Cement & concrete
0.02-1.25 usd/kgConcrete is a complex composite, which can contain all manner of material, including cement, plant, wood, metal and plastic. Using very low cost ingredients, such as industrial by-products, makes it a cost-effective material for large-scale projects and structures.
Ordinary concrete contains a mix of Portland cement (OPC), aggregates and sand. When mixed with water, the cement forms a paste that fills the voids between the sand and aggregate, binding them together. The hardening process, which is the result of a chemical reaction with water, is known as hydration.
Regular Portland cement is made by cooking limestone, sand and clay in kilns at 1,450 degC. Even though it has relatively low kgCO2/kg, it is consumed in such huge volumes that it accounts for around 8% of global CO2 emissions according to The World Economic Forum. Around 50-60% the CO2 emissions come from the limestone as it decomposes in the kiln to form reactive lime (reactive calcium oxide, RCC). The remainder of the CO2 comes from burning fossil fuels to heat the kiln. It is not practical to achieve the high temperatures required for the kiln using electricity, which makes it difficult to reduce the carbon footprint using current technology.
Cement-based building products and concrete are available in eight basic forms, each with its own unique quantities and advantages:
– Ready-mixed, which includes cement, water, sand and aggregates. It is used for casting and pouring.
– Pre-cast concrete, which is produced in a factory and includes everything from masonry blocks and decorative trim to very large reinforced concrete structures.
– Cement-based products that are not strictly classed as concrete, but share many of the same qualities. They are typically a mix of cement, water, sand and perhaps lime. Examples include mortar, render, terrazzo and grout.
– Cement-based products mixed with special fibres or additives to create products such as roof tiles, countertops and construction boards.
– Polymer concrete uses plastic to replace the cement, either partially or entirely. In this case, the concrete hardness through a process of polymerisation. Polymer concrete is typically much more expensive, but has some desirable benefits for certain applications.
– Biocement is made with calcium carbonate (CaCO3) produced by microorganisms and is being explored as a sustainable alternative to Portland cement. Through a process of microbiologically induced calcium carbonate precipitation (MICP), microorganisms react with chemical components to produce minerals suitable as binding agents. As well as having potential as a building material, it is used for reinforcing soils, such as is important for transport infrastructure and sea defences. Compared to OPC it can reduce carbon emissions by up to 90%.
– LC3 cement has a reduced carbon footprint (30-40%) as a result of partially substituting (20-70%) the clinker used in production with calcined clay and limestone. The reduction is the result of reducing the firing temperature and avoiding the decomposition of limestone, which is responsible for a significant proportion of the CO2 of cement production. Clay is calcined (heated to around 800 degC, as opposed to 1,450 degC for regular cement) to make it suitable. It is widely available and compatible with modern cement manufacturing processes. While clinker is a waste product from burning coal and steel furnaces, it is not always available close to the cement factories, and the processes that generate it have come into question over sustainability concerns.
– Low-carbon cement (green cement) is produced using various techniques such as with renewable energy (fuel from biomass, for example); using Portland Limestone Cements (PLCs) and supplementary cementitious materials (SCMs) in the mix; and with carbon capture, such as harnessing industrial CO2 emissions in the production process, or injecting CO2 back into concrete to strengthen it.
Search similar materials
Find better material
Reinforced concrete (RC) is made with steel bars (rebar), rods or mesh. The steel is a means of providing tensile strength, as well as improved shear and sometimes compressive strength, enhancing the overall performance and durability of concrete. Known as deformed reinforcement, the surface of rebar is ribbed, or textured, to increase surface area and provide a strong mechanical interlocking with the concrete when it hardens. The most common type of rebar is produced from mild steel, and may include a mix of down-cycled steels. Stainless steel is also used, such as lean duplex grade 2101 (UNS S32101, DIN 1.4162), because it provides high performance for the cost.
The rebar is graded according to tensile strength, such as Grade 40 / Grade 280 (40,000 psi / 280 MPa), Grade 60 / Grade 420 (60,000 psi / 420 MPa) and Grade 80 / Grade 550 (80,000 psi / 550 MPa). Any higher than 550 MPa and the tensile strength of the rebar will not contribute to increased compression capacity in the concrete. High-strength reinforcing bars (HSRB), considered to be anything over around 400 MPa, allow for the reduction of the amount of steel required, and slimming the cross-section of the concrete. Grade 100 / Grade 690 (100,000 psi / 690 MPa) is only suitable for special seismic systems such as bars, columns and floors.
Reinforced concrete may also be graded according to compressive strength, such as in UK system, as follows:
– RC20/25 (25 MPa) used in light commercial and domestic projects.
– RC25/30 (30 MPa) suitable for wearing surfaces and trolleys for example.
– RC28/35 (35 MPa) used in commercial slabs, agriculture, paths, driveways and seafronts.
– RC32/40 (40 MPa) is found in large industrial floors, warehouses and sheds.
– RC40/50 (50 MPa) for heavy duty traffic in industrial and commercial settings.
The performance of reinforced concrete is enhanced with prestressing (pre-tensioning) or post-tensioning (post-stressing). These techniques make the most of the tensile strength of steel combined with the compressive strength of concrete; allowing for longer spans than is possible with ordinary reinforced concrete.
Prestressed concrete is made by holding the rebar under tension (stretching) while the concrete hardens. The steel is relaxed once the concrete has fully hardened, putting the structure under compression and eliminating the potential for cracking under normal conditions. Post-tensioning is the process of applying tension to the rebar after the concrete has hardened, putting the structure under compression. While prestressing is typically carried out in a factory to produce precast parts, post-tensioning is used on side for large structures such as box-girder bridges and floor slabs. The steel cables used in these types of construction have tensile strength in the region of 1,800 MPa; and the concrete a compressive strength of 40-70 MPa.
To reduce the weight of long spans, voids may be incorporated by adding expanded polystyrene (EPS) blocks before pouring the concrete. This technique is used to make box sections, for example.