Globally the idea of sustainability has now taken a centre stage. The environmental concern focuses on overexploitation of natural resources such as limestone, clay, etc and release of harmful gaseous substance into the global environment from the cement industries worldwide. The effect of these gases in the atmosphere has resulted in a rise of earth atmospheric temperature which has led to global warming creating catastrophic situations such as flooding, droughts and cyclones for human existence on earth.
The construction materials industry is under increasing pressure to reduce the energy used in the production of Portland cement clinker and the associated greenhouse gas emissions. Further, Portland cement is not the ideal binder for all construction applications, as it suffers from durability problems in particularly aggressive environments. Several alternative binders have been available for almost as long as Portland cement, yet have not been extensively used, and new ones are being developed.
In this article we will discuss about the alternative binders for construction. Some of them are analysed below.
Calcium aluminate cements
Calcium aluminate cementsare cements consisting predominantly of hydraulic calcium aluminates. Alternative names are “aluminous cement, “high-alumina cement” and “Ciment fondu” in French. They are used in a number of small-scale, specialized applications. The main active constituent of calcium aluminate cements is monocalcium aluminate (CaAl2O4, CaO · Al2O3, or CA in the cement chemist notation). It usually contains other calcium aluminates as well as a number of less reactive phases deriving from impurities in the raw materials. Rather a wide range of compositions is encountered, depending on the application and the purity of aluminium source used.
A geopolymer is realised by activation of an aluminosilicate prime material having a low calcium content with an alkaline solution at room temperature. Regarding kaolinite-based raw materials, the production of geopolymer cement comprises in two main procedures: calcination and geopolymerisation. The calcination process consists in the thermal treatment of the raw material, which leads to dehydroxylation of clay mineral into a high-energy and nearly amorphous meta-state. One of the main reason to use claybased geopolymer binders as cement alternative is the fact that the dehydroxylation process occurs at lower temperatures than the burning of cement clinkers. After the addition of the alkaline solution to the meta-clay, the geopolymerisation is initiated and is based on three complementary processes, namely, the depolymerisation of the minerals into monomers by condensation of the alkaline component, the formation of oligomers and the polycondensation into a geopolymer ribbon by combination of the oligomers into a covalently bonded network. The synthesized geopolymers show interesting characteristics like good mechanical properties, high strength and good durability. Davidovits presented in the example of an aluminosilicate geopolymeric cement based on calcined kaolinite clays, blast furnace slag and silica fume which were activated using alkali-silicates. The resulting cement hardened at ambient temperature with early compressive strength of around 20 MPa (after 4 hours). The final 28-day compressive strength range from 70 to 100 MPa.
Supersulfated cement consists of granulated slag mixed with 10 to 15 percent hard-burned gypsum or anhydrite (natural anhydrous calcium sulfate) and a few percent of portland cement. The strength properties of supersulfated cement are similar to those of portland cement, but it has an increased resistance to many forms of chemical attack.
The components proportion of SSC usually is 75% ~ 75% slag, 10% ~ 20% of sulfate class (such as dihydrate gypsum and anhydrite, etc.) and 1% ~ 5% content of alkaline elements (such as clinker, calcium hydroxide, etc.). With good properties and simple manufacturing technique and low cost, SSC is a kind of energy conservation and environmental friendly cement. Contrary to the ordinary cement, SSC has the advantage of lower heat of hydration, control thealkali aggregate reaction (AAR) because of the higher content of granulated blast furnace slag that is useful to restrain AARand good sulfate resistance.
At present the sulfate cement has very good sales market, which has been applied into multiple fields, such as sewage treatment plant, pool, water concrete, industrial workshop floor mass concrete, concrete pile and other aspects. Recently the main research of SSC is focused on the use of industrial waste residue in SSC, the physical mechanics performance of SSC, which showed that SSC has lower strength in the early age but can overtake the strength of ordinary cement.
