# Geomaterials: Latest Advances in Materials for Construction and Engineering Applications
Gypsum is calcined to produce plaster products and is an essential component in cement manufacture. Blended with ground cement powder, it controls the hydration kinetics of the aluminates in the clinker, preventing 'flash set'. Limestone is an important geomaterial, both as a building stone and as a source of calcium in cement production and through simple calcining is the precursor for lime products (CaO and Ca(OH)\({}_{2}\)). Zeolites should be considered amongst modern geomaterials, reflecting their use as ion exchange media in permeable reactive barriers (PRBs). This rather niche application is part of a much wider field of subsurface barrier construction to control the flow and quality of groundwater and, particularly, to constrain pollutant migration. Permeable reactive barriers allow the passage of ground water through chemically active media. Zeolites can sorb specific ions from a solution, whereas zero valent iron controls the redox potential of mobile solutions. Crushed limestone in PRB applications is used to raise the pH of moderately acid ground waters which may then precipitate metal ions from solution. An example of a non-permeable sub surface barrier is curtain wall construction. A trench is excavated into which a mineral mixture is poured. One example is a Portland cement--blast furnace slag and clay mixture--which never truly hardens. It remains compliant, setting to form a very low permeability semi-solid, which restricts the flow of groundwater through it. Recent decades have seen the concept of resource efficiency become important to society and the economy. The re-use of industrial wastes and by-products is of mounting importance, driven in part by corporate social responsibility and in part by the rising taxation on waste disposal. This has led to the inclusion of many new materials in the geomaterials field. To this can be added spent foundry sands, ceramic wastes from many sources, waste sea shells, crushed brick and concrete wastes, highway 'blacktop' planings and a wide range of solid combustion products, all of which are reused in construction. Demolition waste has been used as bulk fill and highway sub-base layers for many years. The inclusion of supplementary cementitious materials has been driven by the desire of the cement industry to reduce the embedded carbon of cement, following a report by the World Business Council for Sustainable Development in 2002. This highlighted the CO\({}_{2}\) emissions released by the cement industry and the need for change. The solutions were considered by [PERSON] [9] who described low energy cement linkers:  
1. Pozzolan-based cements. 2. Calcium (sulfo)aluminate-based cements (made from low-grade alumina sources). 3. Calcium sulfate-based cements. [PERSON] compares the pros and cons of each approach and concludes that \"_Based on the present analysis, the most promising low-CO\({}_{2}\) alternative cementing systems appear to be those that make use of large amounts of either natural or artificial pozzolans or those that effectively stabilise hydrated calcium sulfates (e.g., as ettringite)_\". The United Nations Environment Programme commissioned a study ([PERSON] et al. 2016) [10] which also considers a range of options. Most promising of these is the use of calcined, kaolinite clays blended with (and importantly, replacing) much of the Portland cement clinker. The technology, known as LC\({}^{3}\), includes limestone flour in the mixture and is proving to be a step-change in CO\({}_{2}\) reduction from cements, as the raw materials are widely available and need simple processing. [PERSON] and [PERSON] [11] similarly considered options for CO\({}_{2}\) reduction focusing on \"_the potential to blend clinker with alternative materials, and on the use of 'novel cements'--two levers that can reduce the need for clinker itself by lowering the proportion of clinker required in particular cement mixtures_\".