Experimental study on strength improvement of weak subgrade clay soil using copper slag and hydrated lime as an activator

Roads constructed on weak sub-grade clay soils with high plasticity may be affected by the behaviour of the clay soils. Weak sub-grade clay soils undergo volume change due to the changes in the moisture content which triggers cracking, settlement, heaving and damage of the road pavement by breaking up the pavement. Stabilizing these types of weak sub-grade clay soil is required to overcome the swelling of the clay soil and increase the soil strength and improve the performance. The main aims of this research were to study the effect of copper slag (CS) without and with hydrated lime on the engineering behaviour (compaction, swelling potential, plasticity characteristics and unconfined compressive strength) of the composite and to identify the reaction products of the stabilized soil through X-ray diffraction and scanning electron microscopy. To accomplish these aims, extensive laboratory tests were conducted on various mixes (from 2 % up to 10 % of CS by dry weight of the weak subgrade clay soil and from the ratios of 9:1 up to 7:3 replacements by hydrated lime) which were prepared and cured under two conditions (35 ° C with relative humidity of 90 – 100 % referred to as (CC1) and 45 ° C with a relative humidity of 50 – 60 % referred to as (CC2) for up to 74 days. The plasticity characteristics and compaction properties were determined soon after mixing while the unconfined compressive strength (UCS) and the swelling potential were determined after longer curing periods.

In general, the study showed that the engineering properties (plasticity, unconfined compressive strength and swelling potential) improved with CS addition and with increasing temperature and curing period. The addition of copper slag and hydrated lime to the soil resulted in an enhancement of about 76 % increase in strength of the composite soil reported in the study. For the compaction test, the maximum dry density (MDD) reduced while the optimum moisture content (OMC) increased with increment in CS and hydrated lime content.

The main alterations in the swelling and the unconfined compressive strength are due to the development of new cementitious materials. The analytical work verified two main reactions when hydrated lime and CS were added to the weak subgrade clay soil, the hydration of CS activated by hydrated lime yield calcium aluminium silicate hydrate paste and hydrotalcite type of phase and the clay soil-hydrated lime reaction yield calcium silicate hydrate, calcium alumina hydrate and calcium alumino silicate hydrate. The scanning electron microscopy showed that aluminium silicate (Al/Si) ratio and amount of Si in the formed calcium silicate hydrate increased significantly with an increase in the curing temperature and curing period which suggests a stable and perfect crystalline form of calcium silicate hydrate. The results indicate that the use CS alone or preferably with hydrated lime significantly decrease the swelling potential and increase the strength of the soil.

The recommendations for further investigations include a study on the effect of cyclic loading of the composite soil and field trials to evaluate or measure the suitability of the composite.