Crushed sand, dune sand or river sand: which sand for concrete in Algeria?

For decades, concrete in northern Algeria was made with alluvial sand dredged from wadis. That resource is finite, its extraction damages riverbeds and water tables, and it has been restricted and policed for years. The result is a market in which clean alluvial sand with a good curve is expensive and uncertain, especially far from the few authorized sources, and in which contractors look to the two alternatives the country has in abundance: the dune sands of the Sahara and the crushed sand of limestone quarries.

Dune sand is rounded, clean and remarkably uniform, which is the problem. Its grains are nearly all in a narrow band of fine sizes, so its fineness modulus is far below the range that concrete standards prefer; a concrete made with it alone needs more water and cement to be workable, shrinks more and tends to lower strength. Algerian research has nevertheless studied it extensively, because it is free and everywhere in the south: used as a corrector blended with a coarser sand, in some cases with fines or additions to complete the curve, it gives acceptable concretes, and it is widely used in mortars and in sand-based materials for roads in the Sahara. The rule of thumb is that dune sand corrects a curve; it rarely makes one on its own.

Crushed sand, also called manufactured sand, is the 0/3 to 0/5 fraction that comes out of a crushing and screening plant, in Algeria mostly from limestone. Its grains are angular and rough, which improves the bond with the cement paste and the mechanical strength of the mix but demands a little more water or admixture for the same workability. Its curve is continuous and can be adjusted at the plant, and it contains a share of fines, the dust of fracture, which is both its weakness and its asset: too many clayey fines harm the concrete, but clean limestone fines fill the voids and improve cohesion.

This is why the methylene blue test, which measures the activity of the clay in the fines, matters as much as the sand equivalent for crushed sand. Algerian studies of conventional concretes and mortars made with crushed limestone sand generally find an optimum around 10 to 15 percent fines, with strength and durability maintained up to about 20 percent in some series, when those fines are limestone dust rather than clay; the specification nevertheless caps the fines at a category fixed by the standard. A plant can also wash or air-classify the sand to bring the fines content to the figure the specification asks for.

For hot-mix asphalt the answer is less ambiguous than for concrete: crushed sand is preferred because its angular grains interlock and resist rutting under heavy traffic in hot weather, which is exactly the Algerian condition; rounded sands make a mix that is easier to lay but deforms sooner. In unbound road layers the sand fraction of a graded aggregate comes from the same crusher as the gravel, so the question rarely arises, whereas in the Sahara sand-based stabilized layers use dune sand by necessity.

The project laboratory makes the choice, with trial mixes, from the sands actually available within an economic distance of the site. What a supplier can do is deliver a sand whose curve, fines content and cleanliness stay the same from the first truck to the last, and provide test results for each delivery period. For a project owner, the useful questions are where the sand comes from, how its fines are controlled, what its sand equivalent and methylene blue values are, and whether supply can hold the rate the site needs.

SARL ETAHG produces high-quality crushed fine aggregates at its own quarry and stone crushing plant at Oued Sdeur near Aïn El Ibel, in the wilaya of Djelfa, with machines suited to projects that require high production rates, and delivers with its own trucks and semi-trailers. Test requirements are agreed before supply. See fine aggregate production and, for the use of the same material in pavements, road construction.