FAQ

Cement is the glue that binds materials together. It constitutes building material mixes:

• Mortar (palitada) -- cement, water, fine aggregate (sand)
• Concrete (concreto) -- cement, water, fine aggregate (sand), coarse aggregates (usually, either G1, 3/4", 3/8" or a combination of each)

Chemical and mineral additives and admixtures may be added to improve concrete or mortar performance.

Admixtures are used to alter the fresh and long-term properties of cement-based mixtures for a specific purpose.

There are two types of admixtures -- mineral and chemical. A mineral admixture commonly used in concrete is fly ash, a by-product of coal-fired power plants. Chemical admixtures have various types -- plasticizers (increase the workability of fresh concrete using less water, therefore, improving later days strengths), accelerators (accelerates strength development of concrete, allowing for faster demolding time), and retarders (delays setting of concrete to increase board life and improve finishes).

The use of admixtures requires the guidance of the manufacturer. The instructions must be followed accurately as overdosing may have deleterious effects in your concrete performance.

Each concrete design mix requires a different amount of water, depending on the expected concrete performance. However, as a rule of thumb, the lower the amount of water used, the better the concrete's strength development is. Therefore, a water to cement ratio of 0.4 is the most desirable. This translates to 16 liters of water per 40kg bag of cement. It is ideal to use 12 liters of water in the initial mix, and adjust accordingly until the desired slump or flow (labnaw) is attained; however, the maximum amount should not be more than 20 liters of water per bag of cement.

Ideally, it is desired to start finishing (smoothing) the concrete surface when bleeding has stopped or when the concrete starts to lose its plastic state (thumb print test is ok). If concrete is finished before bleeding is completed, bleed water will dispersed and this can cause plastic shrinkage cracks.

Overworking of concrete surface must also be avoided. If overworking occurs, especially in high water content mixes, water will be pushed up and can create a week layer. This will reduce the concrete durability -- its resistance to the surface's frictional abrasion, and can lead to flaking.

Usual reinforced concrete structures (residential, pavements and roads) have a standard 1:2:3 mix; this means 1 bag of cement, 2 cubic feet of sand and 3 cubic feet of coarse aggregates.

For mortar, a standard mix of 1 bag of cement to 4 cubic feet of sand is suitable.

Look at the front of the bag on the upper right hand side. There is a (set of) band(s) to determine the cement type (as required by DTI and Philippine National Standards).

• Portland Cement  (Type I, II, III, IV and V) -- single red band
• Blended Cement (Type IP) -- three yellow bands
• Pozzolan Cement (Type P) -- single yellow band
• Masonry Cement (Type N, S and M) -- two brown bands

Another way of determining the type of cement is to look at the lower part of the bag that contains a descriptive box required by DTI and PN Standards.

Adding water to your concrete or mortar mix to regain workability (re-tempering) is not recommended; the reason is that when concrete or mortar starts to set, there is no turning back, adding water may make your mixture look flowable or workable but this is actually superficial and will ultimately cause more problems -- reduced strengths, honey combs, loss of binding (kapak) -- due to excess water.

Masonry cement is supposed to be used only for masonry works -- block-laying, plastering and sometimes for toppings. The focus of masonry cement is not on strength but more on workability and board life; since all concrete structures require minimum compressive strengths, masonry cement will not be able to achieve these strengths.

If your mason, foreman or worker accidentally uses masonry cement for your concrete structures, the best alternative you can do is to first not to remove the support forms until you have consulted your civil engineer to assess the situation.

In ideal situations a bag of cement can be stored for three months; after this, it is required that the cement must be retested to be recertified that it still complies with the minimum and maximum requirements of the standards. However, if you notice hardening in the cement, please call your supplier so the proper personnel from Lafarge can examine and assess your cement's condition.