Combined effect of waste colemanite and silica fume on properties of cement mortar

1 Introduction

Concrete has been the most common material for numerous applications in many fields for many years. It is expected to remain so in the coming decades. Much of the developed world has infrastructures built with various forms of concrete [1]. Cement used as a binder is still an essential material in making concrete, and it is evolving by time. Traditionally, concrete is a composite consisting of the dispersed phase of aggregates (ranging from its maximum size coarse aggregates down to the fine sand particles) embedded in the matrix of cement paste. Modern concrete, however, is more than simply a mixture of cement, water, and aggregates; modern concrete contains more and more often mineral components, chemical admixtures, fibers, etc. [2].

In recent years, various types of materials, such as silica fume (SF), fly ash, bottom ash, and others, which is known as pozzolanic materials, have found extensive use in cement and concrete to increase its performance or to use it for more specific applications [3–10].

Pozzolanic materials are generally able to combine with the hydrated calcium hydroxide [Ca(OH)₂] forming the hydrated calcium silicate (C-S-H), which is the principal responsible for the strength of hydrated cement pastes. Also, an increase in the bulk density of concrete results, as the mixture voids are filled with very small admixture particles (microfiller effect) [11].

SF is a pozzolanic material that is a byproduct of the silicon melting process. It is used to produce silicon metal and ferrosilicon alloys, which have a high content of glassy-phase silicon dioxide (SiO₂) and consist of very small spherical particles. SF is known to produce a high-strength concrete and is used in two different ways: as a cement replacement to reduce the cement content (usually for economic reasons) and as an additive to improve concrete properties (in both fresh and hardened states), such as freeze-thaw resistance, sulfate resistance, alkali-silica reaction, abrasion resistance, and chemical attack [12–14].

In all nuclear installations, concrete is the most commonly used shielding material to protect from neutron radiation. It is a common practice to add boron into concrete to try enhancing γ and neutron shielding characteristics of concretes. Yet, very limited information on the usage of boron in cement production has been reported. Demir and Keles [15, 16] investigated physical and mechanical properties of concrete containing different colemanite proportions. They observed that cement including colemanite waste shield neutrons. Okuno [17] produced a shielding slab using epoxy resin+colemanite combination. Gencel [18] presents γ and neutron shielding characteristics of concretes with colemanite. Concrete or mortar contains a mixture of many light and heavy elements and therefore has good nuclear properties for the attenuation of photons and neutrons. So to shield neutron, light elements (elements with low atom number) have been needed. It is a common practice to add boron to concrete to try enhance the thermal neutron attenuation properties and to suppress secondary γ-ray generation.

Boron is one of the most important underground richness of Turkey, having about 60% of the world boron reserves. Commercial boron ores of Turkey are colemanite, tincal, and ulexite [15]. Colemanite (2CaO·3B₂O₃·5H₂O) is a calcium borate mineral with hardness between 4 and 4.5 and specific gravity of about 2.4 g/cm³. Pure colemanite has a B₂O₃ content of about 51%. Although colemanite has been fairly widely used in radiation shielding concrete, there is another point to consider that concrete used in nuclear applications must have adequate and satisfactory structural and engineering properties such as workability, compressive strength, thermal conductivity, shrinkage, tensile strength, and modulus of elasticity, which is a factor of importance in large stationary installations [19].

The accumulation of industrial wastes not only occupies large land but also causes environment pollution, which has become a serious society problem nowadays. In this respect, byproduct from an industry type can be used as raw material in other industry according to the concept of industrial ecology, which is developed for sustainable future in the world as presented in detail [20, 21].

Many studies have recently been conducted using a combination of the two or more byproducts. Therefore, utilization of SF together with colemanite containing boron provides an interesting alternative. In this present study, the effects of SF and colemanite combinations on the properties of cement mixtures were investigated.

2 Materials and methods

The cement used in all the mixtures was Portland cement, CEM II/A-M (P-LL) 42.5 N. The physical and mechanical properties and chemical analysis of cement are presented in Tables 1 and 2, respectively.

SF was used in this study. The optimum SF replacement percentage was kept constant as 6%. The reason behind that is too low content of SF below 5% of the total cementitious materials does not lead to a high strength of concrete: the volume SF is adequate to cover the surface of all aggregate particles [22]. Also, use of SF up to 5% is not affecting much the water demand of mixture. Over that percent, plasticizer should be used [23]. The physical and chemical properties of SF are presented in Table 3.

