Construction World September 2022

The fairly square shape of the shallow pit allows for big blasts.

building the aggregate must be devoid of any particles,” says Du Preez. Dust on the aggregate surface adversely affects bituminous hot mix pavement performance and decreases the long-term performance of roads. “In order to produce the various products for the road building industry, a washing process is needed to meet the requirements and specification of the saleable products. During this process those fines that are by-products of the washing process are discarded.” The tailings produced as a result of the washing of aggregates are pumped to the unused dolomite pit that is used as a tailings dam and its rehabilitation facility. “We have a closed circuit regarding water within the operation itself,” explains Du Preez as environmental regulations prohibit the release of untreated contaminated water into the stream that runs alongside the site, and it is economically feasible to recover and reuse the same process water again. Rooikraal Quarry does not make use of municipal water as this would be uneconomical given the volume of water needed for washing. Instead, the quarry is dependent on rainwater that is collected in both the dolerite and dolomite pits. “The last time the facility used municipal water was in 2014. Since then seasonal rain has provided us with enough water to run the closed circuit. On the dolerite side there is a sump too and we collect water there and pump the water to the dolomite side,” says Sterley. There is an HDPE pipeline running underground from the unused pit (which functions as a tailings dam) to the

pozzolanic effect (the chemical reaction between reactive silica or alumina present and portlandite formed during the cement hydration in the presence of water) and therefore is ideal for ready-mix and precast customers, it is limited to these markets. “Dolerite has a wider use with which to produce saleable product,” he says. Rooikraal Quarry’s dolerite is crushed to produce an array of material sizes including G6 (base course); ballast; 7,1 mm, 22 mm and 28 mm concrete stone; 10 mm, 14 mm and 20 mm road stone, as well as crusher sand. “Apart from major companies in Gauteng that are involved in the supply of asphalt for road building, Rooikraal supplies its aggregate to the Free State, from Heilbron to Deneysville to Villiers. It has even supplied its product – because of the competent rock that is mined – to some projects in KwaZulu-Natal,” says Sterley. In addition, the quarry supplies material to the construction companies and sister establishments within AfriSam. Aclosedcircuit forwashing “All the products produced by the quarry are washed. The plant was originally designed for the asphalt market. In road

37 CONSTRUCTION WORLD SEPTEMBER 2022