Building materials are commercial products consisting almost entirely of non-renewable mineral resources [1
]. These include Portland cement, which is widely used in building [3
]. In addition to the fact that its production consumes significant amounts of mineral raw materials, it is among the leaders in polluting the environment with carbon dioxide emissions [6
]. To minimize the negative impact, the amount of clinker in the composition of the cements is reduced and replaced with mineral additives of various types [10
Mining waste is mainly used as mineral additives, including quartz, marble, basalt dust, granite, limestone powders, fly ash, etc. Moreover, not just one type of additive is used, but very often several types simultaneously. The chemical and mineralogical composition of cements plays an important role in the kinetics of hydration of the cements with mineral additives [11
It is established that use of quartz dust in the cement composition increases mechanical strength. Combined presence of colemanite with the waste leads to an increase in specific surface area and water consumption, as well as a decrease in strength properties [12
]. Addition of dolomite to the cement composition reduces its water demand, without having a positive effect on the mechanical characteristics. On the contrary, addition of coal industry waste increases microporosity of the materials and boosts their strength [13
]. Replacement of 20% of Portland cement with activated coal mining wastes (kaolinite) slightly reduces mechanical properties of the obtained materials, which, however, are within the requirements of technical documentation [14
]. Besides, coal combustion waste has pozzolanic activity [16
]. Zeolites and bentonites can be added to cement binders to bind the heavy metals in the fly ash [17
]. Compositions with zeolite manifest themselves best, since their mechanical characteristics have increased values [18
The dust generated during the development of mineral deposits has prospective use as a mineral additive. Marble quarries produce a large amount of marble dust, which can be used as an additive in cements. At 5% of its replacement, the best physical and mechanical properties are observed, and they correspond to an improved cement grade [19
]. Limestone dust in the amount of up to 8 mass% used in production of cement pastes and solutions has a noticeable effect on their properties. The setting time of the samples is extended, their density increases, and the mechanical properties improve [20
]. Chalcedony dust, introduced in an amount of 25%, determines gains of strength of the samples at the later time of hardening [21
During gold mining, huge volumes of gold-bearing waste remain in the quarries. They consist of quartz, muscovite, albite, and microcline. Their introduction into the composition of the feed in the production of cements improves the hydration process and increases the strength of commercial products [22
]. Addition of up to 15% heat-treated heavy loams with a specific surface area not exceeding 500 m2
/kg leads to an increase in density, water resistance, and strength of cement compositions [24
Use of magnesium-containing additives in the composition of mixed cements is of interest. Introduction of their optimal amount leads to an increase in strength of the obtained compositions. For basalts, this parameter is up to 12 mass%. Water demand remains at the same level, and process of setting the cement paste slows down [20
]. Increasing the amount of the additive leads to a decrease in strength properties at the early stages of hydration (7 and 28 days). However, at later age (91 days), the situation is reversed: a mixed cement paste has a higher strength than a control sample [28
In addition, basalt crushing waste can be used as raw materials in the production of Portland cement clinker. The obtained clinkers do not differ in their phase composition from non-additive ones, and the properties of Portland cements meet the requirements of the technical conditions [29
]. Basalt powder added to the composition of solutions does not have a positive effect on their quality. It acts as inert filling materials [30
In preparation of binding compositions, it is possible to use mining waste containing magnesium hydrosilicates, such as talc or serpentinite. Mixing of talc with a solution of sodium polyphosphate in the combined presence of magnesia makes it possible to obtain high-strength materials [31
]. Adding the serpentinite to the composition of magnesia cement increases its durability [32
]. Its application as an additive to Portland cement clinker improves the mechanical properties [33
]. In addition to the mentioned above rocks, dunites, wehrlites, and troctolites are promising raw materials for cement production [34
]. Using them as a mineral additive improves the exploitative characteristics of the obtained materials.
The introduced additives affect the hydration capacity of cement compositions in various ways. Fly ash, as well as zeolite and bentonite, do not lead to the appearance of new phases in the hydrated cement, but only accelerate the beginning of the hydration process and its complete finishing [17
]. When 20% of activated clay materials (kaolinite) are added to the mixed cement, an increase of the concentration of alumina phases C4
, and polymerized C–S–H gel is observed [15
Dolomite and coal waste facilitate the formation of C4
crystal phase together with ettringite and C–S–H gel. Moreover, in the latter case its amount is higher, which facilitates the compaction of cements [13
]. Introduction of gold-bearing tailings as silica raw materials also does not lead to formation of new phases during the hydration of cements; however, it boosts the reactivity. The products of hydration are ettringite, calcium hydroxide, and the gel-phase C–S–H [22
]. The applied limestone quarry dust acts as filler, reacting with the products of cement hydration to form calcium bicarbonate. Basalt quarry dust is not only filler, but also a pozzolanic additive. When hydrated in a mixed cement composition, it leads to the formation of an additional amount of calcium hydrosilicates C–S–H, which determines mechanical strength of the material [20
]. In addition, the interaction rate of silica, which is a part of basalts, with Ca(OH)2
, formed as a result of the cement hydration increases [28
]. Chalcedony dust also reacts with portlandite to form calcium hydrosilicates [21
Thus, the search for new types of mineral additives and the study of their influence on the hydration activity of Portland cement is an urgent task.
