5 Durability of mortars

The weight changes and strengths of mortars due to immersion in MgSO₄ solution are presented in Table 6. It can be seen from weight change results, the weight loss in MgSO₄ solution varied between 0.28% – 0.42% while weight showed an increasing trend in the control group because of water absorption. As shown in the results, the compressive strength of samples increased during testing time, regardless of being under sulfate attack or not owing to the further hydration reaction [33]. Flexural strength had an increasing trend up to 30 days of exposing both MgSO₄ solution and tap water. On the other hand, flexural strength showed a decrease for the further exposure which can be explained as degradation because of SO₄^2– ions. The reactions between binder materials and sulfate ions form ettrengite and gypsum and these compounds cause disintegration and cracks in the mortar matrix which reduce flexibility [34].

Table 6
Durability of mortars in MgSO₄ solution

Immersion time (day) In tap water In MgSO₄ solution

15 30 45 15 30 45

Compressive Strength (MPa) 42.63 45.10 44.97 41.16 45.47 47.56

Flexural Strength (MPa) 6.13 7.77 6.93 7.25 8.33 7.96

Weight change (%) 0.07 0.03 0.33 –0.28 –0.32 –0.42

Immersion time (day)	In tap water	In MgSO₄ solution
Compressive Strength (MPa)	42.63	45.10	44.97	41.16	45.47	47.56
Flexural Strength (MPa)	6.13	7.77	6.93	7.25	8.33	7.96
Weight change (%)	0.07	0.03	0.33	–0.28	–0.32	–0.42

4 Conclusions

The usage of boron wastes in the construction industry is an attention getting research area because of the advantages of using waste materials which reduce cement amount in mortar matrix and gaining specialized properties due to boron content.

In the present study, effects of various chemical admixtures on borogypsum containing cement mortar properties were investigated with physical, mechanical and instrumental tests. According to fresh mortar results, chemical admixtures increased the flowability of mortars and ensure workability. When borogypsum content affected initial and final setting time adversely, air-entraining agent shortens the setting time.

The best borogypsum ratio and w/b ratio were determined as 3% and 0.45 which gave optimum compressive and flexural strength and developed strength values up to 196.38% compare to reference mortar. Air-entraining and set accelerating admixtures decreased compressive and flexural strength when 0.4% superplasticizer addition improved the compressive strength of 28-day curing sample as 65.39 MPa.

According to durability test results, the compressive and flexural strength increased during the testing time (up to 30 days) under sulfate attack because of hydration reactions, on the other hand, flexural strength decreased for the further exposure which can be resulted because of degradation.

In conclusion, even borogypsum presence improved both fresh and hardened properties of mortars, addition chemical admixtures -especially superplasticizer- provided compressive strength of 65.39 MPa which can be defined as high-strength mortar according to American Concrete Institute [35].

Footnotes

Acknowledgments

This research was supported by Yildiz Technical University Scientific Research Projects Coordination Department, with project number of 2014-07-01-KAP04.

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