In the study, the colemanite waste was supplied from ETI Mine Works, Inc. (Turkey) and ground by a laboratory mill. B₂O₃ content of colemanite waste was determined by titrimetrical method, and other contents were determined by X-ray fluorescent method. The results of chemical analysis are presented in Table 4. As seen from Table 4, the specific surface of colemanite waste was pretty high when compared with the specific surface of cement.

The specimens were prepared with cement (450 g) +Rilem Cebureau sand (1350 g)+tap water (225 g). The cement±water mixtures were stirred at low speed for 30 s; then, with the addition of sand, the mixture were stirred for 5 min. Three 40×40×160 mm prismatic specimens for compression testing were made from each mixture. The specimens were cured at 20°C with 95% humidity for 24 h and then placed in tap water and cured up to 28 days. The compositions of mixtures are presented in Table 5.

The following tests were done on the produced samples.

The setting times of cement mixtures were determined according to TS EN 196-3 [24] using a vicat apparatus at room temperature. The initial set time occurs when a vicat needle 1 mm in diameter penetrates the sample to a point 5±1 mm from the bottom of the mold. Final setting time is defined as that at which the 5 mm cap ring leaves no visible mark when placed on the surface of the sample.

The Blaine method follows the TS EN 196-6 [25] standard. The Blaine air permeability apparatus allows drawing a definite quantity of air through a prepared bed of cement of definite porosity. The permeability cell is a rigid cylinder made of stainless steel. The specific surface area is in cm²/g.

The specific gravities of the cement mixtures were measured using Air Comparison Pycnometer Beckman 930 according to TS EN 196-6 [25].

The Le Chatelier method of cement characterization is based on using a 30 mm longitudinally split cylindrical mold with two indicators containing the cement paste exposed to boiling water at the atmospheric pressure for 3 h. The cement is acceptable if the distance between the indicators is ≤10 mm (European Committee on Standardization) [24].

The consistence of fresh mortar was made by flow table according to TS EN 1015-3 [26]. Compressive strength test was done on the samples with 40×40×40 mm in size according to TS EN 196-1 [27] on 2, 7, and 28 days.

The freeze-thaw test was carried out according to ASTM C666 [28]. In the test, the samples were immersed into the water at 20°C for 24 h. On the following day, the samples saturated with water were stored in the freezer at -20°C for 24 h. This process was repeated for 20 times. Microstructures from scanning electron microscopy views (Figure 1) and chemical composition of samples produced were evaluated.

Figure 1

Microstructure of 42.5 N Portland cement.

3 Result and discussion

One of the most important effects of additives was observed in the setting times of the mixes. Setting times are of great importance to workability of concrete. The initial and final setting times of the mixes are given in Figure 2. As seen from Figure 2, initial and final setting times of mixes increase. Mix S6 shows 16.7% of improvement in initial setting time with respect to mix P. This can be attributed to that first cement reacts with water resulting hydration products such as CSH and Ca(OH)₂. Setting is related to concentration of C₃S, C₂S, C₃A, and C₄AF in the cement, which reacts with water resulting in hydration products. It is well known that CSH is responsible for setting and strength of cement and concrete. When SF was added to Portland cement, contents of C₃S, C₂S, C₃A, and C₄AF in the cement decrease. Consequently, the initial setting is delayed. According to TS EN 196 and ASTM C150 [29], the initial setting should not be earlier than at least 60 and 45 min, respectively. The initial setting time of cement incorporated SF fulfills the requirement of codes. Addition of waste colemanite to cement increases initial and final setting times up to 455% and 438%, respectively. Recent studies [3, 30, 31] showed that the initial and final setting times of cement paste increased with the use of cementitious material containing boron. The reason behind this is that maybe waste colemanite does not have a pozzolanic activity to contribute to hydration.

Figure 2

Setting times of mixtures.

Figure 3 shows specific surfaces of cement mixtures. It is well known that specific surface is responsible for strength. The higher the fineness, the higher the strength. As seen from Figure 3, specific surfaces of mixtures increase. Mix S has a slightly higher value of about 4% when compared with mix R. Depending on the concentration of waste colemanite in the cement mixtures, specific surface increases up to 16.4%. This is attributed to the specific surface of fineness of SF and waste colemanite, which have higher fineness than that of cement, as reported in the literature [30, 32, 33].

Figure 3

Specific surfaces of mixtures.

