Civil Engineering August 2022 | Vol 30 No 7

Civil Engineering August 2022 33 RESULTS Stringent laboratory tests, according to both ASTM and SANS standards, were used to differentiate the applicability of lithological units for road construction and concrete applications. Geotechnical laboratory testing methodologies included determining particle properties (grading, flakiness index, relative density and water absorption), strength resistance (aggregate impact value, aggregate crushing value and 10% FACT), durability (alkali-silica reaction), road performance (Los Angeles abrasion and polished stone value) and geophysical (radiometric analysis and magnetic susceptibility) and mineralogical properties (X-ray diffraction, thin section analysis and hyperspectral scanning). Test results indicated a number of areas with good quality aggregate based on physical characteristics such as flaki- ness, water absorption, crushing strength, durability, abrasion and polishing action. Relative density and water absorp- tion were mostly in line with the typical expected values for natural crushed aggregates, with the exception of selected samples that tested outside the normal range of values. Strength properties mea- sured using ACV and 10% FACT appeared to be satisfactory for the majority of the field samples, as summarised in Figure 2. Mineralogical techniques are gener- ally underutilised in the assessment of construction materials in South Africa. In order to add to the body of knowledge regarding the assessment of construc- tion materials from this perspective, representative mineralogical assessments were conducted on all lithological units. Results for Karoo dolerite (93 samples) are summarised in Figure 3, representing hyperspectral scanning results, mineral- ogical thin section analysis and XRD. Based on thin section analysis, samples of the Karoo dolerites are microscopically marked by analcite. Pervasive sodic altera- tion characterised by the predominance of albite (occasionally altered to sericite) was observed, generally aggregated as lath- shaped subhedral phenocrysts embedded in olivine, epidote and pyroxene with irregular contacts (Figure 3Ba to f). Trace amounts of biotite, chlorite and potassium feldspar mineralisation were observed. Interstitial grains of clino- pyroxene (mainly augite) and dendritic opaque minerals (mainly magnetite) were seen to be enclosed by a cryptocrystalline mass, as well as larger grains of albite with a subophitic texture (Figure 3Bb and c). CONCLUSIONS It is evident from the integration of afore- mentioned datasets that Karoo dolerite is the primary target for most quarries in the study area. Out of 83 Karoo dolerites tested, 92% were found to be suitable for road building applications and 95% proved satisfactory for concrete applications. The Dwyka Group tillites indicated a suitability of 11% for road building and 6% for concrete applications, while arenaceous rocks of the Natal Group (i.e. sandstone) and the argillaceous rocks of the Ecca Group (i.e. shale and slate) accounted for 3% for road building applications and between 2% and 5% for concrete applications. Radionuclide concentration indices for almost all the plutonic rocks (granites, gneisses, leucogranites, quartz diorites and migmatites) showed that the radia- tion exposure exceeded allowable limits for concrete construction applications. In addition, it is worth mentioning that the relatively high concentration limits of Dwyka tillite can be attributed to the granitoid clasts embedded in the glacial rock matrix. WAY FORWARD The results of this study successfully proved a new approach to identify and delineate hard rock aggregate potential sites using an integrated geoscientific approach. This allowed for the delineation of quarry potential zones on a regional scale, with a view to confirming expected suitable and unsuitable lithological units for hard rock aggregate applications. It is anticipated that this approach will assist in accelerating efforts to deliver world-class infrastructure and focus- sing efforts on the detailed site-specific characterisation of hard rock aggregate sources. Ongoing efforts are undertaken to refine our approach by incorporating 4IR applications for predictive modelling capabilities and fundamental research into aggregate quality improvements – especially in areas where naturally occurring good-quality aggregate is not abundant.  A) Karoo dolerite hand specimen. B) Photomicrograph illustrating sub-ophitic texture. C) Photomicrograph illustrating a transmineral fracture and glomerocryst texture. D) Photomicrograph illustrating localised fractures in quartz and glomerocryst texture in epidote. E) Photomicrograph of sericite altered plagioclase. F) Photomicrograph illustrating glomerocryst and sub-ophitic texture in Orthopyroxene (Opx). (Photomicrographs were taken under transmitted light in XPL). Sample ID Calcite Dolomite Hematite Ilmenite K-feldspar Plagioclase Quartz Mica Chlorite Kaolinite/ Chlorite Clino- pyroxene Ortho- pyroxene Olivine Pyro- phyllite/ Talc Talc Amphibole I/S Interstrati fication Smectite Pyrite Zeolite Karoo dolerite 0 -4 - - 0 -3 0 -7 18 -94 0-29 0-8 0-29 - 0 -80 0 -19 0 -17 0 -7 0 -5 0 -8 0 -3 0 -3 - 0 -3 XRD semi-quantitative results (wt.%) LWR (OWL): Dominant mineral map A B C Figure 3 Summarised mineralogical analysis of Karoo dolerites across southwestern KwaZulu-Natal