The Effect of Using Lightweight Aggregate on Some Properties of Cement Mortar

The aim of this research is to produce lightweight cement mortar with properties better than reference ordinary cement mortar. Porcelanite stone were utilized as lightweight aggregate with a volumetric partial substitution of fine aggregate. The process includes using different percentages (5, 10, 15 and 20 %) of pre-wetted (24hr.) porcelanite to produce lightweight mortar with internal curing. Water curing was used for reference mortar mixture and air curing for the other mixtures of porcelanite substitution. Compressive strength, flexural strength, density and ultrasonic pulse velocity for different ages (7, 14 and 28 days) have been tested. The results show an improvement in the properties of cement mortar especially in replacement percentage of 10 %.


Introduction
Mortar is defined as a mixture of finely divided hydraulic cementitious material, fine aggregate, and water in either the unhardened or hardened state; hydraulic mortar (ASTM C219, 2014). Mortar utilized as binder building blocks (bricks, stones, and concrete masonry units) together, seal the gaps between them, and decoration. Another kinds of mortar includes pitch, asphalt, and clay, were used between bricks. Ordinary Portland cement (OPC) mortar, commonly known as OPC mortar or cement mortar, is created by mixing OPC, fine aggregate and water. The advantages of Portland cement are its hardness, quick setting and fewer skilled workers required in building structures. Cement mortar becomes harder as age increases with continuing of curing. Mortar must be weaker than building blocks, because mortar is easier and less expensive to repair. Porcelanite is one of the most important sedimentary rocks. It is a lightweight aggregate with a white colour, low density and high permeability.
Lightweight concrete (LWC) has been used for structural members of buildings and bridges because it is lighter in weight than normal concrete. LWC permits reducing dead loads and costs of superstructure and foundation. It has better properties of fire resistance, heat and sound insulation than of normal concrete (Slate, F. O., Nilson, A. H. and Martinez, 1986).

Experimental investigation 2.1 Materials
Portland cement (CEM I 42,5 N) (BS EN 197-1, 2011) was used for mortar mixture throughout present work. Also, the compounds of cement were calculated according to Bogue equations(ASTM C150, 2011). Natural sand was used as fine aggregate in all mixtures. The specific gravity for sand was (2.7), absorption was (1.2)%, and the grading was according to (BS EN 196-1, 2005). Porcelanite stone was used as a partial replacement of fine aggregate in all mixes (except for reference mix). The replacement percentages were (5, 10, 15 and 20) % as a volumetric replacement as same grading as fine aggregate. Porcelanite was pre-wetted with water in lab temperature for 24 hrs. to have saturation, as recommended in (ACI 211.2, 2011). Porcelanite specific gravity was 1.68, and absorption was (42) (2) and (3).

Preparation, casting, compaction and curing
After the cleaning of steel moulds the internal faces were thoroughly oiled to avoid adhesion with mortar after hardening. The mixing procedure, Casting and compaction of cubes and prisms were according to (BS 4550-3.4, 1978) and (BS EN 196-1, 2005) respectively. After (24) hours from mixing, the samples were demoulded. Reference mixture specimens were kept in water curing tank until testing. All other mixtures with added porcelanite were cured in the air at lab temperature.

Testing
Compressive strength test was carried out according to (BS EN 196-1, 2005). Three cubic specimens of (7.07) cm were tested at ages 7, 14 and 28 days. Flexural strength test for prismatic specimens (16 cm *4 cm *4 cm) was carried out according to (BS EN 196-1, 2005). The ages of specimens were 7, 14, and 28 days. Density test was performed according to (BS EN 1015-10, 1999) at ages 7, 14 and 28 days. The ultrasonic pulse velocity test is one of the non-destructive tests. Ultrasonic Pulse transit times are measured by direct transmission method. This test is carried out according to (ASTM C597, 2009) on prisms of length [16cm] at ages 7, 14 and 28 days.  Figure (1) shows that there was an increment in the compressive strength with an increase in the percentages of porcelanite replacement up to (10%) and thereafter, it decreased with increasing the percentage of the porcelanite in the mortar. In spite of the fact that the porcelanite aggregate is much weaker than the normal fine aggregate, the increasing in the compressive strength up to (10)% porcelanite replacement may be due to enhanced cement hydration because of using a saturated porcelanite which works as an IC agent, and this improves of the interfacial transition zone and reduces shrinkages that induced the micro cracking.  Figure (2) explains the increment of flexural strength with the increase of percentages of replacement compared to reference mixture (0% porcelanite). In the cases of using percentage of replacement more than (10) %, the flexural strength decreased but still more than reference. Such behaviour is probably due to; firstly, the increase of percentage of lightweight aggregate which it weaker than normal aggregate and secondly, because of the increasing of reserved water in mixture. For all specimens, the results indicated that with the progress of curing age all mortar specimens show a continuous gain in flexural strength.  (3) shows that the density of mixtures with porcelanite replacement percentages of (5 and 10) % were more than reference mixes (except for 5% at age 7 days). The densities of percentages (15 and 20) % were less than other percentage of replacement. At age of 28 days, there was an increase in density with (15)% porcelanite, more than reference and (5)%, which occurs because of the continuity of IC by using the additional water supplied by saturated porcelanite and un-hydrated particles of cement. For specimens which using air curing, the additional water promotes higher degree of hydration which fills the pores with hydration products (Gel) and increases the density.  Figure (4) shows that using porcelanite aggregate as partial replacement with percentages more than (15) % decreases UPV of mortar compared with reference mortar. The decreasing in UPV can be attributed to; firstly, high moisture content within the mortar containing saturated porcelanite aggregate as internal curing materials. Secondly, UPV decreased due to increase the voids within the mortar which have a negative effect on UPV. The percentage of (10) % replacement gave better results than (5) % and reference mixture. The increasing in UPV may be due to improve the density and homogeneity of cement mortar because of the IC which effects on hydration of cement. This hydration filled more micro pores with hydrates.

Conclusions
Based on the results presented in this paper it can be concluded that: -The behaviour of mortar cement with replacement a partial volume of fine aggregate with porcelanite can be used in the fabrications of lightweight boards and thin sections in construction sector without using water curing.