The specific gravities of samples are presented in Figure 4. As seen from Figure 4, specific gravity decreases. This is due to the specific gravities of SF and waste colemanite. Specific gravity varies between 3.15 and 3 g/m³. The results of this study are consistent with those reported in the literature [30, 34].

Figure 4

Specific gravities of mixtures.

Figure 5 shows the water demand of mixtures. As seen from Figure 5, water demand increases depending on the concentration of additives in the mixture. The value changes between 27% belonged to the mix P and 31.95% belonged to the mix S6C12. This is due to the specific surface of cement mixtures. When Figures 3 and 5 are considered together, the effect of mineral additives is seen obviously. To wet all particle surfaces, more water is needed. The result of this study has been reinforced by many researchers [30, 34].

Figure 5

Water demands of mixtures.

Volume expansions of mixtures are presented in Figure 6. As seen from Figure 6, volume expansion of mixtures decreases. Ca(OH)₂, which is a hydration product, combines with SF and forms CSH. Therefore, this prevents Ca(OH)₂ from taking a role in expansion. The effect of SF for expansion is superior to that of waste colemanite. This can be attributed to the pozzolanic activity of SF forming more CSH gels. This result observed other boron additives cement mixtures [35, 36].

Figure 6

Expansions of mixtures.

The flow abilities of mixtures are presented in Figure 7. As seen from Figure 7, flow ability of mixture increases depending on the concentration of mineral additives. As mentioned above, water demand increases depending on mineral additives in the mixture. More water is needed. This results in increment of flow ability.

Figure 7

Flow ability of mixtures.

The compressive strengths of mixtures are presented in Figure 8. As seen from Figure 8, the strengths of mixtures increase by time. When looked at the effect of SF in the long term, the strength increases. However, the effect of waste colemanite is inverse when compared with that of SF both in the short term and long term. The reason behind the strength loss of mixtures containing waste colemanite is that waste colemanite does not have a pozzolanic activity.

Figure 8

Compressive strengths of mixtures.

The freeze-thaw durability of mixtures is presented in Figure 9. As seen from Figure 9, all mixtures lose strength. Microcracks mainly exist at cement paste-aggregate interfaces within concrete even before any loading and environmental effects. When the number of freeze-thaw cycles (FTCs) increases, the degree of saturation in pore structures increases by sucking in water near the concrete surface during the thawing process at temperatures above 0°C. Some of the pore structures are filled fully with water. Below the freezing point of those pores, the volume increase of ice causes tension in the surrounding concrete. If the tensile stress exceeds the tensile strength of concrete, microcracks occur. By continuing FTCs, more water can penetrate the existing cracks during thawing, causing higher expansion and more cracks during freezing. The load-carrying area will decrease with the initiation and growth of every new crack. Necessarily, the compressive strength will decrease with FTCs [37, 38].

Figure 9

Compressive strength of mixtures according to FTCs.

Still, mixtures, except for S6C10 and S6C12, are acceptable according to the ASTM C 666 code. According to the code, more than 30% strength loss is not allowed. As seen from Figure 9, strength losses of SC5 and S6C12 are 39.4% and 53.3%, respectively. Figure 1 presents the microstructure of mixture of PC-42.5 N Portland cement.

The chemical compositions of the produced mixtures were determined and are presented in Table 6. As seen from Table 6, CaO content decreases and B₂O₃ content increases depending on the waste colemanite in the mixture. Other compounds change slightly, which can be ignored.

4 Conclusion

This study presents the effects of supplementary materials, SF and waste colemanite, on the properties of cement mortar. These specific conclusions can be drawn from the results of this study. The general effects of SF and waste colemanite admixture is to retard both the initial setting time up to 16.7% and 212.5% and the final setting time up to 12.5% and 355.6%, respectively. Both admixtures used increase specific surface of cement. Consequently, water demand of cement increases. Also, specific gravity decreases depending on the admixture contents in the cement. The used admixtures decrease the volume expansion of cement. Compared with control, mix P, substitution of SF increases compressive strength of the specimens after 28 days of curing. Early compressive strength of the specimens containing SF and waste colemanite is lower. It decreases gradually for all samples throughout the entire 2, 7, and 90 days of experiment by depending on the waste colemanite in the mixtures. Compressive strength of mixtures decreases after FTCs. Strength losses of mixtures containing 10% and 12% are above the limit allowed according to the code.