High-alumina cement is a rapid-hardening cement made by fusing at 1,500 to 1,600 °C (2,730 to 2,910 °F) a mixture of bauxite and limestone in a reverberatory or electric furnace or in a rotary kiln. It also can be made by sintering at about 1,250 °C (2,280 °F). Suitable bauxites contain 50 to 60 percent alumina, up to 25 percent iron oxide, not more than 5 percent silica, and 10 to 30 percent water of hydration. The limestone must contain only small amounts of silica and magnesia. The cement contains 35 to 40 percent lime, 40 to 50 percent alumina, up to 15 percent iron oxides, and preferably not more than about 6 percent silica. The principal cementing compound is calcium aluminate (CaO · Al2O3).
Gypsum plasters are used for plastering, the manufacture of plaster boards and slabs, and in one form of floor-surfacing material. These gypsum cements are mainly produced by heating natural gypsum (calcium sulfate dihydrate, CaSO4 · 2H2O) and dehydrating it to give calcium sulfate hemihydrate (CaSO4 · 1/2H2O) or anhydrous (water-free) calcium sulfate. Gypsum and anhydrite obtained as by-products in chemical manufacture also are used as raw materials.
The hemihydrate, known as plaster of Paris, sets within a few minutes on mixing with water; for building purposes a retarding agent, normally keratin, a protein, is added. The anhydrous calcium sulfate plasters are slower-setting, and often another sulfate salt is added in small amounts as an accelerator.
Sand cement plastering is a very time consuming process and involves a long process of procuring various materials like sand, cement, water and mixing them in correct portions. Also, you will need a layer of plaster of Paris punning over your sand cement plaster to fill in the gaps and ridges and have a smooth canvas for paint. All in all, this is a long and messy process. Whereas, gypsum does not need that extra step which eventually saves your money, time and energy. Gypsum is a white soft compound that contains hydrated calcium sulphate, and is a widely used material in constructing interiors of living spaces. Gypsum plastering is an effective alternative to sand cement plastering. It is also extremely labour friendly and gives a smoother and designer feels to the walls and ceilings. It is very light in weight in contrast to sand cement plaster, and thus offers more strength when used in false ceilings and other wall designs. Lesser weight on the frames makes them more robust and long lasting, and gives better protection even in case of natural hazards like earthquake.
Pozzolanas are materials which, although not cementitious in themselves, will combine chemically with lime in the presence of water to form a strong cementing material.
Natural and artificial pozzolanas have been used to obtain hydraulic binders for over a thousand years. Hardening of pozzolanic cement pastes can result from the reaction between pozzolana and the lime that is added to the mix as hydrated lime or is produced following hydration of portland cement silicates. The pozzolanic reaction does not alter cement clinker hydration; it complements and integrates the hydration process because it results in a lower portlandite content and an increase in calcium silicate hydrates.
As in the case of pozzolanic cements, for which the current pozzolana content is about one third by weight of cement, the most outstanding variations induced in the behaviour of portland cement can be summarised as follows. Heat of hydration decreases whilst the rate of clinker hydration increases, paste porosity increases and permeability decreases, both portlandite content and Ca/Si ratio in C-S-H decrease and the C-S-H content increases.
Chemical and physical properties of pozzolanic cements eventually affect engineering ones. Early strength of both pastes and concretes decreases while ultimate strength is often found to exceed that of the reference portland cement.
Lime has been an important component of mortars for over 2000 years. The characteristics of hydrated lime provide unique benefits in masonry applications that distinguish cement-lime mortars from other masonry mortar materials.
There are two forms of lime: quicklime and hydrated lime.
Quicklime is produced by heating rock or stone containing calcium carbonate (limestone,marble, chalk, shells, etc.) to a temperature of around 1000°C for several hours in a process known as ‘calcining’ or sometimes simply ‘burning’. It is an unstable and slightly hazardous product and therefore is normally ‘hydrated’ or ‘slaked’, by adding water, becoming not only more stable but also easier and safer to handle.