3. Results and Discussion
The research included determination of pozzolanic activity of the low grade nephrite, which is used as a mineral additive in cements. This type of study is usually conducted using the traditional method based on absorption of lime from lime solution. However, in the work [47
] it was found that the accelerated Chapel method is preferable [48
], which allows us to effectively determine activity of additives of different quality.
Pozzolanic activity of the low grade nephrite determined by the Chapel method was 66 mg g−1. It indicates that this raw material is not inert and can be used in the production of cements with mineral additives.
The influence of the amount of the additive of the low grade nephrite and the time of grinding of the raw material mixture on mechanical properties of composite binders was studied. In addition, the dependence of their strength on the hardening conditions was established.
As can be seen from the data obtained, mechanical activation of the raw material mixture has a significant impact on the strength of cement compositions (Figure 3
The amount of the additive of nephrite-bearing waste in the cement compositions also affects their hydration activity (Figure 4
). The lowest strength properties were observed in samples with 10% of the rock additive, the highest with 30% of the additive. When 40% of the waste was added to the raw material feed, the strength properties of the hydrated samples were practically at the same level and did not depend on the duration of grinding.
It is established that the strength set of binding compositions depends on the conditions of their hardening. From the data in Figure 5
, presented for samples crushed within 10 min, it is clear that for all types of compositions, heat-and-water treatment did not lead to an increase in the hydration activity of cement compositions. The best results were obtained when hardening in normal humidity conditions. The main strength gain occurred during the first 7 days of sample hardening (more than 70% of the 28-day strength). By fourteen days, this property was more than 90% of the 28-day strength.
X-ray phase analysis of samples after 28 days of hardening in normal humidity conditions is shown in Figure 6
An X-ray diffractogram of a binder composition with a 30% mineral additive (X-ray 3) is a combination of minerals that is a direct result of hydration of pure cement (X-ray 2) and its subsequent hardening. Most of the X-ray reflections were located between 25–35 (2θ), which mainly correspond to the phases of Portland cement. In addition, the X-ray showed reflexes related to nephrite minerals (X-ray 1).
In the process of hydration of Portland cement with a mineral additive, the appearance of reflexes of new phases was not observed. Binary cement compositions containing substandard nephrite behaved in the same way as standard Portland cement. The addition of nephrite does not interfere with the process of its hydration.
As a result of the work, new types of cements with the addition of low grade nephrite were obtained and their physical and mechanical properties were studied (Table 5
According to the presented data, it can be seen that cements with the addition of low grade nephrite differ in their physical and mechanical properties from Portland cement. It was found that the compressive strength of the mixed cement was higher than that of the control sample and was 84.2 MPa. Unlike Portland cement, the duration of hardening of which is within 2 h, this process of the obtained material takes 2 h and 54 min. The average density of the samples with the addition of nephrite was lower than that of Portland cement, and was equal to 2256 kg/m3.
The index of pozzolanic activity of the low grade nephrite for compressive strength of cement–sand samples at the age of 28 days of normal-humidity hardening was determined. Depending on the amount of rock admixture in the composition of cements, the activity index (in %) was equal to: at 10–93.6%; at 20–99.1%; at 30–103.5%; at 40–95.0%. The obtained data indicate that addition of the low nephrite into the cement compositions in the amount of 20–30% did not significantly affect the strength of the cement–sand compositions, which were within the strength of the control sample.
Use of the nephrite has some environmental aspects. That is due to a fact that nephrite is composed of tremolite which during milling/grinding generates a highly carcinogenic tremolite dust similar to asbestos. Nephrite dust increases a risk of a lung cancer as well as other types of cancer. So, if tremolite has to be milled during a proposed technological process, then the entire industry needs to be located far from inherited areas and workers need to wear safety equipment (at least masks preventing from highly toxic dusts).
As a result of the conducted studies, it was found that low grade nephrite of the Ulankhodinskoye deposit of the Kharanur massif of the Eastern Sayan can be used as a mineral additive in cement production. It was determined that the studied rocks belong to low-iron tremolites and consist of minerals of the tremolite–ferroactinolite series.
Composite binding materials with the addition of low-grade nephrite were obtained. It is established that their physical and mechanical characteristics depend on the amount of additives, the time of grinding the raw mixture, and the conditions of hardening.
It was found that the introduction of a mineral additive in the form of low grade nephrite does not lead to the appearance of new phases in the hydrated cement, i.e., it does not interfere with the process of its hydration. However, the strength properties of mixed cement increase. Thus, the compressive strength of the obtained sample is 25% higher than that of the standard sample, and is 84.2 MPa.
It should be noted that, in contrast to normal-humidity hardening, heat-and-water treatment does not lead to an increase in the hydration activity of cement compositions. It was determined that the optimal technological parameters for obtaining new types of materials are: the grinding time of the raw mixture—10 min, the amount of additives—30%, and the hardening conditions—28 days of normal humidity hardening.
The physical and mechanical properties of the obtained materials meet the requirements of the national standard of the Russian Federation.
Thus, waste from the extraction of varietal nephrite can be used as a mineral additive in the production of mixed cements. Their involvement in industrial turnover will help to reduce the negative impact of the mining industry and cement production on the environment.