Physical and chemical changes of the compounds occur when cement is mixed with water. This process is known as hydration of the cement. It is through this process that the cement-water paste becomes a firm and hard binding mass – the hydrated cement. During this process, the paste initially sets (stiffens) and hardens (gain strength) with time. The different compounds hydrate at different rates, produce heat over different lengths of time thus contributing differently to the rate of strength development of the hydrated cement. Cement properties can greatly be altered by the presence of moisture or on exposure to air. It is for this reason that it is advised that cement should always be stored in a dry place and should never be exposed to the air and the elements.
2.4 Fine aggregates
Aggregates are inert filler and generally constitute at least three-quarters of the volume of sandcrete. This percentage is even greater in mixes for sandcrete bricks production, especially for lean mixes. Because aggregates are cheaper than cement, it is advantageous to pack as much aggregate in the sandcrete as possible. Such high aggregate content “confers considerable technical advantages on the sandcrete bricks, which has a higher volume stability and better durability than hydrated paste alone†(Neville, 2011).
Aggregates are classified in many ways as follows: (i) according to size -- into fine and coarse aggregates, (ii) source - natural and artificial aggregates, (iii) weight - lightweight and dense aggregate, (iv) particle shape - rounded, irregular, angular and flaky and (v) particle gradation - into well graded, poorly graded and gap graded. Fine aggregates are generally those whose particle sizes fall below 5mm while coarse aggregates are those with particle sizes greater than 5mm. The recent British and European standard (BS EN 12620, 2002) however, puts the dividing line between fine and coarse aggregates at 4mm. Natural aggregates are those formed from naturally occurring materials such as weathering of rocks and include sand, gravel, and crushed rock such as granite, basalt, sandstone and quartzite. Artificial aggregates on the other hand are manufactured and include sintered fly ash, aluminum slag and bloated clay. Lightweight aggregates have oven-dry particle density less than 2000kg/m3 whilst those with densities greater than 3000kg/m3 are classified as heavy weight aggregates. In between these two lie the medium weight aggregate (BS EN 206-1, 2000). A well graded aggregate is one that contains all the different sizes in appropriate ratios. Such aggregates make better concrete as the smaller sized particles can always fill the spaces between the bigger ones, thus creating a more compact structure. Again they make the concrete more workable. Gap graded aggregates have some sizes missing while poorly graded aggregates have disproportionate ratios of the sizes. Both do not make for the “best†concrete in terms of strength and workability. Gradation or particle size distribution of fine aggregates is of great importance in the manufacture of sandcrete bricks. This gradation is obtained through a sieve analysis in which a certain amount of the aggregate is passed through a set of stacked sieves successively arranged with the one having the largest aperture at the top.