To produce dry powdered hydrated lime just sufficient water is added for the quicklime lumps to break down to a fine powder. This material would have a ‘shelf life’ of only a number of weeks, depending on storage conditions. ‘Old’ hydrated lime would have partially carbonated and become a less effective binder. However, if quicklime is hydrated with a large excess of water and well agitated, it forms a milky suspension known as milk of lime. Allowing the solids to settle, and drawing off the excess water, forms a paste-like residue, termed lime putty, which is the form of lime which can be used in building applications to best effect.
Calcium sulfoaluminate cements
Calcium sulfoaluminate cements are well-known alternatives to OPC. They have essentially been developed in China in the 1970s. Designed by the China Building Materials Academy (CBMA), they were intended for the manufacturing of self-stress concrete pipes due to their swelling properties. Calcium Sulfoaluminate (CSA) is a specialty cement used in many applications where high early strength and fast setting development are necessary, such as bridge decks, airport runways, patching roadways, DOT work, tunneling, etc. CSA cements may also be used to manufacture shrinkage compensated or low shrinkage concrete grouts and mortars.
High Early Strength / Rapid Setting: The primary advantage of CSA cements is that concretes, grouts and mortars made with CSA Cement, can easily be formulated to achieve compressive strengths in excess of 5000 psi in just a few hours. With CSA Cement is possible to attain desired 28 day strength in 24 hours. Rapid setting and high early strength gain is critical in situations where an airport runway, a bridge repair or a damaged freeway must be returned to service in a very short amount of time.
The clinker proposed by Lafarge and registered as Aerther® is richer in belite than classical CSA, which allows the use of less expensive raw materials. It consists of a combination of various known chemical reactions of cementitious systems, but overall leads to very different reactivity from Portland cement.
The development of new binders with longer durability is therefore needed. Alkali-activated binders have emerged as an alternative to OPC binders, which seems to have superior durability and environmental impact. Alkali-activated binders, especially those resulting from low-cost industrial by-products, such as coal fly ash or metallurgical slag, represent a sustainable option for cement replacement, though their use is more challenging, due to some technological issues related to workability or curing conditions. This paper presents sustainable alkali-activated mortars cured in room conditions and based on metakaolin, fly ash, and furnace slag (both by-products resulting from local sources) and relevant blends, aiming at their real scale application in the building sector. The effect of binder composition-gradually adjusted taking into consideration technical and environmental aspects (use of industrial by-products in place of natural materials in the view of resources saving)-on the performance (workability, compressive strength) of different mortar formulations.
The binder formulations were developed using the following materials: commercial metakaolin, fly ash (by-product resulting from thermal-power plants) and furnace slag (by-product resulting from metallurgical plants), both from local sources.
AshCrete is a concrete alternative that uses fly ash instead of traditional cement. By using fly ash, a by-product of burning coal, 97 percent of traditional components in concrete can be replaced with recycled material.
Timbercrete is a building material made of sawdust and concrete mixed together. Since it is lighter than concrete, it reduces transportation emissions, and the sawdust both reuses a waste product and replaces some of the energy-intensive components of traditional concrete. Timbercrete can be formed into traditional shapes such as blocks, bricks, and pavers.
Ferrock is a new material being researched that uses recycled materials including steel dust from the steel industry to create a concrete-like building material that is even stronger than concrete. What’s more, this unique material actually absorbs and traps carbon dioxide as part of its drying and hardening process – making it not only less CO2 intensive than traditional concrete, but actually carbon neutral.
The large amount of waste yearly disposed to landfill, the global impoverishing of natural resources and environment, the emergence of carbon dioxide emissions, are some of the motivations driving research institutes and industrial world to move towards sustainable solutions for construction sector. Accordingly, the use of sustainable materials for green buildings construction is an important goal that must be reached in a short time.
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Constructionclimatechallenge.com, Sciencedirect, researchgate, orbilu.uni.lu/bitstream, britannica.com , knowledgepoint.org, LKAB Minerals